Low-carbon indoor humidity regulation via 3D-printed superhygroscopic building components

Abstract

Indoor humidity can significantly impact our comfort and well-being, often leading to the use of mechanical systems for its management. However, these systems can result in substantial carbon emissions and energy precarity. This study offers an alternative: using low-carbon materials that naturally buffer moisture to passively regulate the indoor humidity. A geopolymer composite incorporating industrial waste is implemented via binder jet 3D printing technology. The superhygroscopic nature of the material, combined with the optimal geometry of 3D-printed components, unlocks remarkable potential for passive humidity regulation, achieving a moisture buffering value over 14 g·m⁻²·%RH⁻¹. The use of 3D-printed, geopolymer tiles for surface finishing in a library hosting 15 people was shown to improve annual indoor hygrometric comfort by up to 85%, a performance inconceivable with conventional materials and techniques. Additionally, the environmental impact of these tiles is significantly lower than that of a conventional dehumidification system. This study paves the way for merging highly hygroscopic, low-carbon materials with advanced manufacturing techniques to regulate indoor humidity levels and reduce our dependency on mechanical systems.

In this study, 3D-printed geopolymer tiles were used for passive indoor humidity regulation. With a moisture buffering value over 14 g·m⁻²·%RH⁻¹, in a case study, the tiles improved users’ comfort by up to 85% and with a much lower carbon footprint than conventional dehumidification systems.

Introduction

As we spend most of our lives indoors^1,2, internal environmental conditions significantly impact our health and well-being³. Among various factors, indoor humidity plays a crucial role in determining our comfort, health, sleep quality and productivity^4,5. Low humidity can lead to excessive dryness of mucous membranes and skin, resulting in irritated eyes, itchy skin, and increased respiratory issues^5,6. Conversely, high humidity can intensify the sensation of discomfort from overheating^7,8 and can trigger joint problems⁹, as well as inflammatory rheumatic diseases¹⁰. Avoiding excessive relative humidity helps minimise the survival of bacteria, limits the growth of mould and fungi, and reduces the presence of mites, all of which are allergens that can affect human health^11–13. Additionally, research indicates that important viruses, e.g., influenza and SARS-CoV-2, tend to spread more easily at very low and very high relative humidities (RHs) than at intermediate RH levels¹⁴. For these reasons, maintaining the indoor relative humidity within the range of 40–60% is considered ideal for providing comfortable and healthy indoor conditions, as indicated on the Sterling chart¹¹. This range promotes an environment where occupants can thrive and experience improved well-being.

To maintain appropriate indoor humidity levels, active humidity-regulation systems, which increase building energy consumption, are often employed. A recent study¹⁵ showed that the need for dehumidification can be particularly relevant during the cooling season due to the temperature drop-derived increase in relative humidity. When dehumidification is used in this context, it can retain up to 40% of the energy used for air conditioning¹⁵. Other studies indicate that the use of passive methods to reduce humidification‒dehumidification loads has significant potential for energy savings from mechanical systems¹⁶, specifically, up to 25–30%¹⁷. Given this background and the need for low-carbon passive humidity control strategies, new ideas and alternative solutions to conventional systems are currently needed^15,18.

Porous, hygroscopic building materials offer a chance for the passive regulation of indoor relative humidity, with no need for operational energy. They can absorb moisture from the indoor environment when the humidity increases and release it when the air becomes drier¹⁹. This type of behaviour is particularly suited to spaces that are subjected to the effects of intermittent sources of indoor moisture, such as spaces with temporary human occupation, e.g., bedrooms, offices, and public spaces^20–22. Indeed, when we breathe and sweat, we release water vapour, thus contributing to indoor moisture levels. Hygroscopic materials can absorb this excess humidity and release it after occupation hours, moderating daily peaks in indoor humidity²³. The interest in hygroscopic building materials has increased in recent decades^24,25, with a practical method for quantification developed in the early 2000s, with the concept of the practical moisture buffering value (MBV) of materials and components defined in the context of the NORDTEST project²⁶. Modern advances in materials science and building technologies are starting to offer new options with higher MBVs than conventional building materials. Among others, recent research advancements have shown that geopolymer composites are an innovative solution that offers ground-breaking moisture buffering potential in the field of hygroscopic building materials. As observed in a scientific review²⁷, the highly hygroscopic nature of geopolymer composites has gone unnoticed for a long time. Pioneer studies on the topic were published in 2017–2018^28,29, followed by a few further publications in the years 2022–2023^30–35. In these studies, only three research groups^29–32,35 quantified the moisture buffering value as defined in the NORDTEST protocol, and they all observed MBVs >4 g·m⁻²·%RH⁻¹, which is twice as high as the threshold for classifying materials as excellent for moisture buffering applications²⁶.

Geopolymer composites represent a cutting-edge and environmentally conscious innovation in the construction sector^36,37. They initially emerged as a very promising alternative to ordinary Portland cement-based concrete. The term ‘geopolymers’ was coined by Joseph Davidovits in the 1970s and was applied to a class of solid materials synthesised by activating precursors—aluminosilicate materials, typically metakaolin—with alkaline solutions, usually derived from sodium or potassium silicates³⁸. Through a polycondensation reaction, three-dimensional networks are formed, resulting in a solid and strong geopolymer binder. Compared with the production of conventional cement, the production process requires lower production temperatures and can significantly reduce the carbon footprint of the material³⁹, especially if waste and byproducts are incorporated into the mix formulation^40–43. By repurposing waste and byproducts, geopolymer technology can help reduce landfills and promote a circular economy^44,45. Geopolymer-based materials offer good mechanical properties and durability^46,47, as well as remarkable resistance to chemical attacks, fire, and harsh environmental conditions^48,49. Additionally, they do not emit volatile organic compounds (VOCs)⁵⁰. Geopolymer composites offer a multitude of benefits that make them stand out as emerging nonconventional construction materials. However, most studies have focused on the structural performance of these materials and their potential to replace Portland cement⁵¹. Only a few investigations have considered the materials from the point of view of moisture buffering capacity and therefore their implementation in indoor-exposed, nonstructural elements, which is the purpose of the current study.

This research shows that the combination of the superhyroscopic nature of geopolymer-based materials with optimised 3D-printed geometries offers remarkable humidity regulation potential. The geopolymer composite developed in this study has an MBV that is 4 times greater than that of highly performing conventional building materials, thus offering significant potential for passive humidity control. A custom binder-jetting 3D printing technology is used to produce finishing tiles with a gyroid-based shape. This geometry allows for a high surface area-to-volume ratio. Compared with a full-volume alternative, it reduces material usage and offers an effective solution to increase the moisture buffering ability of the building component. The MBV_3D reached with the 3D-printed component developed here exceeds 14 g·m⁻²·%RH⁻¹, which is greater than that of any solution currently available on the construction market. Moreover, the optimisation of material usage reduces the embodied emission of the 3D-printed component to 40% of that of full-volume applications. Our work adopts a public library as a case study and leverages dynamic hygrothermal simulations to investigate the performance of 3D-printed components under realistic conditions. The results indicate that adopting 3D-printed geopolymer tiles for finishing indoor surfaces can lead to an improvement in annual hygrometric comfort levels of up to 85%. Moreover, the use of 3D-printed gyroid-based tiles is found to have a lower environmental impact than the use of a dehumidification system. This research reveals the potential of merging 3D printing technology and superhygroscopic materials to create low-carbon solutions for healthier indoor conditions and reduce the dependency on mechanical systems.

Results and discussion

3D-printed geopolymer composite

The geopolymer composite implemented in this study relies on quarry waste as an aggregate, potassium silicate as an activator, and metakaolin as a precursor. The composite is 3D printed via a selective binder‒jet approach, a technique that tends to increase the porosity of the material⁵⁰. In the printing process, the activator‒precursor reaction generates a geopolymer matrix that cohesively binds the aggregates, as demonstrated by the scanning electron microscopy (SEM) image in Fig. 1a. The microstructure shown in the figure reveals that significant void spaces, manifesting as pores, are a defining feature of the printed composite. This observation aligns with the results from the total open porosity test, indicating that ~30% of the material’s volume consists of open pores. A high degree of open porosity can be advantageous for enhancing the moisture-buffering performance of a material. Indeed, open pores with radii below 10⁻⁷m are involved in sorption phenomena, whereas pore sizes larger than 10⁻⁸ m contribute to water vapour diffusion transfer⁵², two mechanisms that are closely related to the moisture-buffering capabilities of the material^53,54.

Fig. 1. 3D-printed geopolymer composite.

a SEM image of the cross-section of a binder jet-printed geopolymer composite, showing the presence of significant voids. b Binder jet printer prototype, (c) schematic representation of the printing setup, (d) roughness of the printed surface, (e) prototype of a 3D-printed geopolymer tile, and (f) geometrical details of a 20 x 20 x 4 cm gyroid-based tile. The net exposed area of the tile represents the component’s surface facing the indoor air after application, thus excluding the bottom portion adhering to the support (wall or ceiling) and the sides in direct contact with adjacent tiles. Photographs were taken by the authors.

In this study, binder jetting technology was chosen over extrusion-based methods because of its ability to eliminate the need for support structures for overhangs⁵⁰. Despite their merits, both binder jetting and extrusion methods face challenges in incorporating mid-coarse recycled aggregates and binders with viscosities higher than those of typical cementitious materials^55,56. To address these issues, a prototypical testing setup was developed, as shown in Fig. 1b. The print resolution was adjusted to accommodate midviscous activators and midcoarse aggregates. The working principle of the machinery is schematically illustrated in Fig. 1c. The 3D printing process consists of depositing a layer of powder (mix of precursor and aggregates) on a plane support and proceeding with the jetting of a liquid water solution containing the activator into the designated areas, similar to the concept of inkjet printing⁵⁷. This process is repeated layer by layer until the desired geometry is attained. The residual powder—not selectively bonded into the geopolymer matrix—serves as a support for overhangs and can be reused in subsequent prints. Figure 1d shows the surface of a printed object close to it, revealing the rough effect of adopting coarse aggregates and a moderate printing resolution, which results in a satisfactory geometrical appearance at a full-scale view for tile and panel production, as illustrated in Fig. 1e.

3D printing allows for freedom in the shape of the components and enables the production of geometries with undercuts, which are difficult to implement with conventional casting and milling/subtractive techniques. 3D printing is employed in this study to produce tiles with a geometry based on the gyroid shape⁵⁸. This geometry allows for a high surface area-to-volume ratio, thus reducing material usage and offering an effective solution to increase the moisture buffering ability of the building component while also reducing its embodied carbon. The geometry of the 4 cm thick gyroid-based components developed for application as finishing tiles is shown in Fig. 1f. These tiles have almost 4 times more exposed surface than a full-volume component while requiring a 60% lower volume of material.

Physical and hygrothermal properties of geopolymer composite

Following Kunzel’s numerical formulation of combined heat and moisture transfer in porous building materials⁵⁹, the relevant properties of the geopolymer composite were determined via laboratory testing. The results are compared to those of more commonly adopted finishing materials, namely, an earthen plaster and a lime-cement plaster (Fig. 2a–d). The 3D-printed geopolymer composite has a density similar to that of common plasters and has relatively high capillary water absorption (Fig. 2a, b). The composite offers better thermal performance than the plasters do (Fig. 2c), but its thermal conductivity remains higher than that of thermally insulating composites, exceeding 0.2 W·m⁻¹·K⁻¹⁶⁰. The sorption isotherm (Fig. 2d) and vapour resistance factor (Fig. 2b) indicate great potential for moisture buffering. The material can hold higher quantities of moisture than conventional plasters at any given RH. Furthermore, it can store a greater quantity of moisture when passing from lower to higher humidities, for instance, from 80 to 95% RH. Finally, it has a low resistance to water vapour diffusion, comparable to the earthen alternative, which contributes to promoting a high moisture penetration depth for moisture buffering applications²⁶.

Fig. 2. Comparison of the physical properties, hygrothermal characteristics, and environmental impact of the 3D-printed geopolymer composite, earthen plaster, and lime-cement plaster.

a The chart shows the open porosity (P₀) and dry bulk density (ρ) of the three materials. b The chart presents their water vapour resistance factor (μ) and capillary water absorption coefficient (A_w). c The chart illustrates their thermal conductivity (λ) and specific heat capacity (c_p). d The chart shows their sorption isotherms. e The chart shows the MBV of the building materials reported in the literature, in comparison to the MBV of the materials considered in this study. The grey shaded area represents ‘excellent’ MBV performance, defined as values above 2 g·m⁻²·%RH⁻¹. Legend: I-Concrete²⁶, II-Fired clay bricks²⁶, III-Gypsum plaster²⁶, IV-Gypsum board²⁶, V-Cement-stabilised rammed earth⁶¹, VI-Earthen plaster¹⁹, VII-Rammed earth (not stabilised)⁶¹, VIII-Compressed earth blocks (not stabilised)⁶², IX-Highly hygroscopic clay plaster²⁵, X-Hemp-lime composite⁶³, XI-Flax-lime composite⁶³, XII-Hemp fibre panel⁶⁴, XIII-Cellulose panel⁶⁴, XIV-Wood fibre panel⁶⁴, and XV, XVI-Geopolymer composites^31,35. f Charts illustrating the environmental impact of materials and components in terms of GWP for a functional unit of 1 kg of material or a covered surface area of 1 m². The error bars in (a–e) represent the standard deviation of measurements performed on 3 samples of the geopolymer composite. The error bars for μ in (b) and for the first point of the sorption isotherm in (d) are not visible, as they are <0.5 and ~1 kg m⁻³, respectively. The data on the earthen and lime-cement plasters are sourced from¹⁹ and the WUFI database⁶⁵. The source data are provided in the Source Data file.

The 3D-printed geopolymer composite developed in this research has an MBV of 6.1 g·m⁻²·%RH⁻¹. This indicates a strong potential for humidity regulation, particularly when contrasted with building materials presented in the literature, as illustrated in Fig. 2e. Earthen composites^25,61,62, and biobased products⁶³, such as hemp, cellulose, and wood fibre panels⁶⁴ typically have MBVs in the range of 1.5–3 g·m⁻²·%RH⁻¹, and they are thus considered to offer good to excellent performance for indoor moisture regulation. The geopolymer composite under consideration has an MBV more than twice as high as those of these other solutions. In Fig. 2e, the MBV of the geopolymer composite is compared with that of two alternative common solutions: an earthen plaster¹⁹ and a painted lime-cement plaster⁶⁵. The earthen plaster has moderate performance, with an MBV of 1.5 g·m⁻²·%RH⁻¹. No paint is considered since leaving earthen plasters exposed is a common practice. The painted lime-cement plaster has limited performance, with an MBV of 0.36 g·m⁻²·%RH⁻¹. In this case, the lime-cement plaster itself has a moderate MBV, which is strongly reduced by the use of paint with a s_d value of 0.2 m—a type of paint that is commonly adopted in construction^66,67.

The MBV of the 3D-printed geopolymer composite is remarkably high not only in comparison with that of conventional solutions but also in relation to the most recent advancements in hygroscopic building materials. Recent studies have reported earthen materials, sunflower concrete, starch-paper, hemp-clay, biowaste-gypsum, and other biobased composites with a maximum MBV of 3.7 g·m⁻²·%RH⁻¹^62,68–73. Three studies published in 2023 offered solutions with competitive and even higher performance than the geopolymer composites did, with the MBV reaching 8–10 g·m⁻²·%RH⁻¹. This level of MBV is obtained by introducing desiccant salts or superabsorbent biopolymers into mineral matrices^74,75 or by using biowaste and sodium silicate mixtures⁷⁶. Despite the moisture-regulating potential of these solutions, a few questions can be raised. The salt used in the first study was lithium chloride, which is 10 times more carbon intensive than ordinary Portland cement is^77,78; thus, concerns related to environmental sustainability might arise. The same question can be raised regarding the study in ref. ⁷⁶, due to the massive use of sodium silicate as a glueing agent in biocomposites. In ref. ⁷⁵, the implemented solution appears more environmentally friendly, but the research is still quite preliminary, and the material needs to be further investigated considering higher humidity levels, specifically above 70–80% RH, which can occur in indoor environments^79,80. Finally, a new class of porous materials, namely, metal‒organic frameworks (MOFs), has recently emerged⁸¹. Originally developed for humidity extraction in dehumidification systems—akin to zeolites and silica gel—recent attention has shifted towards their potential for humidity regulation in indoor environments. These materials, when conceptualised as panels, can achieve an MBV above 7 g·m⁻²·%RH⁻¹^82,83. However, research outcomes are provided only from numerical studies, not from real applications. Furthermore, the preparation process using solvents such as dialkyl formamides or pyridine and metals such as zinc or cobalt makes their scale-up potential in the construction industry very difficult. In contrast, the geopolymer composite presented here requires sand, calcined clay, and potassium silicate. It can be produced directly at room temperature and does not require any type of treatment. Despite the challenges associated with MOFs, a novel aluminium MOF-based autonomous humidity control material (MOF-AHCM) has recently been reported to offer high stability, nontoxicity, and potential for scale-up via green synthesis methods⁸⁴. Moreover, this solution has been observed to yield an MBV as high as 12 g·m⁻²·%RH⁻¹.

Given this background, the combination of the superhyroscopic nature of the geopolymer composite developed, together with the optimised 3D-printed geometry presented in Fig. 1f, offers unprecedented moisture regulation potential. 3D-printed geopolymer tiles can achieve an MBV_3D of 14.3 g·m⁻²·%RH⁻¹, as reported in Fig. 2e—more than that of any solution currently available on the construction market. This MBV_3D refers to the moisture buffering value per unit area covered by tiles, an essential metric for comparing the performance of different finishing components per square metre of surface coverage. The 4 cm thick 3D-printed tiles have a net exposed surface of 3.45 m² per square metre of coverage. This favourable ratio allows the component to reach an MBV_3D that is more than double the MBV of a full-volume component made of the same material. Nonetheless, when the total exposed surface of the 3D-printed geometry is considered, the MBV_3D per unit of exposed area decreases to 4.1 g·m⁻²·%RH⁻¹. This value is ~35% lower than the MBV measured with full-volume samples of geopolymer composite in the standard NORDTEST procedure. This limitation is due to the reduced thickness in the cross-section of the 3D-printed tiles, which ranges between 15 and 25 mm. This geometry does not fully exploit the moisture penetration depth of the material, which is estimated to be ~50 mm, according to the results provided in Supplementary Fig. 8. This characteristic of the 3D printed geometry leads to an MBV_3D per net exposed surface that is lower than the MBV measured for the full-volume material application. Nonetheless, the geometrical design selected is considered an appropriate compromise to balance the desired reduction in embodied emissions through low material usage with high exploitation of the moisture buffering capacity of the material.

Embodied emissions in materials and 3D-printed components

3D printing allows for the efficient use of resources through optimal design strategies, thus promising a reduction in material usage. Nonetheless, to achieve significant carbon reduction in additive manufacturing, merely reducing material usage is insufficient. A focus should be placed on using materials with low embodied emissions and enhancing multifunctionality through geometric complexity⁸⁵. In this study, the dual functionality of geopolymer tiles as finishing and moisture-regulating agents diminishes the need for supplementary elements, namely, mechanical systems for humidification and dehumidification, thereby reducing the operational and embodied emissions associated with the production, installation, use and end-of-life of these systems. At the material level, the geopolymer composite has a global warming potential (GWP) of 0.26 kg CO_2-eq per kg of material, which is higher than that of conventional lime-cement plaster and earthen plaster but much lower than that of the phenolic-based materials often used in 3D printing. The main emissions are due to the use of potassium silicate solution and, to a lesser extent, metakaolin. This outcome is consistent with the literature, which indicates that the activator is often the main contributor to embodied emissions in geopolymers, followed by the precursor^40,42. Figure 2f shows that the reduction in embodied emissions provided by the avoided landfill of quarry waste is noticeable, but it provides only a marginal benefit from a carbon perspective. Nonetheless, it is relevant for resource depletion aspects in relation to aggregate consumption^86,87, especially because of the criticality of the sand used in conventional 3D printing⁸⁸. Further carbon reduction can be envisioned if the activator and precursor are substituted, partially or totally, with waste and byproducts. Alternative activators, such as rice husk ash, waste glass, nanosilica, and microsilica, can be synthesised from silica-rich waste resources⁴¹. Ground granulated blast furnace slag and fly ash from coal combustion in power plants are often used as substitutes for raw precursors⁴⁴. Although these precursors have low embodied emissions and support the circular use of resources, their availability is relatively limited⁸⁹. Ground granulated blast furnace slag and fly ash sources of adequate quality for use as alternative cementitious materials were reported by Scrivener, John, and Gartner⁹⁰ to be able to support only ~15–25% of cement consumption globally. In the same study, the estimated global availability of clay for calcination was much greater than the current production of ordinary Portland cement in terms of Mt/year. Hence, this study considers metakaolin due to its greater availability, especially in the European context⁹¹, where the gradual phase-out of coal power plants⁹² and the transformation of the steel industry towards electric arc furnaces⁹³ are forecasted to further reduce the supply of fly ash and ground granulated blast furnace slag in the future. Furthermore, compared with slag and fly ashes, metakaolin is more appropriate for binder jetting due to the faster geopolymerisation reaction. Such fast reaction speed is required by the additive manufacturing technique adopted, unlike in a conventional casting process. The contribution of the electricity and machine consumption of the 3D printer is much less relevant than the embodied emissions in the materials used in the mix design of the geopolymer composite. When the functional unit is a covered square metre (with components 4 cm thick), the environmental impact of 3D-printed geopolymer tiles is significantly lower than that of full-volume applications of geopolymer composite (e.g., for conventional plastering). The benefit of efficient material usage is evident, with 3D-printed components exhibiting 60% lower embodied emissions than the full-volume use of geopolymer composite, as shown in Fig. 2f. This benefit is so relevant that the 3D-printed geopolymer tiles have lower GWP per covered m² than lime-cement plaster does (5% lower, including the metal fastening system, 15% lower considering only the 3D-printed tiles). In contrast, the 3D-printed tile still has a significantly higher GWP than earthen plaster does. Overall, the benefit of 3D printing with an optimised geometry is highly relevant for reducing embodied emissions. By integrating secondary products to partially replace both the activator and precursor in the mix design, embodied emissions from the geopolymer components could be further substantially reduced compared with those documented in this study; this suggests the potential for further decreases in embodied emissions in the future.

Mechanisms driving the material’s superhygroscopic nature

The superhygroscopic nature of the geopolymer composite arises from its high capacity to absorb moisture from the surrounding environment and retain it as free or loosely bound water inside the composite’s matrix. This behaviour can be attributed to a combination of physical adsorption and chemical interactions: first, moisture adsorption in the pore structure due to in-pore condensation⁹⁴; and second, the presence of hygroscopic free alkaline activators in the pores, including free alkali and free silicates^95,96. Finally, the presence of hydroxyl groups in the geopolymer structure is mentioned in the literature^92,93. Hydroxyl groups can form hydrogen bonds with water molecules, thus providing a mechanism for water absorption into the chemical structure of the geopolymer matrix. The relevance of these three different aspects to the hygroscopic behaviour of the material was evaluated through thermogravimetric analysis (TGA) coupled with mass spectrometry (MS). The thermogravimetric analyses were run from 30 °C to 200 °C. This range is relevant for the release of free and weakly bound water contained in the geopolymer composite^95,97, thus offering a means to quantify the moisture stored in the different samples. The range of temperatures selected is based on the literature and is supported by the results obtained from running TGA tests up to a temperature of 1000 °C on moist and oven-dried samples (Fig. 3c and Supplementary Fig. 6). To quantify the contributions of the microstructure, free activators, and hydroxyl groups to the hygroscopic moisture storage capacity of the geopolymer composite, the following samples were tested: a solid 3D-printed geopolymer composite, a powdered composite, a powdered composite after the removal of free alkaline activators, and an unreacted powder (metakaolin and aggregates). The samples were tested after 8 h of conditioning at 33% RH or 75% RH and 23 °C in a climatic chamber. The results are reported in Fig. 3a. The TG results revealed that the quantity of moisture stored in the solid geopolymer composite was significantly greater than that stored in the powdered composite at both 33% RH and 75% RH. This outcome indicates that the participation of the microstructure in the hygroscopic behaviour of the composite is highly relevant. Comparison of the powdered composite before and after the removal of free alkaline activators showed that the removal had little effect on the moisture retained at 33% RH. In contrast, the removal significantly reduced the moisture storage capacity at 75% RH. This outcome indicates the relevant participation of free alkaline activators in moisture absorption under high relative humidity conditions. Finally, the unreacted powder has a much lower ability to absorb moisture than the alkali-removed powder does, highlighting the importance of sodium silicate and the geopolymer matrix for the hygroscopic behaviour of the material. The latter observation suggests that the hydroxyl groups of the formed geopolymer might play a relevant role in the hygroscopic behaviour of the composite, although other mechanisms related to the geopolymer matrix and the presence of sodium silicate in the mixture cannot be excluded. The moisture capacity in the range of 33–75% RH, i.e., the difference in moisture storage capacity between the two RH levels⁶², is reported in Fig. 3b. The results show that the microstructure, alkali activators, and geopolymer matrix significantly contribute to the moisture capacity of the material. This outcome explains the higher moisture buffering value of the geopolymer composite than that of more common building materials. Indeed, in conventional building materials such as lime- and cement-based mortars, the hygroscopic behaviour is considered to be primarily due to physical adsorption within the microporous structure^98,99, leading to a moisture buffering ability that is typically limited or moderate^25,100. Earthen materials often exhibit a greater capacity for moisture buffering, typically ranking between good and excellent^62,101. The higher performance of earthen materials can be explained by the fact that in addition to physical adsorption, the chemical interaction between clay minerals and water molecules also plays a significant role. Clay minerals are typically composed of layers of silica tetrahedra and alumina octahedra, and they normally have a negative charge due to isomorphous substitutions within these layers¹⁰². The negative charge, with the help of interlayer cations in some clays, attracts water molecules and increases the characteristic affinity of clay minerals for water. This interlayer absorption of moisture influences the hygroscopic capacity of earthen materials⁶². Similarly, the super hygroscopic nature of the geopolymer composite can be explained by the fact that different mechanisms contribute to the hygroscopic capacity of the material. Specifically, the porous structure supports physical adsorption, while the presence of alkali activators and the affinity of the geopolymer matrix for water are responsible for chemical absorption.

Fig. 3. Moisture release from samples conditioned at different relative humidity levels or oven-dried, accelerated carbonation of the geopolymer composite, and CO₂ release from the carbonated geopolymer composite.

a TGA results for the solid geopolymer composite (SolidGP), powdered geopolymer composite (PowderGP), powdered geopolymer composite after removal of free alkaline activators (DisGP), and unreacted metakaolin-sand mixture (UnreactedPGP) for samples conditioned 8 h at 33% RH and 75% RH. b Moisture capacity in the range of 33 − 75% RH, with the percentage reduction quantified and rounded to the nearest 5%, based on the TGA results above. c Moisture release from oven-dried and moist (conditioned at 80% RH) SolidGP samples. The top half of the figure shows the TGA results, while the bottom half displays the MS data for water release detected during the test. The charts indicate that the free and weakly bound water in the moist sample is primarily released below 200 °C (grey area in the chart). d Mass gain during the accelerated carbonation test on the 1 year-old geopolymer composite (error bars represent the standard deviation of measurements performed on 3 samples). e TGA-MS results for a 1 year naturally carbonated SolidGP sample, with Peak 1 and Peak 2 representing CO₂ release events detected during the analysis. Peak 1 arises from the decomposition of carbonate in the quarry waste (marble aggregates), and Peak 2 is associated with the geopolymer matrix. The source data for (b) and (d) are provided in the Source Data file.

Maximum carbonation potential

Geopolymer matrices undergo carbonation when in contact with air as a consequence of the reaction between alkaline materials and the carbon dioxide (CO₂) present in the air. For low-calcium-content metakaolin-based geopolymers, depending on the exposed CO₂ concentration, the carbonation products are mainly carbonate and bicarbonate formed by alkali metals¹⁰³. This process of carbonation has significant implications across multiple dimensions. It provides increased mechanical performance. It removes CO₂ from the air, thus contributing to the reduction of greenhouse gas in the environment. Additionally, carbonation consumes the free alkali in the geopolymer matrix, reducing the potential occurrence of efflorescence in the composite. Figure. 3d shows the mass gain observed during the accelerated carbonation test performed on 1 year-old geopolymer composite samples. Mass stability was reached within the first 2 days, suggesting that the 1 year-old samples were already close to full carbonation. For this reason, the carbonation potential was evaluated by considering a sample of a 1 year-old 3D-printed geopolymer composite that was naturally carbonated. The TGA-MS results are reported in Fig. 3e. Two distinct peaks of CO₂ release can be observed, with the peak in the temperature range of 650–950 °C coming from the decomposition of carbonate in the quarry waste (marble aggregates) and the peak in the range of 520–650 °C being associated with the geopolymer matrix. The CO₂ released from the geopolymer composite can be attributed to the formed carbonates in the matrix, and its amount is estimated to be ~4 mg of CO₂ per g of composite. Information on the calculations performed is reported in the methods section. The carbonation potential is verified via additional evaluations based on accelerated carbonation. When exposed to elevated CO₂ concentrations, the geopolymer matrix can carbonate in a way that leads to the conversion of metal alkali or carbonate forms into bicarbonate forms¹⁰⁴. This was verified by TG results from tests on accelerated-carbonated geopolymer composites, which revealed that the release peak at 580 °C observed in the naturally carbonated composite disappeared, whereas a new early peak appeared at ~150 °C, as shown in Supplementary Fig. 3. Depending on the initial carbonate product, the mass change can be roughly converted into CO₂ absorption, which yields values ranging from ~2.3 to 3.6 mgCO₂·g⁻¹. This carbonation potential is in the same range as that observed from the TGA‒MS results for the naturally carbonated geopolymer composite. The carbonation capacity is hence estimated to be between 2.3 and 4.0 mgCO₂ per g of composite. Given that the production of the geopolymer composite releases the equivalent of 0.26 kg CO₂-eq per kg of material, the recapture of CO₂ through carbonation only reduces the material’s carbon footprint by 0.9–1.5%. The carbonation potential is negligible and thus not considered when accounting for the environmental impact of the geopolymer composite. Relatedly, carbonation can reduce the moisture buffering capacity of geopolymer composites due to the reduction in free alkali activators and the change in the microstructure of the material¹⁰⁵. Previous studies considered a similar geopolymer composite with an MBV of ~4.5 g·m⁻²·%RH⁻¹ and evaluated the change in MBV after efflorescence removal and subsequent accelerated carbonation³⁵. Efflorescence was induced by partial immersion in water for 1 month as in ref. ¹⁰⁶, whereas accelerated carbonation was performed by exposing the samples to a 10% CO₂ concentration for 1 month following ref. ¹⁰⁷. The results indicate that almost 20% of the MBV can be lost due to alkali removal, and an additional 20% decrease can be registered after subsequent accelerated carbonation. These outcomes indicate that further studies are needed to clarify the durability of the moisture buffering performance of geopolymer composites under typical indoor environmental conditions. In the 3D-printed composite considered in this study, full efflorescence is very unlikely to occur since no efflorescence was observed in the samples left in the archives and laboratories exposed to indoor air for >2 years. Thus, a lower rate of MBV loss might be expected under exposure to typical indoor conditions. However, even considering a loss of 40% in the MBV, the 3D-printed geopolymer composite evaluated here would have an MBV above 3.5 g·m⁻²·%RH⁻¹, which is greater than that of most of the hygroscopic materials available in the construction industry.

Moisture regulation performance under realistic conditions

The dynamic hygrothermal simulations performed in this study enable the evaluation of the effects of 3D-printed geopolymer tiles on the indoor relative humidity of a public library. These are compared with other finishing solutions and a dehumidification system. The case study considered is a consultation room located in the Municipal Library of Porto, in Porto, Northern Portugal. The variation in the indoor relative humidity is strongly influenced by the extension of the interior surface covered in the 3D-printed tiles, as displayed in Fig. 4a. The influence of other finishes on the annual indoor humidity level is also considered, namely, a 4 cm full-volume geopolymer composite, an earthen plaster with good moisture buffering capacity, and a painted lime-cement plaster with a limited MBV. The results obtained in terms of a decrease in indoor hygrometric discomfort during occupation hours are reported in Fig. 4b. The painted lime-cement plaster has little ability to regulate the indoor humidity level, resulting in a limited reduction in discomfort even when it is applied on the full surface of the ceiling and walls, which is approximately −10% in hours of discomfort. Earthen plaster has better regulation ability, reducing discomfort hours by up to 20% while decreasing the intensity of annual discomfort by almost 55%. The superhyroscopic nature of the geopolymer composite, corresponding to an MBV that is 4 times greater than that of earthen plaster, results in much better performance. Using the 4 cm full-volume geopolymer composite led to a reduction of up to 60% in the discomfort hours and up to 90% in the annual intensity of discomfort. For the same covered areas, a full-volume layer of geopolymer composite offers greater benefits than 3D-printed tiles do. Specifically, 4 cm thick tiles lead to a reduction in hours of discomfort of up to 30% and a decrease in the annual discomfort intensity of 75%. The lower performance of the tiles is due to the slenderness of the printed parts and the lower volume of material used in the component. When the thickness of the 3D-printed components is increased from 4 to 5 cm, the tiles are able to perform comparably to the full-volume application, with a reduction in discomfort intensity of ~85%. These outcomes demonstrate that materials with good to excellent moisture buffering abilities can effectively help mitigate hygrometric discomfort in intermittently occupied spaces. Further details regarding the efficacy of the different finishing solutions in mitigating hygrometric discomfort across different seasons are available in Supplementary Fig. 12. In this regard, the results show a greater ability of hygroscopic finishings to reduce hygrometric discomfort in winter than in summer. This outcome aligns with the lower rate of natural ventilation that characterises the library during the winter. The literature indeed indicates that the humidity-regulation effect of moisture-buffering materials becomes less relevant when ventilation rates increase^66,108,109. Notably, the efficacy of moisture-buffering materials in enhancing indoor hygrometric comfort is also influenced by other building-specific variables, such as the use of heating, ventilation, and air conditioning (HVAC) systems, the external climate, indoor moisture loads and room volume^22,110. Consequently, the use of hygroscopic materials can be instrumental in improving users’ comfort in real-world applications, but this strategy should be employed when deemed appropriate for the specific context under study.

Fig. 4. Impact of different finishing solutions on the indoor relative humidity and hygrometric discomfort in the case study.

a Graph showing the annual variation in the indoor relative humidity in the original case study and after the application of 3D-printed geopolymer tiles to portions of the ceiling surface, the entire ceiling surface (A_c), and the full surface of both the ceiling and walls (A_cw). The illustrations in the corner of the figure depict the geometry of the 3D-printed geopolymer tiles and schematic views of the areas considered for tile coverage, specifically, 1/8, 1/4, and 1/2 of the ceiling area; the entire ceiling area; and the combined surface of both the ceiling and walls. The orange region in the graph highlights the range of relative humidity considered optimal for indoor hygrometric comfort (40%–60% RH). b The charts compare the uncomfortable operational hours (indicating the duration of hygrometric discomfort during library operating hours), and the hygrometric discomfort index (representing the intensity of discomfort during library operating hours) under different scenarios. The scenarios considered are the original case study and the case study after applying various finishing solutions to cover part or the entire surface of the ceiling and walls. In the charts, the benefits of using hygroscopic and superhygroscopic materials to cover the entire surface of the ceiling and walls are highlighted. The source data for (b) are provided in the Source Data file.

Environmental impact of active and passive humidity control

Figure 5 shows the improvement in the annual hygrometric comfort levels obtained with different interior finishing solutions, together with the resulting carbon emissions. The improvement in hygrometric comfort is calculated from the results of dynamic hygrothermal simulations, as previously outlined, considering the percent reduction in annual hygrometric discomfort intensity. The environmental impact is evaluated considering a cradle-to-grave approach. Evaluations account for a service life of 30 years for plasters and 3D-printed tiles. The life cycle assessment (LCA) of the dehumidification system considers commercially available dehumidifiers for the evaluation of operational consumption. The annual dehumidification loads are calculated via numerical simulations. The embodied, maintenance and end-of-life emissions of the dehumidification system are estimated based on the detailed LCA evaluations of dehumidification systems provided in ref. ¹¹¹, which are based on manufacturer-provided data. The service life of the dehumidifier employed is estimated to be 15 years. Complete information on the LCA results is provided in the Supplementary Data file. Figure 5 indicates that higher comfort levels require higher embodied emissions with each single finishing solution. This results from the need for a larger exposed surface of material to achieve greater benefits, with a consequently higher volume of materials applied and higher total embodied emissions. In terms of performance, the conventional use of painted lime-cement plaster (limited MBV) results in higher embodied emissions and lower comfort benefits than all other solutions do. In contrast, earthen plaster (moderate MBV) has the lowest embodied impact, with a relevant improvement in indoor comfort. The best performance reachable with earthen plaster is an improvement of ~55% in annual indoor hygrometric comfort. A much greater performance can be achieved with the superhygroscopic geopolymer composite. It allows for improvements of ~85% and 90% with 3D-printed tiles and full-volume applications, respectively. Despite both solutions having high performance, 3D-printed tiles carry much lower embodied emissions than a full-volume geopolymer composite layer does. Thus, the use of 3D-printed tiles is a passive moisture regulation strategy that allows for high performance with moderate embodied emissions. This approach strikes a balance between the modest benefits achievable with natural earthen plaster, which has a very low environmental impact, and the superior performance of the geopolymer composite, which, however, comes at a relatively high environmental cost. Compared with the use of an active humidity-regulation system, such as a conventional dehumidifier, 3D-printed geopolymer tiles are of particular interest. The adoption of 3D-printed tiles to reach an improvement of 85% in indoor comfort has a 40% lower environmental impact than employing a dehumidification system to actively achieve equivalent indoor hygrometric comfort levels over a 15 year time span—considering embodied, operational and end-of-life emissions. A much greater reduction, i.e., more than 70% in GWP is observed when comparing 3D-printed tiles with the use of a dehumidification system over a 30 year time span. The ecoefficient nature of the 3D-printed optimal geometry emerges at high comfort levels, i.e., ~85% hygrometric comfort improvement. In this context, the use of a full-volume geopolymer composite has a comparable environmental footprint to the use of a dehumidification system for 15 years, whereas 3D-printed tiles successfully achieve this comfort level with relatively low carbon emissions.

Fig. 5. Improvement in indoor comfort through active and passive humidity-regulation solutions and their associated environmental impact.

Reduction in annual hygrometric discomfort achievable with various finishing solutions applied to partial or full surfaces of the ceiling and walls of the case study and their environmental footprint (embodied GWP). For comparison, the environmental impact of using an active dehumidification system is also provided, considering 15 or 30 years of use (embodied and operational GWP). The improvement in hygrometric comfort is quantified through numerical simulations relative to the baseline case study, in which the walls and ceiling finishings have negligible moisture buffering capacity. The source data are provided in the Source Data file.

Trade-offs between comfort, health, and sustainability

The production of geopolymer components involves exposure to highly alkaline solutions, posing threats of alkali burns to the eyes and skin of construction operators^112–115. The concerns remain restricted to the early fabrication phase: after the alkaline solution has fully reacted with the precursor, the resulting geopolymer composite becomes safe for direct contact. Within this framework, 3D printing technologies introduce significant health advantages for construction professionals. Notably, by allowing a machine to handle the process, we effectively bypass the health hazards associated with direct human contact with highly alkaline materials. This solution provides an environment where the potential of these materials can be fully realised without compromising human safety. A schematic overview of the comfort, embodied carbon, and health of construction operators is provided in Fig. 6 for the four finishing solutions considered in the study. The ternary diagram suggests that while moving towards high-tech constructions, we must not lose sight of the merits of traditional materials. Low-tech solutions, such as earthen plaster, still possess inherent value in today’s construction landscape. Their exceptionally low carbon footprint, combined with the safety they offer to construction operators from health hazards and their contribution to moderate improvements in indoor comfort, underscores the point that sustainability is not solely achieved through futuristic materials or high-tech solutions. Overall, both 3D-printed geopolymer tiles and earthen plaster have emerged as viable and balanced choices when considering comfort, health, and sustainability.

Fig. 6. Ternary diagram showing the potential of different finishing solutions to provide healthy working conditions for construction workers, enhance indoor comfort, and minimise embodied carbon.

In the diagram, the four finishing solutions considered in the study—earthen plaster, lime-cement plaster, geopolymer composite, and 3D-printed geopolymer tiles—are qualitatively compared using scores ranging from 0 to 100. Indoor comfort is estimated based on the reduction in annual hygrometric discomfort intensity observed in numerical simulations (the greater the reduction is, the higher the score). The environmental impact is evaluated based on the GWP associated with applying the finishing to 1 m² of surface (a higher GWP results in a higher environmental impact score). Health reflects the conditions for construction workers (lower health hazard risks lead to a higher score). The areas shaded in green, light blue, and grey highlight the ranges corresponding to low health hazard risks for construction workers, high potential to passively improve indoor hygrometric comfort, and high embodied carbon in finishing solutions, respectively. The area marked by a dotted line and labelled as “(+) Health (+) Comfort (+) Low carbon” indicates the range of solutions considered to have a positive ranking in the three aspects. The source data are provided in the Source Data file.

Implications of the study

In our study, a composite that integrates industrial waste in a geopolymer matrix is implemented via advanced binder jet 3D printing technology. The synergy of the superghyroscopic nature of the material and the efficient geometric design of the 3D-printed components led to remarkably high humidity-regulation performance with reduced material usage. The 3D-printed tiles developed in this study are designed for finishing the interior surfaces of buildings and offer significantly lower embodied emissions than full-volume finishing solutions, such as geopolymer composites used for plastering, owing to the optimised geometry. Through a validated hygrothermal simulation model, 3D-printed geopolymer tiles are shown to offer substantial enhancement in indoor hygrometric comfort through their passive moisture regulation capacity. An improvement of 85% in annual hygrometric comfort is shown in a library room—a performance that remains inconceivable to reach with conventional building materials and manufacturing techniques. The cradle-to-grave carbon footprint associated with the use of 3D-printed tiles for indoor humidity regulation is 40% lower than that resulting from the use of a conventional dehumidification system to reach the same comfort level over a period of 15 years and 70% lower than that resulting from the use of a conventional dehumidification system for 30 years. The findings of this research underscore the importance of combining high-tech manufacturing techniques with innovative materials to develop low-carbon components with the ability to passively regulate indoor humidity levels.

The unprecedented moisture buffering ability of the components developed in this study enables a paradigm shift in how we approach the challenge of indoor hygrometric comfort, moving away from a predominant reliance on mechanical systems towards more passive, material-driven solutions. This technology can play a key role in spaces where the installation or use of mechanical systems is restricted, for example, in buildings protected by preservation mandates or in areas subjected to energy precarity. The implementation of 3D-printed, multifunctional, geopolymer tiles represents a significant step towards hybridisation in construction. This approach, combined with minimised embodied emissions, paves the way for a more sustainable construction industry.

Methods

Materials

A standard 100 g batch of the geopolymer composite mixture consisted of aqueous potassium silicate solution (29 g), metakaolin (14 g), and aggregates (57 g). The potassium silicate solution is highly alkaline, posing risks to the skin, eyes, and respiratory system. Metakaolin dust can cause respiratory issues if inhaled. Proper handling of these materials requires the use of protective equipment such as gloves, masks, and adequate ventilation.

In the composite, the geopolymer matrix is derived from the reaction between metakaolin (Metastar501 from Imerys) and the potassium silicate (K₂SiO₃) solution (Geosil 14517 from Woellner, with a composition of 23.54% SiO₂, 21.48% K₂O, and 55% H₂O, as specified by the supplier). The aggregates are sourced from the waste of a local marble quarry. The samples were sieved to obtain 5% fine aggregates (up to 0.3 mm in diameter), 25% medium aggregates (0.3–0.6 mm), and 70% coarse aggregates (0.6–2.0 mm) by weight. The intended geopolymer binder composition has a Si/Al molar ratio of 1.8, K/Al ratio of 1.0 and H₂O/K₂O ratio of 15. The Si/Al ratio significantly affects the microstructure and mechanical properties of geopolymers^116,117, and a Si/Al ratio of 1.8 is considered suitable for providing high mechanical strength¹¹⁸. The K/Al ratio is related to the polymerisation reaction¹¹⁹. One would usually aim for a ratio higher than one to reach a full reaction; however, such a high alkali content also leads to a relatively high efflorescence level. In this study, a ratio of 1.0 was chosen to have a sufficient reaction level to reach the targeted compressive strength and consuming most of the alkali in the polymerisation process to minimise free alkali and reduce efflorescence. Finally, H₂O/K₂O is related to the alkalinity of the solution, which controls the reaction speed and viscosity of the solution¹²⁰. An H₂O/K₂O ratio of 15 aligns with previous research¹²¹ and was found to be suitable for the 3D printing technology adopted in the study.

The 3D printer is based on a 3-axis Cartesian system and uses a pressure-operated dispensing system. For each layer, the powder distribution system deposits the powder bed of precursor and aggregates, which were previously prepared and mixed according to the mix design. The liquid distribution system jets the liquid solution, containing the activator, on the powder bed. Layer by layer, this process continues, moving the distribution system upwards by the targeted layer thickness (2 mm). The liquid is pressurised in a dedicated vessel and dispensed through multiple nozzles by individually controlling the respective solenoid valve.

Procedures for physical and hygrothermal characterisation

All tests were performed on samples produced via 3D printing (binder jet technology). The measurement of the open porosity and dry bulk density was based on the standard procedure outlined in ASTM C642:2021¹²². To ensure representative results, the three samples tested were extracted from different types of 3D-printed samples, and they had volumes of ~10, 13, and 23 cm³. They were dried at 105 °C until a constant mass was reached, with gravimetric changes below 0.1% over 24 h. The samples were then left under water for 24 h under controlled environmental conditions (23 °C, 50% RH – climatic chamber). Excess surface water was removed with a damp cloth, and the mass of the saturated samples was measured via hydrostatic and gravimetric weighing. The open porosity and dry bulk density were indirectly determined from the results. The resistance to water vapour diffusion was assessed via the dry cup method according to standard ISO 12572:2016¹²³. Three disk-shaped samples with a diameter of 20 cm and a thickness of 2 cm were tested. The samples were sealed with wax on a metal cup that had been prefilled with a calcium chloride desiccant, CaCl₂, up to 1 cm below the top edge. The samples were then placed in a climatic chamber set to 23 °C and 50% relative humidity. They were weighed every 24 h until a constant vapour flux was observed. The vapour resistance factor was calculated via Fick’s law based on the last three measurements taken under steady vapour flux conditions. The sorption isotherms were measured with the climatic chamber method defined in ISO 12571:2021¹²⁴. Three disk-shaped samples with a diameter of 7 cm and a thickness of 1.5 cm were tested. They were dried in an oven at 105 °C until a constant mass was achieved (gravimetric changes below 0.1% over 24 h). The samples were then sealed along their lateral and bottom surfaces with aluminium tape and placed in a climatic chamber. The chamber was set to maintain the environmental conditions of 23 °C and constant relative humidity. The relative humidity steps considered were 40%, 60%, 80%, 90%, and 95%. The samples were kept at each constant RH level until equilibrium with the environment was reached (gravimetric changes below 0.1% over 24 h), after which the RH was increased to the next defined step. The sorption isotherm was obtained by plotting the equilibrium volumetric water content against the relative humidity during the adsorption process. Capillary water absorption was evaluated through a partial immersion test, following the standard EN 1015-18:2002¹²⁵. The samples used were prismatic (160 × 40 × 40 mm³) and were split in half via a 3-point flexural test device. For each sample, one half was used for the test. The lateral surfaces of the samples were sealed with aluminium tape and then conditioned at 23 °C and 50% relative humidity until a constant mass was achieved (gravimetric changes below 0.1% over 24 h). A tray filled with demineralised water was placed in the chamber, and the samples were positioned on lateral pedestals so that the water level was 5–10 mm above the tested surfaces. The masses of the samples were measured after 10 min, 1 h, 1.5 h, 3 h, 24 h, and 48 h. The contact surface was geometrically determined using a calliper. The change in water content per unit area was plotted against the square root of time to obtain the water absorption curves. Based on these capillary absorption curves, the water absorption coefficient (A_w) was determined as the slope of the linear portion of the absorption process, which corresponded, in this case, to the slope of the line between the points measured at 0 and 10 min. The free water saturation content was measured as the volumetric water content in the samples at the end of the test once mass stability was reached (gravimetric changes below 0.1% over 24 h). The thermal conductivity and specific heat capacity were tested with a KD2 Pro device¹²⁶, which is based on a transient line heat source method. To account for the potential variability of the material in the printing process, different types of samples were tested, namely, one disk (diameter of 20 cm, 2 cm thick) and two cuboid samples (10 x 10 x 5 cm³). Materials were dried in an oven at 105 °C until a constant mass was achieved and then conditioned at 23 °C and 50% relative humidity in a climatic chamber. In both the drying and conditioning phases, equilibrium was considered reached when gravimetric changes of <0.1% were recorded over 24 h. The test was then conducted under the same environmental conditions, with the measuring equipment placed inside the climatic chamber alongside the samples during the measurement. The NORDTEST protocol¹²⁷ was followed to measure the moisture buffering value (MBV) of the geopolymer composite. Three cuboid samples, each with an exposed area of 10 × 10 cm² and a thickness of 5 cm, were tested. The samples were dried in an oven (105 °C) until a constant mass was achieved and then conditioned at 23 °C and 50% RH in a climatic chamber. In both the drying and conditioning phases, equilibrium was considered reached when gravimetric changes of <0.1% were recorded over 24 h. Aluminium tape was used to cover the lateral surfaces and the bottom surface, leaving only the upper surface exposed to moisture exchange with the environment. The samples were placed in a climatic chamber and subjected to cycles of 8 h at 33% RH and 16 h at 75% RH, at a constant temperature of 23 °C. The cycles were repeated until three consecutive quasi-steady cycles were achieved, defined as changes of <5% in weight amplitude between successive daily cycles. The masses of the three cuboid samples were measured five times during the high RH period, including at the beginning and end of this period. The MBV was calculated based on the first and last weight measurements during the high RH period over three quasi-steady cycles. The NORDTEST protocol was also used to measure the MBV_3D of a 3D-printed, gyroid-shaped sample. In this study, the term MBV refers to the moisture buffering value of solid elements, whereas MBV_3D refers to 3D-printed components with a gyroid-based geometry. For comparison, the MBVs of an earthen plaster and a painted lime-cement plaster were derived from dynamic hygrothermal simulations, with 5 cm thick components assumed to ensure consistency with the tested geopolymer composite samples. The test results are presented as the averages of measurements taken from three samples, with the standard deviation indicating data variability. Owing to limitations in the production process, the MBV_3D was measured on a single 3D-printed gyroid-shaped prototype. Therefore, this value is based on a single measurement, and no standard deviation is provided. Additional information on the test methods is available in the Supplementary Information, under the section ‘Supplementary Methods’.

Thermogravimetric analysis and mass spectrometry

TGA‒MS was employed to measure the moisture content in samples of 3D-printed geopolymer composite, solid and powdered (crushed with a pestle), powdered composite undergone treatment for free alkaline activator removal, and unreacted powder (metakaolin and aggregates). The removal of free alkaline activators was performed by immersing the samples in distilled water and periodically measuring the pH of the water until a constant value was reached. The samples were conditioned for 8 h at 23 °C, 75% RH and 23 °C, 33% RH, in a climatic chamber before testing. The measurements were conducted on a Netzsch STA 449 C instrument connected to a 403 C Aëolos quadrupole mass spectrometer under a N2 atmosphere, with a gas flow of 50 mL·min⁻¹ and a heating rate of 10 K·min⁻¹. Samples with volumes in the range of 50–350 mm³ were used. The same test device used for the TGA‒MS tests was also adopted to evaluate the CO₂ absorbed by the carbonated geopolymer‒composite samples. The volumes of the samples tested for CO₂ absorption quantification were in the range of 120–350 mm³. The carbonation potential of the 3D-printed geopolymer composite was semi quantified based on the TGA‒MS results, as detailed in the Supplementary Information, under the section ‘Supplementary Methods’. The sample considered for CO₂ absorption quantification (from carbonation) was a 1 year naturally carbonated sample. The accelerated carbonation test was performed in a CO₂ chamber with a 3% CO₂ concentration at 20 °C and 60% RH. Tile samples with a size of 10 x 10 x 5 cm³ were tested after preconditioning at 20 °C and 60% RH. The mass of the samples was periodically measured until a stable mass was attained, which was defined as a mass change of <0.2% over 24 h.

Dynamic hygrothermal simulations

Dynamic hygrothermal simulations offer an advanced numerical tool that accounts for the complexity of buildings, including human occupancy, use of HVAC systems, natural ventilation, infiltration, heat and moisture transfer in building materials, and outdoor dynamic weather conditions¹²⁸. This type of simulation is largely employed to study the efficacy of hygroscopic building materials for the passive regulation of indoor environments under realistic conditions^{109,129–137}. In this study, numerical simulations were performed via WUFI Plus V.3.2.0.1⁶⁵. WUFI Plus is a whole-building simulation software that relies on a holistic hygrothermal model¹³⁸, combining thermal building simulations and hygrothermal envelope calculations. Envelope calculations include moisture transfer in liquid and vapour forms and the hygroscopic behaviour of building materials.

The geopolymer composite was modelled via numerical simulations according to the data obtained via physical, hygric, and thermal characterisation. The 3D-printed tile was modelled as a not-visualised plane object with the same exposed area and equivalent volume. To validate this approach, the behaviour of the tile under NORDTEST MBV testing was simulated. A comparison of the numerical and experimental results validated the method, as illustrated in Supplementary Fig. 9. The approach was found to be suitable for representing the moisture buffering ability of 3D-printed components in numerical simulations.

The case study considered is a room located in the Municipal Library of Porto, Porto, in northern Portugal. The building is characterised by massive granite masonry walls plastered and rendered at the interior and exterior surfaces, respectively. The room is a reading space with monumental dimensions, and it can host >60 users for book consultation. The indoor hygrothermal conditions were monitored for >1 year, and the data collected were used for the calibration and validation of the parameters of the numerical model, as documented in ref. ¹³⁹. This procedure aims to guarantee the accuracy of the numerical modelling and the choice of input parameters^140–142. Based on the input used in the validated model, average conditions were considered in this study in terms of heating usage and ventilation patterns, whereas the occupancy was modelled as medium-high, considering 15 users during opening hours. This choice was made to analyse the effectiveness of hygroscopic materials to moderate indoor humidity levels in spaces with relatively high occupation. All the simulations are performed considering a 2 year running period. The first year is used to reach realistic initial conditions, and the second year supplies the results for the study.

In the whole-building simulations, the 3D-printed tiles are modelled via a not-visualised plane object that interacts with the indoor air through one exposed surface. For coherence and ease of comparison, the plasters are modelled via the same simplification and account for 4 cm of thickness. To validate the reliability of this simplification, the results obtained by applying the plasters directly on the surface of the walls and ceiling were compared to those obtained using the not-visualised plane object simplification. The comparison is provided in Supplementary Figs. 13 and 14. Since the results are comparable, the simplified not-visualised objects approach is considered suitable for modelling the plasters in this study.

The simulated scenarios consider the use of different finishing solutions to cover 1/2, 1/4 or 1/8 of the ceiling area, the complete ceiling area, and the entire area of the ceiling and walls. The hourly results of the indoor air relative humidity are compared to the reference range for hygrometric comfort, 40–60% RH. Uncomfortable operational hours [h] are quantified by accounting for the sum of occupation hours with an RH falling out from the hygrometric comfort range during 1 year. The hygrometric discomfort index [h·%RH] is defined by quantifying the sum of hours of discomfort multiplied by the corresponding hygrometric distance from the limit of comfortable RH, thus representing the cumulative intensity of discomfort for 1 year. The hygrometric comfort improvement [%] represents the percentage reduction in the discomfort index obtained with the moisture buffering components or dehumidification system in comparison to the conditions in the case study with no moisture buffering components (original case study).

Environmental assessment

In accordance with the EN15804:2019 guidelines¹⁴³, a life cycle assessment is employed to evaluate the environmental impacts associated with the use of different components. The life cycle inventory (LCI) data are retrieved from the Ecoinvent Database v3.8 (2021). The functional units considered are 1 kg of material and material used to cover 1 m² of surface. The system boundaries include stages A1 to C4 of the LCA within the “cradle-to-grave” framework, including embodied emissions within the ‘cradle-to-gate’ framework, as well as the operation phase and end-of-life. The examined materials were considered to be recycled at the end of their lifetime. The embodied impact of materials is considered by using the “market value” of single components of the mix design from the Ecoinvent LCI database. The energy used for the powder mixing of materials, the additional energy needed for manufacturing 3D-printed tiles, and the embodied emissions in the metal fastening system for the tiles are considered in the evaluation. The LCA accounts for the use of waste for aggregate supply in the geopolymer composite, considering the avoidance of landfilling. The results are evaluated in terms of global warming potential measured in kg CO₂-equivalent emissions, estimated via the IPCC 2013 impact assessment method. The service life of tiles and plasters is estimated to be 30 years as described in the literature^144,145. The LCA is also used to evaluate the “cradle-to-grave” environmental impact of using a dehumidification system to provide comfortable indoor humidity levels. The operational energy needs are estimated from dynamic hygrothermal simulations, and the correlated environmental impact is evaluated according to the LCI data for local electricity. The embodied, maintenance, and end-of-life emissions of the mechanical system within the 15 years of service life are also considered. The service life is estimated based on the literature¹¹¹. The environmental impact of the dehumidification system is compared to that of the finishing materials needed to achieve the same improvement in hygrometric comfort.

Author contributions

Magda Posani: Conceptualisation; methodology; material characterisation; numerical simulations; LCA; writing—original draft, review, and editing. Vera Voney: Material development; writing—review and editing. Pietro Odaglia: 3D printing and technological implementation; writing—review and editing. Yi Du: TGA-MS analyses and results interpretation for evaluating the mechanisms behind the superhygroscopic nature of the geopolymer composite; carbonation quantification; writing—review and editing. Anastasija Komkova: Supervision of cradle-to-grave LCA for materials and dehumidification systems; writing—review and editing. Coralie Brumaud: Supervision of material development; writing—review and editing. Benjamin Dillenburger: Supervision of technological implementation; writing—review and editing. Guillaume Habert: Conceptualisation; methodology; supervision; writing—original draft, review, and editing.

Peer review

Peer review information

Nature Communications thanks Dagmawi Mulugeta Degefu, Menghao Qin, and the other, anonymous, reviewer for their contribution to the peer review of this work. A peer review file is available.

Data availability

The data supporting the findings of this study are included in the main text, Supplementary Information, and Supplementary Data files. Source data are provided with this paper. Raw data can be obtained from the corresponding author upon request. Source data are provided with this paper.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s41467-024-54944-1.