High-Phosphogypsum Bricks: Microstructural, Mechanical, and Radiological Optimization According to Clay Type for Circular Construction

Abstract

This study explores the valorization of phosphogypsum (PG) in clay-based composites for sustainable production of bricks, addressing key factors such as drying behavior, mechanical performance, microstructure, porosity, and radiological safety. The analysis of drying kinetics reveals a rapid mass loss during the first 120 min, with intermediate PG–clay ratios (50:50) promoting optimal pore connectivity. Mechanical testing of fired composites indicates that green clay–PG (50:50) has the greatest performance, with a Young’s modulus over 2000 MPa and a maximum load capacity of 4000 N. Additionally, porosity decreases from ∼40% in the unfired condition to ∼10% after firing. Microstructural analysis confirms well-connected sintering necks and a cohesive clay–PG network. Radiological assessment indicates radium-equivalent activity of ≈50 Bq/kg for green clay composites, safely below the regulatory limit of 370 Bq/kg, while yellow PG formulations approach 257.6 Bq/kg. A multi-criteria optimization identifies the PG50–G50 (green clay) formulation as optimal, providing a balanced combination of mechanical strength, structural integrity, low porosity, and radiological safety. These findings indicate that fired PG–clay composites represent a viable route for producing high-performance, eco-friendly construction materials, supporting waste valorization, circular economy concepts, and advancing sustainable building practices.

1. Introduction

The rapid expansion of the phosphate industry has generated a significant environmental concern in the management of phosphogypsum (PG), a by-product formed during the wet-process synthesis of phosphoric acid by sulfuric acid assault on phosphate rocks. Globally, the production of phosphoric acid generates enormous quantities of PG, typically estimated at about 4–5 tons for every ton of phosphoric acid produced. − As a result, hundreds of millions of tons of phosphogypsum are produced annually across the world and are often kept in enormous open-air stacks near manufacturing sites.

This issue is especially serious in nations with a large phosphate industry, such as Tunisia. The large volumes of PG produced each year lead to the formation of extensive storage sites, where the material is either accumulated in open dumps or, in some cases, discharged into the sea. Such practices can have significant environmental consequences, including contamination of soil and water through leaching processes, as well as atmospheric emissions. In addition, phosphogypsum may contain trace levels of heavy metals, residual acids, and naturally occurring radionuclides inherited from the phosphate ore, raising concerns about its long-term environmental and health impacts.

Within the broader framework of sustainable development and the transition toward a circular economy, the valorization of industrial by-products has become a major research priority. By converting industrial wastes into valuable secondary resources, this approach reduces environmental impacts, conserves natural resources, and supports the development of more sustainable and resource-efficient materials. , In this context, the reuse of phosphogypsum (PG) across multiple application pathways represents a promising strategy for mitigating environmental impacts while promoting resource efficiency. PG can be integrated into various sectors, including construction materials, soil amendment in agriculture, environmental remediation, and other industrial processes. − Such diverse valorization paths help to reduce the buildup of PG in storage locations, lessen the need for virgin raw materials, and convert a large-scale industrial by-product into a valuable secondary resource. As a result, the multiuse of phosphogypsum promotes waste reduction, resource conservation, and the creation of sustainable industrial ecosystems based on circular economy principles.

Despite this potential, the large-scale utilization of phosphogypsum in construction materials faces several scientific and technological challenges. One of the main concerns is related to the mechanical stability of the resulting products. The incorporation of PG can significantly influence the physical and mechanical properties of bricks, including density, compressive strength, and durability, due to its specific mineralogical composition and its behavior during processing. − Furthermore, we investigated the interactions between phosphogypsum and clay minerals. Furthermore, interactions between phosphogypsum and clay minerals have a significant impact on the material’s microstructural evolution, particularly in terms of pore structure, particle bonding, and phase transformations, all of which influence the mechanical performance of the final product. ,,

Another important aspect is the radiological safety. Phosphate rocks naturally contain radionuclides, mainly from the uranium decay series, as well as thorium and potassium isotopes. During the production of phosphoric acid, these radionuclides can become partially concentrated in phosphogypsum (PG), which may therefore exhibit higher specific radioactivity than conventional construction materials. ,

When PG is used in building materials, it is essential to evaluate radiological parameters such as radionuclide activity concentrations and radon exhalation rates. − According to European regulations, particularly the EURATOM Basic Safety Standards Directive 2013/59 and the recommendations of the European Commission, building materials should not lead to an additional radiation exposure exceeding 1 mSv per year for the public, and indoor radon concentrations should remain below the reference level of 300 Bq·m^–3. Therefore, the safe use of phosphogypsum in construction requires a comprehensive radiological examination to verify conformity with these guidelines.

The feasibility of phosphogypsum-based construction materials is also closely linked to their microstructural characteristics. The crystalline structure of calcium sulfate dihydrate (CaSO₄·2H₂O), which constitutes the main component of PG, together with the presence of residual impurities such as phosphates, fluorides, and organic matter, can significantly influence the porosity and cohesion of the material. Detailed microstructural investigations are therefore required to understand how these phases evolve during compaction and drying processes, as well as how they impact the structural integrity of the finished bricks.

Ultimately, these microstructural characteristics play a decisive role in the mechanical performance of the material. Parameters such as compressive strength and Young’s modulus are critical for determining the suitability of bricks made from solid waste for construction. Understanding the links between microstructure, mechanical behavior, and radiological characteristics is thus critical for assessing the feasibility and safety of these materials.

Although numerous studies have investigated the use of phosphogypsum in cementitious or ceramic materials, most have focused on relatively low substitution ratios. Research exploring the utilization of high phosphogypsum contents remains limited, particularly with regard to the combined assessment of microstructural characteristics, mechanical properties, and radiological implications. Such an integrated approach is critical for achieving optimal material performance while ensuring environmental sustainability and radiological safety. , Moreover, the influence of clay type in combination with phosphogypsum has received little attention, despite the key role of clay minerals in governing plasticity, consolidation behavior, and final mechanical properties. , Furthermore, the incorporation of reinforcing fibers or supplementary cementitious materials, such as fly ash, has been reported as an effective strategy to enhance the mechanical performance of phosphogypsum-based bricks by improving strength, toughness, and crack resistance while promoting the valorization of industrial by-products.

Thus, the present study explores the development of construction bricks incorporating exceptionally high phosphogypsum contents, reaching up to 90 wt %. Particular attention is devoted to the influence of clay mineralogy through a systematic comparative investigation of two clay types originating from distinct depositsgreen clay and yellow clay. By integrating radiological assessment, microstructural analysis, and mechanical characterization, this work aims to provide a comprehensive understanding of the relationships between composition, structure, and performance. The ultimate objective is to improve the microstructural, mechanical, and radiological properties of high-phosphogypsum bricks and to evaluate their potential as sustainable building materials within circular construction strategies.

2. Materials and Methods

2.1. Raw Materials

The phosphogypsum (PG) used in this study is a raw by-product originating from a wet-process phosphoric acid manufacturing plant located in the Gafsa phosphate basin in southern Tunisia. In this industrial process, phosphate rock interacts with sulfuric acid to create phosphoric acid, with phosphogypsum serving as the primary solid residue. The collected PG appears as a fine, moist powder characterized by a predominantly calcium sulfate dihydrate composition (CaSO₄·2H₂O). Its chemical composition and physical characteristics are consistent with those commonly reported in the literature for phosphogypsum derived from sedimentary phosphate deposits. , Like most industrial PG, it may also contain minor impurities inherited from the phosphate ore and the production process, including residual phosphates, fluorides, trace metals, and naturally occurring radionuclides.

In addition to phosphogypsum, two natural clays from local Tunisian geological resources were selected as complementary raw materials for the brick formulation. The first material is yellow clay (Y) collected from the southwestern region of Tunisia within the Gafsa basin. The second material is a green clay (G) extracted from the central region of Tunisia, specifically from the Jebel Mrhila near Sidi Bouzid. These clays are widely recognized for their favorable ceramic properties, particularly their good plasticity, workability, and suitable behavior during the thermal treatment. Clay materials are traditionally used in construction due to their ability to regulate indoor humidity and their significant thermal inertia, which contribute to improved thermal comfort and durability of building materials. Their incorporation in phosphogypsum-based formulations is therefore expected to enhance both the structural integrity and processing behavior of the composite materials.

Through this preparation procedure, a set of phosphogypsum–clay bricks with varying compositions and clay mineralogy was produced, enabling a systematic investigation of the relationships between raw material characteristics, microstructural evolution, mechanical behavior, and radiological properties.

The chemical analysis results are reported in Table , which presents the oxide compositions of phosphogypsum (PG), green clay (G), and yellow clay (Y). As predicted, the phosphogypsum sample is dominated by calcium and sulfate components, indicating the presence of the calcium sulfate phases. In contrast, both clays exhibit higher contents of silica and alumina, which are typical of aluminosilicate minerals. Iron oxide is also present in noticeable amounts in the clays, contributing to their characteristic coloration, while influencing their thermal behavior during firing.

1. Chemical Composition of the Raw Materials Determined by XRF (wt %).

oxide	PG	GC	YC
SiO2	7.23	44.50	49.64
Al2O3	N. D.	11.02	6.10
Fe2O3	<0.01	1.61	0.66
CaO	38.63	13.87	13.00
MgO	1.38	3.12	5.12
SO3	46.51	0.45	1.85
K2O	<0.01	0.82	0.88
Na2O	0.56	1.31	0.77
P2O5	<0.01	1.01	0.24
TiO2	<0.01	0.27	0.08
LOI	5.68	22.00	21.65

The chemical composition of the raw materials used was determined by X-ray fluorescence (XRF) analysis. This approach quantifies the constituents of materials using their distinctive spectra. The XRF results confirm that phosphogypsum is mainly composed of calcium sulfate dihydrate (CaSO₄·2H₂O), commonly known as gypsum. Minor elements related to impurities originating from the phosphate ore or the industrial process may also be detected.

The X-ray diffraction patterns of the green (G) and yellow (Y) clays reveal a mineralogical assemblage dominated by aluminosilicate clay minerals, accompanied by accessory crystalline phases such as quartz and feldspars. These constituents play a fundamental role in controlling the plasticity of the raw materials, their phase transformations during firing, and the evolution of the brick’s microstructure. Consequently, the combined chemical and mineralogical characterization of phosphogypsum (PG), G clay, and Y clay provides a sound basis for understanding their interactions throughout shaping and thermal treatment and for explaining the resulting microstructural development and mechanical performance of the phosphogypsum-based bricks.

2.2. Brick Formulation

Conventional fired clay bricks are expected to exhibit adequate mechanical strength, dimensional stability, and durability to satisfy the requirements of construction applications. Typically, these materials present an open porosity ranging from 15 to 35%, water absorption between 8 and 20%, and a Young’s modulus generally within the range of 2–15 GPa. These properties are strongly influenced by the nature of the raw materials, their composition, and the firing conditions. To evaluate the effect of clay type and composition on the properties of the resulting bricks, a series of mixtures was prepared with varying mass proportions of phosphogypsum (PG) and clay. Initial formulations were produced without firing to assess the feasibility of manufacturing bricks with high PG content while maintaining sufficient mechanical stability and structural integrity.

Two types of clays were used: green clay (G) and yellow clay (Y), sourced from the Gafsa Basin and Djebel Mrhila regions, as previously mentioned. The clay content in the mixtures was progressively increased to examine its role in enhancing the cohesion and overall performance of PG-based materials during firing. Mixtures containing up to 90 wt % phosphogypsum were included to investigate the maximum incorporation of PG achievable in brick production.

Table summarizes the representative compositions investigated in this study. The samples were labeled according to the relative proportions of phosphogypsum and clay, as well as the clay type used in the mixture. The formulations include PG contents ranging from 50 to 90 wt % combined with green or yellow clay, allowing the evaluation of the effects of PG incorporation and clay nature on the resulting brick properties.

2. Weight Percent Composition of Phosphogypsum and Clay in the Brick Formulations.

sample	PG (wt %)	clay (wt %)
PG90-G10	90	10 (green clay)
PG70-G30	70	30 (green clay)
PG50-G50	50	50 (green clay)
PG70-Y30	70	30 (yellow clay)
PG50-Y50	50	50 (yellow clay)

These formulations provide a systematic framework for assessing the effect of clay type and content on the overall performance of PG-based bricks. By altering the quantities of phosphogypsum and clay, it is able to see how clay contributes not only to mechanical stabilization but also to microstructural refinement and radiological mitigation. The PG90-G10 formulation, with its very high phosphogypsum content, serves as a test case for pushing the limits of PG incorporation while maintaining structural integrity. In contrast, the 70–30 and 50–50 mixtures allow examination of how increasing clay content enhances matrix cohesion, reduces porosity, and improves mechanical properties, offering insight into the optimal balance between PG and clay for safe and durable construction materials.

The dry components of each combination were rigorously homogenized to ensure that PG and clay particles were distributed evenly. Water was then gradually added, resulting in a plastic, moldable paste with adequate workability to shape into bricks. This meticulous preparation is crucial because it provides uniform material qualities, eliminates internal flaws, and allows for valid comparisons among formulations. The technique also enables the subsequent characterization of mechanical performance, microstructural development, and radiological behavior, resulting in a thorough understanding of how formulation factors impact the final material.

2.3. Sample Preparation

The phosphogypsum–clay bricks were made using a well-regulated, multi-step preparation technique that ensured compositional uniformity, structural integrity, and repeatable performance (Figure ). First, the raw materials, including phosphogypsum and chosen clays, were properly dried and finely ground to achieve a homogeneous particle size, which is required for consistent mixing, reduced segregation, and improved particle bonding during compaction and burning. For each composition, the dry powders were rigorously mixed using a mechanical mixer until a consistent texture and color were achieved, ensuring that each sample included a homogeneous distribution of PG and clay. Water was gradually introduced into the mixture under continuous stirring to form a plastic, cohesive paste with optimal workability. The water content was carefully adjusted to ensure appropriate flexibility for molding while avoiding excessive porosity, which might compromise the mechanical performance of the finished bricks. In this context, water was considered a key parameter, used in a proportion equivalent to that of phosphogypsum to obtain a workable and homogeneous mixture.

1.

Schematic workflow of phosphogypsum–clay brick preparation.

The prepared paste was poured into prismatic molds measuring 4 × 4 × 16 cm³, corresponding to standard dimensions for mechanical testing of construction materials. The molded specimens were then air-dried for 72 h to gradually remove moisture and limit the danger of microcracking during further thermal treatment. This process is critical for ensuring the bricks’ dimensional stability and structural integrity.

Once dried, the bricks underwent a firing process in a laboratory muffle furnace at 1140 °C for 2 h, followed by controlled cooling to room temperature. This firing temperature was selected based on previous studies of clay-based ceramic composites, which indicated that this thermal level promotes stability of mineral phases, effective sintering, and the development of strong particle bonding, all of which are critical for achieving the desired mechanical strength and durability. During firing, the phosphogypsum and clay components undergo complex mineralogical transformations that reduce porosity, enhance cohesion, and improve the overall microstructural integrity of the bricks. The combination of controlled grinding, thorough mixing, precise water addition, high-pressure compaction, careful drying, and optimized firing results in homogeneous, mechanically robust, and microstructurally stable bricks, providing a reliable platform for systematically evaluating the effects of phosphogypsum content and clay type on their physical, mechanical, and radiological properties.

2.4. Microstructural Characterization

The microstructural characterization of the phosphogypsum–clay bricks was performed to gain detailed insights into their internal structure and to correlate these features with their mechanical behavior. Understanding the arrangement of mineral phases, pore distribution, and particle bonding is critical for interpreting the strength, durability, and overall performance of the bricks.

The primary observation technique employed was optical microscopy using a Leica DM100 microscope, allowing for high-resolution visualization of the internal microstructure. Thin sections of the brick samples were prepared to investigate the spatial distribution of phosphogypsum and clay phases, as well as to identify microcracks and examine the interparticle interfaces before and after the firing process.

To enhance the analysis, Huygens software was employed for the mineralogical filtering and phase segmentation. This advanced image processing tool enables precise separation of individual mineral phases within the microstructure, facilitating quantitative knowledge of phase distribution, particle size, and connection within the material matrix.

Furthermore, ImageJ software was utilized to quantify pore characteristics, including pore size distribution, porosity fraction, and pore connectivity. By examining numerous micrographs from each sample, a statistical description of the pore network was developed, which directly influences mechanical properties such as compressive strength, Young’s modulus, and durability under environmental loading.

The combination of these approaches yields a complete image of the internal structure, allowing correlations to be established between microstructural parameters (such as particle arrangement, porosity, and phase distribution) and measured mechanical performance. This integrated approach is essential for optimizing the formulation of high-phosphogypsum bricks and ensuring that their structural and functional qualities match those of building requirements.

2.5. Mechanical Testing

The mechanical performance of the phosphogypsum–clay bricks was systematically evaluated through uniaxial compression tests to determine their structural suitability for construction applications. These tests provide key information on the strength, stiffness, and ductility of the material, which are directly influenced by the composition, microstructure, and porosity of the bricks.

Compression tests were carried out using a Lloyd LR 5K universal testing machine, which provides precise control of the applied load and accurate measurement of the resulting deformation. Each prismatic specimen was positioned between the platens of the testing device, and a constant displacement rate was applied until failure occurred. Load-strain curves were constantly recorded during the experiment, allowing us to characterize the samples’ elastic and post-elastic behavior. Several essential mechanical characteristics for each formulation were found using the load-strain data. The Young’s modulus (E) was obtained from the initial linear portion of the curve and represents the stiffness of the material and its resistance to elastic deformation under applied stress. The maximum compressive strength (σ_max) corresponds to the peak stress reached before fracture and reflects the ultimate load-bearing capacity of the brick. Finally, the strain at failure (ε_f) was determined from the deformation at the rupture point, providing information about the ductility and the energy absorption capability of the material.

These parameters allow a quantitative comparison of the effects of phosphogypsum content and clay type on the mechanical performance of the bricks. In addition, by correlating the results with the microstructural features observed via microscopy and image analysis, it is possible to establish relationships between the internal structure (pore distribution, particle bonding, mineral phases) and mechanical behavior. This integrated approach ensures that the formulations with a high phosphogypsum content not only meet structural requirements but also provide insight into modifying the composition for increased strength, stiffness, and durability.

2.6. Radiological Assessment

The radiological safety of the phosphogypsum–clay bricks was evaluated at the Centre National des Sciences et Technologies Nucléaires (CNSTN) to assess the suitability of materials containing up to 90 wt % phosphogypsum for residential construction. This evaluation is critical due to the presence of naturally occurring radionuclides in phosphate rock, which can become concentrated in phosphogypsum during phosphoric acid production. ,

The radiological characterization was performed by high-resolution γ-ray spectrometry using a coaxial high-purity germanium (HPGe) detector with a relative efficiency of 53%, coupled to a digital multichannel analyzer (MCA) for spectrum acquisition and processing. This configuration allows precise determination of the activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K (Bq/kg). These radionuclides were determined, as these radionuclides constitute the standard reference for phosphogypsum characterization in Tunisia and are recommended by the IAEA and UNSCEAR for evaluating radiological hazards associated with building materials. The measured activities were subsequently used to calculate the principal radiological indices, including the radium equivalent activity (Ra_equiv), external hazard index (Hex), absorbed γ dose rate, annual effective dose, and γ activity concentration index (Iγ). Although other naturally occurring radionuclides, such as ²³⁸U, ²³⁵U, ²¹⁰Pb, and ²¹⁰Po, have been reported in previous studies, including, they were beyond the scope of the present work, which focuses on the radionuclides routinely monitored for external radiological hazard assessment.

To maximize phosphogypsum valorization, the investigation focused on the formulation containing 90 wt % PG and 10 wt % clay. Three variants were analyzed: PG green (raw), PG green (fired), and PG yellow (fired). After drying, the bricks were mechanically ground and sieved to eliminate heterogeneities and obtain a fine, isotropic powder suitable for radiometric analysis, thereby minimizing self-absorption effects and ensuring uniform radionuclide distribution.

For γ spectrometry, representative powder samples were sealed in 100 cm³ high-density polyethylene containers with a standardized filling height of 5 cm, ensuring constant apparent density, reproducible measurement geometry, and high analytical precision. The experimentally determined activity concentrations were used to evaluate the compliance of the developed bricks with the internationally accepted radiological safety criteria for construction materials.

The calculated radiological indices were finally compared with the limits established by the European Union Basic Safety Standards for building materials. In addition, the radioactivity index previously reported for Tunisian phosphogypsum (≈0.97), which remains below the regulatory limit of 1.0 specified by the European Commission, further supports the feasibility of incorporating high proportions of phosphogypsum into fired clay bricks. Overall, this radiological assessment demonstrates that high-PG bricks can satisfy internationally accepted safety requirements while promoting the sustainable valorization of phosphogypsum in construction applications.

3. Results and Discussion

3.1. Drying Kinetics of Samples

The drying behavior of the PG–green clay mixtures was monitored over 300 min prior to firing (Table ). All samples began at 100% relative mass, and a rapid decrease in mass was observed during the first 120 min, corresponding to the evaporation of free water. After approximately 150 min, the mass of all compositions reached a quasi-steady plateau, indicating the completion of the primary drying stage and the retention of structural water bound to the clay or PG matrix.

The final plateau values show a clear dependence on the clay fraction: samples richer in phosphogypsum (PG) reach lower residual mass (M _∞ ≈ 52%), whereas formulations with higher green clay content retain more water (up to M _∞ ≈ 58%), reflecting the hygroscopic nature of the clay and its ability to slow moisture release. Notably, the 70% PG–30% green clay mixture exhibited the highest mass loss, suggesting enhanced pore connectivity and more efficient moisture transport.

To describe the drying kinetics quantitatively, an empirical formulation was proposed. Let (x) denote the fraction of green clay in the mixture (0 ≤ x ≤ 0.5). The mass evolution M(x,t) can be approximated by a first-order exponential model:

$$M(x,t)=M_{\infty }(x)+(100-M_{\infty }(x))e^{-k(x)t}$$

The plateau mass M _∞ and the drying rate k depend on the clay fraction. For the clays used in this study, these parameters can be expressed as follows:

$$\mathrm{YC}:M_{\infty }(x)=52+14x;k(x)=0.012-0.006x$$

and

$$\mathrm{GC}:M_{\infty }(x)=58-16x;k(x)=0.01+0.01x$$

This model captures both the influence of the clay content on the residual moisture and the deceleration of drying as the bound water fraction dominates. The exponential behavior reflects the two-stage drying process: rapid free water evaporation followed by slower structural water removal (Figure ).

2.

Effect of phosphogypsum content on the drying behavior of PG–clay bricks over time.

3.2. Microstructural Evolution

The qualitative findings from the picture analysis are well corroborated by direct optical microscope data. An examination of the unfired specimens (Figure ) reveals pronounced differences in microstructural behavior between the two clay–phosphogypsum (PG) systems. The sample labels correspond to the following formulations: a: PG with 50% Yellow Clay, b: PG with 30% Yellow Clay, c: PG with 10% Yellow Clay, d: PG with 50% Green Clay, e: PG with 30% Green Clay, and f: PG with 10% Green Clay.

3.

Optical microscopy of unfired and fired specimens: (a, a′) PG–50% Yellow Clay; (b, b′) PG–30% Yellow Clay; (c, c′) PG–10% Yellow Clay; (d, d′) PG–50% Green Clay; (e, e′) PG–30% Green Clay; and (f, f′) PG–10% Green Clay. Unprimed and primed labels denote unfired and fired specimens, respectively.

The yellow clay samples (A, B, and C) have discrete black spots on the surface that correspond to apparent porosity. Notably, the density of these voids rises with increasing PG concentration, implying a direct relationship between filler fraction and pore volume. The yellow clay matrix displays limited cohesiveness and a weakly formed clay–PG interface, resulting in fragmented grain distribution and a weak network of solid bridges. These properties show that the yellow clay is less successful at encapsulating PG particles and producing a continuous binder network in the unfired condition.

In contrast, green clay specimens (D, E, and F) display a significantly more cohesive microstructure (Figure ). Dark central zones, which correspond to bonding sites, are readily apparent and related to areas of increased cohesiveness within the matrix. The distribution of these zones decreases slightly with increasing PG content, but overall, the green clay promotes dense packing and effective encapsulation of PG grains. This favors the formation of solid bridges between particles, resulting in a more interconnected microstructure and improved structural integrity. The 50–50 PG–green clay mixture (D) exhibits the most extensive distribution of cohesive zones, reflecting optimal grain distribution and superior filler–binder interaction. These findings emphasize the importance of clay mineralogy: green clay enhances cohesion, optimizes grain connectivity, and supports solid bridge formation even at high PG substitution levels. While these assessments are qualitative, they give a solid knowledge of the microstructural principles that influence porosity and particle bonding, which will be measured in the following studies.

Upon firing at 1140 °C, the influence of both clay type and PG content on structural evolution becomes more pronounced (fired specimens), Figure . Green clay formulations exhibit great visual uniformity, suggesting enhanced thermal reactivity and the development of a stable vitreous phase that efficiently encapsulates PG grains. This vitrification contributes to a reduction in overall porosity and reinforces the solid bridges between filler and binder, stabilizing the internal microstructure.

Conversely, at 90% PG, a critical binder threshold is exceeded: the clay volume is inadequate to effectively cover and bind the phosphogypsum particles, resulting in a fractured microstructure with discontinuous solid bridges and a disordered porous network. This phenomenon highlights the fundamental role of clay as a structural stabilizer and a mediator of the microstructural integrity during sintering.

The evolution of surface porosity as a function of PG content is illustrated in Figure . Across all formulations, the porosity increases with PG addition due to thermal decomposition of gypsum, which progresses from dihydrate to hemihydrate and then to anhydrite.

4.

Evolution of surface porosity for unfired and fired samples measured by ImageJ analysis.

The release of water vapor during this process generates additional voids. Image analysis confirms that firing acts as the primary consolidation mechanism, reducing porosity from over 12% in the unfired state to below 5% after sintering. Yellow clay exhibits an increase in porosity with PG addition in the unfired state, reflecting poor filler encapsulation, while green clay demonstrates minimal residual porosity (∼2%) after firing, even at high PG content. These results underscore the combined influence of PG content, clay type, and firing state on the microstructural evolution of the ceramics.

To quantify these effects, the total porosity was measured in triplicate for all formulations. A three-way ANOVA was conducted to rigorously evaluate the effects of clay mineralogy, PG content, firing state, and their interactions (Figure ).

5.

ANOVA analysis of the statistical significance of parameters affecting sample porosity.

All factors were highly significant (p < 0.001). The firing state exerted the dominant effect (F > 175), reflecting the critical role of thermal transformations in driving densification, solid bridge formation, and microstructural consolidation. Clay type was the second most influential factor (F ≈ 50), confirming that initial mineralogical composition strongly affects matrix cohesion and grain distribution. PG content had the smallest isolated effect (F ≈ 10); however, interactions were substantial. The Clay × PG interaction (F ≈ 25) demonstrates that the response to PG dosage is strongly dependent on clay mineralogy, with green clay facilitating superior encapsulation and bridge formation compared to yellow clay. Firing-related interactions (Clay × Firing, PG × Firing) also influenced the final porosity, illustrating that the effectiveness of thermal consolidation is modulated by both filler and binder composition.

The findings further reveal that green clay serves as a superior stabilization matrix, promoting optimal grain distribution, solid bridge formation, and low residual porosity even at high PG substitution. Its cohesive interaction with PG, enhanced by firing-induced vitrification, leads to a denser, more homogeneous ceramic structure. These findings not only confirm the qualitative observations from optical microscopy but also give statistically confirmed proof of the importance of clay type, filler content, and firing state in determining microstructural development, porosity, and structural integrity of PG–clay ceramics.

3.3. Chemical and Mineralogical Composition

The initial stage of this study focuses on a qualitative assessment of the specimens’ microstructural changes, using advanced image processing techniques. This approach allows for a detailed visual analysis of surface and internal features that may be affected by abrasion or thermal treatment, providing insight into the evolution of the material’s microstructure before and after testing. ,

Photomicrographs processed with Huygens software were segmented into four color shades, enabling the precise identification of the main mineralogical phases in both raw and fired samples. Each color corresponds to a dominant phase or specific mineral constituent, allowing qualitative and semi-quantitative comparison of microstructural evolution under consistent imaging conditions.

The initial phase of this study focused on a qualitative assessment of microstructural changes by using advanced image processing techniques. Specimens were photographed under a microscope before and after abrasion, and the color segmentation based on a four-color viridis palette (violet, yellow, sky blue, and white) provided detailed insights into surface and internal features affected by thermal treatment or mechanical stress.

Figure presents the specimens in their raw and fired states. Phase composition was estimated via pixel-based image analysis, with each color corresponding to specific optical contrasts associated with different mineral phases or microstructural domains. Pixel counting allowed a semi-quantitative estimation of the relative proportions of these phases (Figure ). Specimens were prepared with varying proportions of phosphogypsum (PG) and clay: a: PG with 50% Yellow Clay, b: PG with 30% Yellow Clay, c: PG with 10% Yellow Clay, d: PG with 50% Green Clay, e: PG with 30% Green Clay, and f: PG with 10% Green Clay.

6.

Mineralogical phases in raw and fired specimens revealed by Huygens-corrected optical microscopy images: (a–f) raw specimens and (a′–f′) corresponding fired specimens.

7.

Pixel-counting-based semi-quantitative analysis of mineral phases: (a, a′) PG–50% Yellow Clay; (b, b′) PG–30% Yellow Clay; (c, c′) PG–10% Yellow Clay; (d, d′) PG–50% Green Clay; (e, e′) PG–30% Green Clay; (f, f′) PG–10% Green Clay. Unprimed and primed labels denote raw and fired specimens, respectively.

Based on the combined XRF chemical composition and XRD mineralogical characterization, the color distribution observed in the raw samples can be interpreted as reflecting the initial phase assemblage governed by the relative proportions of the major oxides and crystalline constituents. The oxide composition of the clay–PG mixtures, particularly SiO₂, Al₂O₃, CaO, MgO, and SO₃, influences the stability and interactions of the mineral phases during processing. In particular, MgO can promote the formation and stability of silicate and aluminosilicate phases and may act as a fluxing oxide during heating, whereas sulfate originating from phosphogypsum is expected to influence the decomposition pathways and reactions with the surrounding oxide matrix. Accordingly, Sky-blue regions indicate gypsum, white areas correspond to hemihydrate-like phases likely formed during storage, yellow regions represent alumina-rich clay minerals (kaolinite, illite, montmorillonite), and small violet zones correspond to quartz, consistent with measured fractions.

During firing, thermally activated reactions progressively modify the initial mineral assembly. Sulfate-bearing phases from phosphogypsum undergo decomposition, while clay minerals experience dehydroxylation and structural transformation. The released species subsequently interact with the major oxides identified by XRF, including SiO₂, Al₂O₃, CaO, and MgO, promoting the formation of new crystalline phases such as calcium and magnesium silicates, aluminosilicates, and mixed oxide phases. At higher temperatures, partial melting of the most reactive components leads to the formation of a viscous glassy phase, which fills residual pores, improves particle cohesion, and promotes the densification of the fired brick matrix.

In the fired samples, violet represents residual quartz, sky-blue corresponds to residual anhydrite (CaSO₄), and white represents the ceramic phases (anorthite, gehlenite, and wollastonite) generated during firing through reactions between phosphogypsum and aluminosilicate constituents of the clay (Figure ). The increasing proportion of ceramic phases with an increasing clay content reflects the enhanced formation of thermodynamically stable reaction products, whereas the remaining anhydrite indicates incomplete transformation of calcium sulfate during sintering. Residual quartz persists as a relatively inert phase due to its limited reactivity under applied firing conditions. These phase assemblages demonstrate the mineralogical evolution of the composites, revealing the conversion of the raw materials to a chemically stable ceramic matrix through dehydration, solid-state reactions, crystallization of new phases, and partial preservation of the original minerals. This approach demonstrates the direct link among composition, thermal treatment, and resulting microstructure, highlighting the influence of clay type and PG fraction on phase evolution and structural integrity of the composites.

8.

XRD pattern of the fired PG50-G50 brick.

3.4. Mechanical Performance

Uniaxial compression tests on phosphogypsum–clay composites reveal pronounced differences in mechanical response between raw (unfired) and thermally treated (fired) samples. In the unfired state, the load–displacement curves show relatively low stiffness and strength. The apparent Young’s modulus (E) ranges from 29 MPa for PG90-G10 to 81 MPa for PG50-G50, while the maximum stress (σ_max) spans from 1.38 to 2.19 MPa (Table ).

3. Young’s Modulus and Maximum Load of PG–Clay Samples before and after Firing.

sample	state	E (MPa)	F max (N)
PG90–G10	unfired	29	220
PG50–Y50		45	250
PG50–G50		81	350
PG90–G10	fired	120	300
PG50–Y50		667	1400
PG50–G50		2000	4000

This low initial stiffness is attributed to the granular nature of phosphogypsum, the high porosity of the matrix, and the weak interparticle cohesion provided by the unfired clay. The observed deformation is mainly due to grain rearrangement, particle sliding, and progressive closure of voids, so the initial slope of the curves reflects an apparent modulus rather than the intrinsic elastic stiffness of the composite (Figure ).

9.

Load–strain curves of unfired PG–clay specimens.

After firing, the mechanical response improves substantially. The fired composites exhibit a significant increase in both Young’s modulus and compressive strength, highlighting the role of consolidation mechanisms during sintering. For instance, PG50-G50 reaches E = 2000 MPa and σ_max = 53.3 MPa, whereas PG50-Y50 shows E = 667 MPa and σ_max = 18.7 MPa. This improvement is mainly due to the formation of “sintering necks” within the clay phase, which bond phosphogypsum grains and enhance load transfer. In contrast, PG90-G10 shows only moderate improvement after firing (E = 120 MPa, σ_max = 4.0 MPa), as the low clay fraction prevents the development of a continuous load-bearing network, leaving the PG skeleton dominant in carrying applied loads (Figure ).

10.

Load–strain curves of fired PG–clay specimens.

The type of clay used as a binder strongly influences the mechanical performance. Green clay provides a higher stiffness and strength due to its greater plasticity and reactivity during firing, promoting the formation of cohesive sintering necks and strong interparticle bonds. Yellow clay provides some consolidation, but the network of bonded grains remains less connected, resulting in lower stiffness and strength compared to green clay composites with a similar PG content.

The phosphogypsum fraction is another critical factor. High PG content composites (e.g., PG90-G10) show limited mechanical improvement even after firing because the granular PG phase dominates load transfer, and the clay is insufficient to form a reinforcing network (Figure ). In contrast, intermediate PG–clay ratios (50–50 PG–G or PG–Y) allow an optimal balance between filler and binder, enabling the clay matrix to create a continuous load-bearing network that enhances both stiffness and strength (Figure ). This shows that mechanical properties depend more on the connectivity of the clay network than on the mere presence of clay.

11.

Load–strain characteristics of PG90–GC10 before and after firing.

12.

Load–strain characteristics of PG50–GC50 before and after firing.

The observed mechanical behavior correlates directly with the microstructural features. Microscopic images show that unfired composites contain large, discontinuous pores and weak interparticle contacts, explaining the low apparent moduli and limited strength. Fired composites reveal sintering necks between clay and PG grains, reduced porosity, and improved connectivity, directly accounting for increased stiffness and compressive strength. Composites with insufficient clay retain isolated particle contacts, limiting mechanical enhancement, whereas those with sufficient clay develop a cohesive network that is capable of effectively distributing stresses.

Thus, the mechanical properties of phosphogypsum–clay composites depend on the combined effects of clay type, PG fraction, and thermal treatment. Unfired composites exhibit low apparent modulus and strength due to porosity and weak interparticle cohesion. Firing promotes consolidation, with green clay yielding the greatest improvements in stiffness and compressive strength. Mechanical performance is best at middle PG-clay ratios, where the combination of filler and binder promotes grain connection and sintering neck formation, lowers porosity, and increases load transmission. Although the mechanical performance of PG-based composites remains lower than that of conventional fired bricks, the incorporation of phosphogypsum represents a promising alternative for the sustainable reuse of this industrial by-product, particularly in applications where environmental valorization is prioritized. This apparent constraint might be addressed in future advancements by using reinforcing phases, such as metallic fibers, which may improve fracture resistance, load transfer mechanisms, and overall structural dependability of PG-clay composites. These results confirm the critical role of phase composition and microstructural connectivity in determining the mechanical behavior of fired phosphogypsum–clay composites.

3.5. Radiological Safety Assessment

Nuclear safety is a critical parameter in the validation of bricks containing high mass fractions of phosphogypsum (PG), as these materials can contain naturally occurring radionuclides. Figure presents the measured specific activities of ²²⁶Ra, ²³²Th, ⁴⁰K, and uranium in PG–Green Clay, as well as the calculated radium equivalent activity (Ra_equiv), which provides an integrated metric for assessing potential external radiological exposure. Ra_equiv is particularly useful as it combines the contributions of the three main radionuclides according to standard formulas to ensure comparability with regulatory limits.

13.

Comparison of natural radionuclide activities and radiological indices in PG–Green Clay samples before and after firing.

The activity of ²³²Th remains largely unaffected by both the clay type and thermal treatment, indicating its stability within the phosphate matrix. ⁴⁰K shows a minor increase after firing in green PG, which may be associated with the reorganization of the aluminosilicate matrix, but no significant difference is observed between the fired PG–green clay mixtures. Uranium activity remains consistently below detection limits (<LD) across all formulations, demonstrating minimal contribution to overall radiological risk.

The radium equivalent activity Ra_equiv provides a comprehensive index for radiological safety. PG–Green Clay formulations exhibit a low Ra_equiv of 50.3 Bq/kg, well below the regulatory limit of 370 Bq/kg. PG–Yellow Clay formulations, in contrast, reach 257.6 Bq/kg, approaching the safety threshold (Figure ). This contrast emphasizes the importance of clay in diluting the radionuclide content.

14.

Radiological characterization of PG–Yellow Clay specimens: non-fired vs. fired.

The observed decrease in the radionuclide activity after clay incorporation and firing can be explained by several complementary mechanisms. The addition of inert clay reduces the radionuclide concentration per unit mass through a dilution effect, while the comparable radiological behavior observed before and after thermal treatment confirms the compliance of the developed bricks with recommended safety criteria. Furthermore, firing may promote partial degassing of volatile species and redistribution of radionuclides within newly formed crystalline and glassy phases. Such phase transformations could contribute to the immobilization of radionuclides within the ceramic matrix, thereby limiting their mobility and reducing their potential contribution to the external γ exposure.

A comparative analysis of green PG and yellow PG reveals significant differences arising from the geological and sedimentary origin of the phosphate ores. In the unfired state, green PG exhibits ²²⁶Ra activity of 123.9 Bq/kg, while yellow PG shows a substantially higher value of 280–300 Bq/kg, reflecting its naturally higher radionuclide content. Firing exerts a substantial effect on radionuclide distribution, especially for ²²⁶Ra. In green PG–clay mixtures, ²²⁶Ra decreases drastically from 123.9 to 16.5 Bq/kg after thermal treatment. This reduction is attributed to the dehydration of gypsum during firing, which releases structural water and reduces radium retention within the crystalline matrix. In yellow PG, however, ²²⁶Ra remains relatively high at 225.4 Bq/kg after firing, underscoring the higher initial concentration and limited mitigation through thermal processing alone.

Other radiological indices, such as the external hazard index (H_ex) and the α index, confirm this trend, with green PG formulations consistently showing values well below unity, the safety criterion for both indices. The dilution effect of clay is particularly pronounced for ²²⁶Ra, the primary contributor to Ra_equiv, while the concentrations of ²³²Th and ⁴⁰K remain largely unaffected by the clay proportion.

Therefore, these results confirm that PG–Green Clay formulations are fully compliant with international radiological safety standards, including TENORM exemption limits. The combination of low specific activity, favorable Ra_equiv, and effective dilution by clay establishes green PG as the preferred choice for industrial brick production. By simultaneously optimizing structural and radiological performance, this formulation ensures safe handling, long-term stability, and minimal exposure risk for both workers and end-users.

3.6. Multi-Criteria Optimization

To determine the best formulation for industrial applications, phosphogypsum–clay composites must be developed while balancing several performance requirements, such as mechanical strength, microstructural integrity, and radiological safety. A comprehensive assessment of several formulations with varied phosphogypsum (PG) fractions and clay types indicates significant trade-offs among these requirements.

From a mechanical perspective, fired composites with intermediate PG–clay ratios (50–50 PG–G) exhibit the highest Young’s modulus (E = 2000 MPa) and maximum compressive strength (σ_max = 53.3 MPa), indicating excellent stiffness and load-bearing capacity. In contrast, high-PG formulations with low clay content (PG90-G10) show only limited improvement in modulus and strength despite firing, due to insufficient binder connectivity within the matrix. Although yellow clay provided moderate reinforcement, its performance remained consistently lower than that of green clay, particularly regarding stiffness and structural integrity. ,

Regarding microstructural criteria, microscopic analysis shows that an optimal connectivity between phosphogypsum grains and the clay binder is critical. PG50-G50 samples display a well-connected network of sintering necks and reduced porosity, promoting a uniform stress distribution and minimizing defect formation. In contrast, PG90-G10 and PG50-Y50 compositions display discontinuous bonding, higher residual porosity, and weaker intergranular cohesion, thereby limiting their mechanical potential, despite partial densification during firing.

Radiological considerations also contributed to the selection of the optimal formulation. Green PG–clay-based composites exhibited the lowest radium equivalent activity (Ra_equiv ≈ 50 Bq/kg), well below the recommended limit of 370 Bq/kg, whereas yellow clay formulations showed higher values (≈257.6 Bq/kg). This behavior is mainly associated with the dilution effect of clay incorporation and its influence on radionuclide distribution. Therefore, green clay provides the most balanced solution, combining improved mechanical consolidation with enhanced radiological safety for high-phosphogypsum construction materials.

The PG50-G50 formulation was chosen as the best composition after considering the mechanical, microstructural, and radiological factors. It strikes the optimal compromise between great mechanical performance, a dense and cohesive microstructure, and minimal radiological danger (Figure ).

15.

Multi-criteria optimization of phosphogypsum–clay fired composites.

4. Circular Construction Implications

The optimization of phosphogypsum–clay composites has significant implications for circular construction practices and the valorization of industrial by-products. , The identification of an optimal formulation, such as PG50-G50, demonstrates that high-performance, structurally stable bricks can be produced while simultaneously addressing environmental and industrial challenges. By incorporating a high fraction of phosphogypsuma major by-product of phosphate fertilizer productioninto construction materials, these composites provide a pathway for industrial valorization, transforming what would otherwise be a waste stream into a valuable building resource.

The use of phosphogypsum in brick production directly contributes to waste reduction. Large quantities of PG are generated annually, often stockpiled in uncontrolled heaps, posing environmental and radiological risks. By integrating PG into composite bricks, the material’s inherent mineral content is stabilized within a fired matrix, mitigating leaching and radiological concerns, as confirmed by the multi-criteria optimization results. This approach not only diverts waste from landfills but also reduces the environmental footprint of conventional brick manufacturing by replacing virgin raw materials with a recycled industrial by-product. ,

These composites exhibit circular economy concepts of resource efficiency and material recirculation. The combination of PG and reactive clay allows for regulated microstructural consolidation, resulting in long-lasting, fire-resistant, and mechanically strong bricks suited for load-bearing applications. The fact that the optimal formulation (PG50-G50) achieves high mechanical performance, low residual porosity, and outstanding radiological safety demonstrates the viability of using waste-derived materials in mainstream construction. This correlates with larger sustainability aims, increasing resource circularity, reducing the level of extraction of virgin resources, and minimizing waste output.

Furthermore, the development of PG–clay bricks demonstrates strong potential for ecological construction, offering an alternative to traditional cement- or clay-only bricks. Their low energy requirements for firing, combined with the utilization of industrial by-products, make these composites a promising candidate for green building applications. , By combining mechanical performance, controlled microstructure, and safety concerns, optimized PG-clay composites offer a potential avenue for converting industrial waste into high-value construction materials.

5. Conclusions

This study demonstrates the feasibility of valorizing phosphogypsum (PG) as a secondary raw material for the development of sustainable clay-based construction composites. The combined assessment of drying behavior, mechanical performance, microstructure, and radiological safety provides a comprehensive understanding of the factors governing the final material quality. Intermediate PG–clay ratios were found to favor homogeneous drying by promoting efficient moisture removal and balanced pore connectivity, thereby reducing the risk of structural defects before firing.

Among the investigated formulations, the 50:50 PG/clay composite exhibited the best overall performance, achieving a Young’s modulus close to 2000 MPa and improved mechanical strength after firing. These improvements are closely related to microstructural evolution, characterized by reduced porosity and the formation of well-developed, interconnected sintering necks that strengthen particle bonding and overall cohesion. In contrast, PG-rich formulations with insufficient clay content exhibit limited densification and weaker mechanical integrity, highlighting the key role of clay fraction in consolidation processes.

Radiological assessment revealed differences between raw and fired samples due to dehydration, degassing, phase transformations, and matrix densification during firing, which can modify the radionuclide distribution. Considering the low penetration depth of α particles, the evaluation focused on ²²⁶Ra, ²³²Th, and ⁴⁰K, which are the main contributors to external γ exposure in construction materials. Other naturally occurring radionuclides, such as ²³⁸U, ²³⁵U, ²¹⁰Pb, and ²¹⁰Po, were not included in the present assessment. The firing process further contributed to radionuclide stabilization within the densified ceramic matrix, reducing its potential mobility and leaching.

A multi-criteria optimization approach identifies the PG50–G50 formulation as the most suitable compromise, providing a balance between mechanical performance, structural integrity, and radiological safety. This optimized composition combines the beneficial contribution of clay to ceramic bonding and matrix densification with a high phosphogypsum incorporation rate while maintaining acceptable structural and environmental performance. Although the mechanical properties achieved remain lower than those of conventional fired clay bricks, this limitation can be addressed through reinforcement strategies, such as the addition of fibers, ashes, or metallic particles, to further enhance the mechanical behavior of the composite matrix. Moreover, this approach offers a major advantage due to its ability to incorporate significant amounts of phosphogypsum into construction materials, providing an effective solution for reducing phosphogypsum stockpiles, minimizing the consumption of natural raw materials, and promoting sustainable waste valorization. These findings emphasize the industrial potential of PG–clay composites as environmentally responsible construction materials aligned with circular economy principles.

Glossary

Nomenclature

PG

phosphogypsum

LOI

loss on ignition

XRF

x-ray fluorescence

Ra_equiv

radium equivalent activity

M∞

residual mass

x

fraction of clay (wt d%)

E

Young’s modulus

σ_max

maximum compressive strength

ε_f

strain at failure

F _max

maximum load

The authors confirm that the data supporting the findings of this study are available within the article.

The authors declare that no specific funding was received for this research or for the preparation of this manuscript.

The authors declare no competing financial interest.