Impact of alkaline activator concentration on mechanical properties and microstructure of a ternary blended one-part geopolymer cement

Abstract

Geopolymers offer a sustainable alternative to Portland cement (PC), which significantly contributes to global greenhouse gas emissions. One-part geopolymers (GP), which are synthesized by mixing solid precursors and dry activators with water on-site, present a promising alternative to conventional cement. This study investigates the impact of alkaline activator dosage (6–16%) on the mechanical properties and microstructure of a ternary blended one-part geopolymer cement, incorporating diatomite, feldspar, and ground granulated blast-furnace slag (GGBS) as raw materials. The materials were first evaluated for pozzolanic reactivity through strength activity index, lime saturation, and Frattini tests. Results revealed that activator dosage significantly influenced geopolymer performance. While all mixes exhibited minimal workability, the mechanical properties improved up to an optimal activator dosage. The mix with 10% activator and a sodium silicate (SS) to sodium hydroxide (NH) ratio of 1.5 demonstrated the highest compressive strength (46 MPa), split tensile strength (4.69 MPa), and flexural strength (7.448 MPa) after 28 days of curing. Microstructural analysis showed a dense, well-formed structure at the optimum mix, while lower activator dosages led to a less compact structure, and higher dosages caused brittleness and cracking. This study highlights the importance of optimizing activator dosage for enhanced geopolymer performance.

Introduction

Cement is becoming more prevalent in global construction, serving as an essential building material. The production of this material poses a major threat to the environment due to its enormous carbon dioxide emissions into the atmosphere. Approximately 5% of global CO₂ emissions are attributable to the cement sector¹. This occurs as a result of two variables in the production process. The principal reason is the conversion of limestone into clinker. Moreover, combusting the raw materials with coal leads to the emission of greenhouse gases². These emissions ultimately intensify global warming, a considerable threat to the biosphere. Besides its effect on climate change, the cement mill releases pollutants such sulphur dioxide and nitrogen oxides, resulting in deteriorating air quality. These contaminants can induce respiratory issues in persons. Nitrogen oxide also plays a key role in the formation of acid rain and smog³. Cement production is anticipated to rise in the next years because of the increasing need for infrastructure projects⁴. This poses a major challenge to achieving a sustainable future. The ecological impacts of cement manufacturing need the advancement and implementation of more sustainable methods and alternative materials.

Geopolymer is developing as a feasible substitute for PC owing to its detrimental effects. It provides a diminished carbon footprint with around an 80% decrease in CO₂ emissions^5,6. Geopolymers offer greater strength and durability when compared to PC, which is necessary for the increased lifespan of buildings. Certain chemicals such as sodium hydroxide, potassium hydroxide, sodium silicate, and potassium silicate activate the geopolymerization process^7–10. The activator elevates the pH of the system and induces the dissolution of silica and alumina from the precursors. Subsequently, a Si–O–Al chain is created through polycondensation¹¹. The network structure of a geopolymer is the defining characteristic that contributes to its strength and endurance. The geopolymers can be cured at either room temperature or higher temperatures, based on the desired characteristics and the speed of the chemical reactions. Despite the favorable performance of geopolymers, they nevertheless possess certain drawbacks such as handling of hazardous chemicals. To mitigate this, researchers are currently placing greater emphasis on one-part geopolymers which employ pre-mixed solid activators, allowing for easier on-site preparation similar to PC¹². This eliminates the necessity of managing risky liquid activators, hence boosting the safety of workers on building sites. Nevertheless, two-part methods, which typically utilize liquid alkaline activators, have demonstrated superior potential compressive strengths (CS). Recent study indicates that the difference in strength between one-part and two-part geopolymers is becoming less^13,14.

The strength of GPs is mostly figured out by the composition of the raw ingredients employed. Fly ash, Ground Granulated Blast Furnace Slag (GGBS), metakaolin, red mud, laterite, dolomite, etc. are some of them^15–19. The raw materials must possess a high concentration of reactive silica and alumina. Recently, diatomite, a naturally formed sedimentary rock abundant in silicon dioxide, has gained significant appeal due to its distinctive features and various applications^20–22. Some studies have explored the incorporation of diatomite in geopolymer systems. Diatomite is primarily composed of amorphous silica, often exceeding 80% by weight, and possesses an inherently porous, low-density microstructure that promotes the dissolution of silicate species under alkaline activation. This high surface area facilitates the formation of aluminosilicate gels critical to the geopolymerization process. Researchers have found that thermal treatment, particularly calcination between 500 and 700 °C, for a duration of 1–3 h significantly enhances the reactivity of diatomite, converting residual crystalline phases into a more amorphous and soluble form of silica, which improves its performance as a geopolymeric source material^23–25. Several investigations have demonstrated that diatomite can be used either as a primary precursor or a partial replacement for conventional materials such as fly ash or metakaolin. Ke et al. (2015) reported improved strain capacity and reduced density in red mud-based geopolymers modified with diatomite²⁶. Similarly, Ye et al. (2016) showed that the incorporation of diatomite enhances the structural flexibility of geopolymer binders²⁷. Although Phoo-Ngernkham et al. (2013) observed a decrease in compressive strength and Young’s modulus with increasing diatomite content, this trade-off was linked to improved ductility and thermal insulation properties, which are advantageous in lightweight construction applications²⁸. Furthermore, diatomite’s microstructural properties contribute to thermal resistance, fire insulation, and lightweight performance, making it suitable for environmentally friendly cement alternatives. Recent developments in one-part geopolymer technology have also considered diatomite as a dry precursor compatible with solid activator systems, thus facilitating safer and more practical field applications²⁹.

Feldspar is a group of tectosilicate minerals widely present in the Earth’s crust and typically composed of alumina (Al₂O₃) and silica (SiO₂) frameworks with alkali (Na, K) and alkaline earth (Ca) cations. Its chemical composition and crystalline structure make it a viable precursor in geopolymer systems. Recent studies have highlighted its latent geopolymeric potential when finely ground and subjected to alkaline conditions. A study by Nana et al. (2024) investigated feldspar-activated geopolymers and found that alkaline activation leads to reduced water absorption and enhanced mechanical strength, particularly flexural and compressive strength. The findings demonstrated that feldspar could participate effectively in aluminosilicate gel formation when combined with appropriate activators, making it suitable for structural applications³⁰. Aghabeyk et al. (2022) reported that feldspar-based geopolymers showed excellent performance in immobilizing heavy metal ions such as Cd²⁺, Co²⁺, and Zn²⁺. This functional property points to a broader applicability of feldspar geopolymers beyond traditional construction, including environmental remediation and waste stabilization³¹. Patel and Agrawal (2021) demonstrated that substituting 20% of fly ash with feldspar powder in geopolymer concrete resulted in strength values that met structural performance requirements³². This suggests feldspar can be used as a partial or supplementary precursor to reduce dependence on industrial waste materials like fly ash, which are subject to supply fluctuations. Kamseu et al. (2021) explored feldspathic solid solutions in geopolymer composites and highlighted their superior mechanical strength and dimensional stability. Their study confirmed that feldspar contributes to the formation of a denser, cohesive aluminosilicate matrix when activated under optimal conditions³³.

GGBS is a cementitious material that is produced as a by-product of iron and steel manufacture. It has been utilized for over a century as a supplemental cementing agent, mostly in concrete. GGBS possesses both cementitious and pozzolanic characteristics and is consisting of calcium silicate and calcium aluminosilicate, which have been melted. It can be effectively utilized as a binder in making geopolymers. George et al. (2022) discovered that geopolymer concrete based on GGBS demonstrated excellent CS, split tensile strength (STS), and FS, along with endurance in harsh settings³⁴. Patare et al. (2019) provided additional evidence showing the CS of geopolymer concrete was positively correlated with both the proportion of GGBS and the concentration of NH, up to a specific threshold³⁵. Mishra et al. (2024) conducted a thorough examination of the CS and microstructure characteristics of GP binder systems based on GGBS. The study emphasized the potential of these systems for sustainable construction purposes³⁶. Manimaran et al. (2015) also documented the exceptional CS of geopolymer concrete composed of GGBS and fly ash, indicating its appropriateness for use in structural applications³⁷. Saludung et al. (2018) and El-Hassan et al. (2018) discovered that the CS of geopolymer specimens rose as the GGBS concentration increased. Saludung specifically observed a noteworthy rise of up to 100 MPa^38,39.

Despite the promising attributes of these materials, there is currently no systematic study that explores their integration into a one-part geopolymer system, offering significant advantages in safety, handling, and field applicability over traditional two-part systems. The development of such a material would represent a breakthrough in low-carbon, high-performance construction technology, especially if it can simultaneously achieve high strength, low density, and improved thermal resistance. This study addresses this critical research gap by investigating, for the first time, a ternary blend of diatomite, feldspar, and GGBS within a one-part geopolymer framework. The work specifically focuses on optimizing the alkaline activator dosage and evaluating its influence on key mechanical properties, microstructural development, and functional performance characteristics. The main goal of this study is to develop a new type of sustainable one-part geopolymer cement that is structurally strong and makes effective use of underutilized natural and industrial materials. Given the current global push for eco-efficient construction materials, the successful development of such a binder would offer meaningful contributions to both scientific knowledge and industry practice.

Significance of work

The main objective of this study is to develop a one-part geopolymer cement using a combination of GGBS, diatomite, and feldspar. This specific combination of materials has not been previously investigated in a one-part geopolymer system. The selection of raw materials in this study was guided by their unique and complementary chemical compositions and functional roles in promoting efficient geopolymerization. These materials, when used together, form a synergistic blend that enhances both the reactivity and performance of the one-part geopolymer system, making the formulation not only structurally viable but also environmentally and functionally advantageous. GGBS was chosen as the primary binder due to its well-established behavior in alkali-activated systems. Its high calcium content and amorphous structure facilitate rapid dissolution and early strength gain through the formation of C–A–S–H gel. However, relying solely on GGBS can lead to issues such as flash setting at higher dosages, which necessitated the inclusion of additional materials to balance its reactivity. Diatomite, a lightweight, highly porous, and amorphous silica-rich material, was incorporated to address this. Upon calcination, diatomite becomes more reactive, contributing silica to the geopolymer network while also improving microstructural refinement and thermal properties. However, the low specific gravity of diatomite leads to increased water demand, which can negatively impact workability. Therefore, its content was restricted to 10% by weight of the binder to maintain acceptable mixing and flow characteristics. Feldspar, on the other hand, is introduced for its fluxing behavior and alkali contribution. Composed of SiO₂, Al₂O₃, and alkali oxides (Na₂O, K₂O, and CaO), feldspar supports the breakdown of aluminosilicate networks and enhances the dissolution of precursors under alkaline conditions, thereby improving gel formation and promoting geopolymerization even at ambient temperatures.

Materials

This study utilized GGBS, diatomite, and feldspar. GGBS was procured from JSW Cement, Calicut, Kerala; diatomite from Haritson Minitech Pvt Ltd, Jaipur, India; and feldspar from Shri Giriraj Mineral, Rajasthan, India. X-ray fluorescence spectroscopy (XRF) was employed to determine the oxide content of the materials, as illustrated in Fig. 1.

Fig. 1.

Oxide composition of raw materials; (a) GGBS, (b) Diatomite and (c) Feldspar.

Figure 2 depicts the major mineral composition and Fig. 3 shows the particle morphology. XRD reveals that GGBS is more amorphous compared to feldspar and diatomite. SEM image revealed that GGBS and feldspar consisted of sharp, angular particles, whereas diatomite comprised cellular particles. The specific surface areas of GGBS, diatomite, and feldspar were 428 m²/kg, 522 m²/kg, and 476 m²/kg, respectively, while their densities were 2.76, 2.02, and 2.6 g/cc, respectively. Diatomite was determined to be less dense than other raw materials. The solid activator was synthesized by combining sodium hydroxide (NH) with a purity of 98% and anhydrous sodium silicate (SS) in powdered form. Fine aggregate conforming to zone II was used for mortar preparation. The particle size distribution is displayed in Fig. 4 and Table 1 lists the properties of the fine aggregate.

Fig. 2.

XRD patterns of GGBS, Diatomite and Feldspar.

Fig. 3.

SEM image of; (a) GGBS, (b) Diatomite and (c) Feldspar.

Fig. 4.

Particle size distribution of fine aggregate.

Table 1.

Properties of fine aggregate.

Parameter	Result
Water absorption	0.7%
Fineness modulus	2.5
Specific gravity	2.56
Bulk density	1.7 kg/l

Mix proportion and method of mixing

Table 2 presents the component ratios of the geopolymer mortar. The ratio of GGBS, diatomite, and feldspar was established by a series of trials (70:10:20). Diatomite was calcined at 600 °C for 2 h to enhance its reactivity. The calcination temperature was identified through Thermo Gravimetric Analysis (TGA) and XRD. A water-to-binder ratio of 0.45 was maintained. Six distinct activator dosages (6%, 8%, 10%, 12%, 14%, and 16%) and two SS/NH ratios (1.5 and 2) were used. GGBS, diatomite, feldspar, NH, and SS were blended in a ball mill for 10 min. Water was subsequently incorporated and mixed for 5 min. After creating a homogeneous paste, Fine aggregate was incorporated and mixed for another 3 min to achieve uniformity. The Fig. 5 illustrates the mixing process. To maintain consistency, all specimens were stored in an oven at 35 °C until the testing day.

Table 2.

Mix proportion of mortar.

MIX ID	GGBS (kg/m3)	Diatomite (kg/m3)	Feldspar (kg/m3)	Sodium hydroxide (kg/m3)	Sodium silicate (kg/m3)	Sand (kg/m3)	Activator dosage (%)	SS/NH	w/b
M1	470.4	67.2	134.4	16.128	24.192	2016	6	1.5	0.45
M2	470.4	67.2	134.4	13.44	26.88	2016	6	2	0.45
M3	470.4	67.2	134.4	21.504	32.256	2016	8	1.5	0.45
M4	470.4	67.2	134.4	17.92	35.84	2016	8	2	0.45
M5	470.4	67.2	134.4	26.88	40.32	2016	10	1.5	0.45
M6	470.4	67.2	134.4	22.4	44.8	2016	10	2	0.45
M7	470.4	67.2	134.4	32.256	48.384	2016	12	1.5	0.45
M8	470.4	67.2	134.4	26.88	53.76	2016	12	2	0.45
M9	470.4	67.2	134.4	37.632	56.448	2016	14	1.5	0.45
M10	470.4	67.2	134.4	31.36	62.72	2016	14	2	0.45
M11	470.4	67.2	134.4	43	64.512	2016	16	1.5	0.45
M12	470.4	67.2	134.4	35.84	71.68	2016	16	2	0.45

Fig. 5.

Work flow diagram.

Test methods

Characterization of diatomite by using thermo gravimetric analysis (TGA) and X-ray diffraction analysis (XRD)

In order to perform TGA analysis on raw diatomite, controlled heating settings were used to continually record the mass loss in response to temperature fluctuations. By tracking weight variations brought on by the release of volatiles such carbonates, organic compounds, and water, TGA gives information on the thermal breakdown of diatomite. These components gradually vanish as the temperature rises during the calcination process, with discrete mass loss phases that correlate to certain thermal occurrences. The test was conducted by using HITACHI-STA7200 equipment. The sample was heated to 900° at a heating rate of 5° per second, and the corresponding mass loss was noticed. The main mineral compositions were then determined by XRD analysis of both calcined and non-calcined diatomite.

Test on pozzolanic property

Calcined diatomite (CD), feldspar and GGBS were individually assessed for its pozzolanicity by using different tests such as strength activity index, lime saturation, and Frattini test.

Strength activity index

This test was conducted as per ASTM C 311⁴⁰. A total of 500 g of Ordinary Portland Cement was combined with 2.75 times the amount of sand and 242 ml of water to create cubic mortar samples with 50 mm edges. The workability of the control mix was assessed based on flow values according to ASTM C1437⁴¹. A blend including pozzolana was created by substituting 20% of OPC with pozzolana. The amount of water to be added was determined by conducting a flow table test to achieve a control flow value within a 5% tolerance, as specified in the recommendations. Following a 24-h period, the cubes were demolded and placed in lime water to undergo curing. The CS of both the control specimens and the specimens incorporating pozzolana was assessed on the 7th and 28th days.

Lime saturation

The lime solution for this experiment was created by combining 2 g of lime with 1 g of distilled water. A 75 ml volume of a lime solution that was completely filled with calcium hydroxide was measured, and then 1 g of pozzolana was added to it. The container was tightly sealed and placed in the oven, where it was maintained at a temperature of 40 °C for a period of 7 days. Following a 7-day period, the sample was filtered, and the resulting liquid was analyzed using titration to determine the amount of hydroxide and calcium ions present.

Frattini

The Frattini test is a chemical technique employed to measure the pozzolanic potential of a material. Pozzolans, when exposed to moisture, chemically interact with calcium hydroxide (lime) produced during the hydration of PC, resulting in the formation of fresh cementitious compounds. The test was conducted as per BSEN 196-5⁴². A mixture of 2 g of pozzolana and 18 g of OPC was dissolved in 100 ml of water. The solution was stored in tightly sealed polyethylene bottles and maintained at 40 °C for a period of 8 days. After 8 days, the substance underwent vacuum filtration utilizing a sealed Buchner funnel and a filter paper with a nominal pore size of 2.7 µm. The liquid that passed through the filter was left to cool to the temperature of the surrounding room. A volume of 50 ml of the filtrate was selected and subjected to titration using 0.1 mol/l HCl. This was done by introducing a methyl orange indicator to ascertain the concentration of OH⁻ ions. The endpoint was the transition from the color yellow to orange. To ascertain the concentration of Ca⁺ ions, the pH of the titrated sample was modified to 12.5 by employing a solution of NH. The Patton and Reeder indicator was introduced and subjected to titration using a 0.03 mol/l EDTA solution. The endpoint was determined as the transition from a pink color to the color blue. The measured concentration of Ca⁺ and OH⁻ was compared to a standard value to analyze the extent of lime consumption by the pozzolan. A higher lime concentration signifies lesser pozzolanic activity, as the substance has undergone chemical bonding with lime to create new, stable molecules.

Test on cement

The density of the cement was assessed in accordance with IS 4031 (Part 11)⁴³. Fineness was determined using Blaine’s air permeability method in compliance with IS 4031 (part 2)⁴⁴. Soundness was evaluated according to IS 4031 (Part 3)⁴⁵. Initial and final setting times were determined according to IS 4031 (Part 5)⁴⁶.

Test on mortar

Flow table test was employed to assess the workability of the mortar (ASTM C 1437)⁴⁷. Mechanical properties were evaluated through compressive strength (ASTM C 109)⁴⁸, flexural strength (ASTM C 348)⁴⁹, and split tensile strength tests (ASTM C 496)⁵⁰. All strength measurements were conducted on the 3rd, 7th, and 28th days. The CS was determined using cube specimens of 50 × 50 × 50 mm. Prisms measuring 40 × 40 × 160 mm were utilized to evaluate FS. Cylindrical specimens of 100 mm in diameter and 200 mm in length were used for the measurement of STS. The measurement of ultrasonic pulse velocity was conducted in accordance with ASTM C 597⁵¹.

Microstructural characterization

The mineral phases of the geopolymer mortar were characterized using XRD. A Bruker Kappa Apex II X-ray diffractometer was employed to get the XRD patterns. The patterns were generated via Ni-filtered copper radiation at a current of 40 mA and a voltage of 45 kV. The scanning range spanned from 10° to 90°. Fourier transform infrared spectroscopy (FTIR) was conducted using a PerkinElmer Frontier spectrometer to qualitatively assess the functional groups throughout the range of 400–4000 cm⁻¹ at a resolution of 4 cm⁻¹. Scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) were utilized to assess the morphology of the geopolymer mortar. The ZEISS Sigma scanning electron microscope was utilized to analyze the microstructure.

Results and discussions

TGA and XRD

The thermal behavior of the raw diatomite was examined using TGA, as shown in Fig. 6. The TGA curve reveals a series of mass losses up to 600 °C, which are associated with the progressive release of water and structural transformations within the silica network. An initial weight loss of approximately 3.93% below 200 °C is attributed to the evaporation of physically adsorbed moisture. This is followed by a 2.28% loss between 200 and 400 °C, which corresponds to the dehydroxylation of silanol (Si–OH) groups⁵². A further 2.17% weight reduction between 400 and 600 °C is linked to the breakdown and structural reorganization of the opaline silica matrix. Above 600 °C, the material exhibits minimal mass change (< 1%), indicating that most of the volatile and hydroxyl-containing species have been eliminated⁵³. This plateau in the TGA curve suggests that further thermal treatment does not induce significant additional decomposition or mass loss.

Fig. 6.

TGA curve for diatomite.

Figure 7 represents the XRD diagram for non-calcined diatomite and diatomite calcined at 400 °C, 600 °C and 800 °C for 2 h. Q stands for quartz, and C for cristobalite. Significant variations in the crystalline structures of calcined and non-calcined diatomite are revealed by XRD analysis, which reflects the effect of heat exposure on the mineral composition of the material. Although diatomite is primarily amorphous, the raw material used in this study exhibited minor crystalline components, particularly quartz and cristobalite, as identified by XRD. Upon calcination at 600 °C, the intensity of these crystalline peaks decreased, indicating increased disorder and the formation of more amorphous silica, which is favorable for alkali activation in geopolymer synthesis. At 800 °C, the XRD pattern exhibits sharper and more intense peaks corresponding to cristobalite and quartz, suggesting recrystallization of previously amorphous silica. This transformation reduces the availability of reactive silicate species for geopolymer gel formation, as crystalline silica has limited solubility in highly alkaline environments. These findings align with literature reports on the thermal behavior of diatomaceous silica^21,26.

Fig. 7.

XRD of calcined and non-calcined diatomite.

Strength activity index (SAI)

Figure 8 depicts the CS obtained for CD, feldspar and GGBS in contrast to the control mix on the 7th and 28th days. In order to be classified as pozzolanic, a material must demonstrate a CS of at least 75% of the control specimens when evaluated using the SAI test. The test results indicate that CD, feldspar, and GGBS achieved strength activity indices of 41.7%, 74.27%, and 92% on the 7th day, and 43%, 78.5%, and 94% on the 28th day, respectively. The results demonstrated an enhancement in CS from the 7th day to the 28th day, attributed to the additional pozzolanic reaction happening during this interval. However, CD failed to get the necessary strength even after 28 days. The strength measured for diatomite was significantly lower to the required value for classification as pozzolanic according to the guidelines in ASTM C 618.

Fig. 8.

Compressive strength of mortar cubes in strength activity index test.

Frattini

This test assesses the capacity of the material to generate more calcium silicate hydrate, which is a crucial element that enhances the strength and durability of cementitious mixtures. In the Frattini test, the standard saturation curve represents the equilibrium concentration of calcium and hydroxide ions in the presence of excess Ca(OH)₂. If a test material is pozzolanic, it will react with Ca(OH)₂ and consume both Ca²⁺ and OH⁻ ions. This leads to a decrease in their concentrations, causing the data point to fall below the saturation curve. Thus, when the values for CaO and OH⁻ lie below the curve, it confirms that the material is pozzolanic, as it actively participates in hydration reactions to form cementitious gels such as C–S–H or C–A–S–H.

The amounts of CaO and OH⁻ in each sample were determined using titration and are indicated in the Fig. 9. The values found for all the samples fall markedly below the saturation curve, indicating the pozzolanic character of the materials. The CaO concentrations were 5.8 mmol/l for CD, 12.71 mmol/l for feldspar, and 4.32 mmol/l for GGBS. The hydroxide ion concentration for CD, feldspar, and GGBS was 1.006 mmol/l, 12.71 mmol/l, and 1.8 mmol/l, respectively. A drop in the concentration of CaO and OH⁻ ions within the solution indicates that these ions have been extensively used up due to their reaction with the pozzolanic material. The drop-in ion concentration suggests that the pozzolanic material has successfully undergone a reaction with calcium hydroxide to produce more calcium silicate hydrate, which enhances the cement-like characteristics of the mixture.

Fig. 9.

Results of Frattini test.

Lime saturation

The lime saturation test is vital for assessing the pozzolanicity of materials, a key factor in determining their efficacy in improving the strength and long-term performance of concrete. The test evaluates the reaction between a material and lime when exposed to water to see if it has the required qualities to enhance cement reactions and enhance the performance of concrete. One of the primary benefits of this approach is the integration of pozzolanic material directly into the fully saturated lime solution. As a result, it is possible to achieve full reactivity of pozzolanic material. Figure 10 illustrates the results of the lime saturation test. Along with the test results, the diagram also contains the standard curve, which is used as a reference to measure pozzolanicity. A material is said to be pozzolanic if the concentration of calcium ions and hydroxide ions falls below the standard curve. The calcium ion concentration of CD, feldspar, and GGBS was 0.226, 2.938, and 0.1 mmol/l, respectively. The hydroxide ion concentration in CD was 3.22 mmol/l, feldspar was 24.25 mmol/l, and GGBS was 1.5 mmol/l.

Fig. 10.

lime saturation test result.

Discussion on pozzolanic tests

In the SAI test, only GGBS and feldspar exhibited acceptable strength performance in accordance with ASTM C311, which specifies a minimum of 75% of the control compressive strength to confirm pozzolanicity. While feldspar achieved 74.27% strength at 7 days which is slightly below the required threshold, it surpassed the benchmark at 28 days, confirming its progressive reactivity and latent pozzolanic nature. On the other hand, CD exhibited compressive strength values of 41.7% and 43% at 7 and 28 days, respectively, and did not meet the minimum requirement. However, it is important to note that the water demand for the mixtures varied significantly due to differences in specific gravity and powder volume. Specifically, CD, having a low specific gravity, required 360 ml of water compared to 242 ml used for the control to achieve similar flow properties. The water content in the control mixture was established following the code ASTM C 311 recommended utilizing 242 ml of water for the control mixture, whereas the water requirement for the pozzolana mixture was determined by flow table test, ensuring that the flow of the pozzolana mixture remained within 5% of that of the control mixture. The higher water-to-solid ratio in the CD mixture likely contributed to reduced compressive strength. This outcome illustrates a limitation of the SAI test in evaluating materials like diatomite, where water demand significantly influences strength outcomes and may underrepresent their reactivity in real-world geopolymer conditions.

In contrast, the Frattini test provided more direct insight into the pozzolanic behavior of the materials by evaluating their ability to consume calcium hydroxide in solution. All three materials exhibited data points below the saturation curve, confirming their capacity to react with Ca(OH)₂ and form secondary calcium silicate hydrate (C–S–H) or calcium aluminosilicate hydrate (C–A–S–H). For feldspar, while the CaO concentration slightly exceeded the hydroxide ion concentration, both values remained below the standard solubility curve, indicating that the material had engaged in pozzolanic reactions. The marginally elevated CaO concentration may be attributed to residual unreacted lime or slow-reacting mineral phases, rather than a lack of pozzolanic activity. This reinforces feldspar’s viability as a complementary precursor, especially in blended systems where long-term reactivity is desirable.

The lime saturation test further substantiated the pozzolanic potential of the raw materials. All three precursors resulted in Ca²⁺ and OH⁻ concentrations well below the saturation curve, indicating their ability to consume lime in a saturated solution. However, feldspar exhibited a relatively high hydroxide ion concentration (24.25 mmol/L) compared to CD (3.22 mmol/L) and GGBS (1.5 mmol/L). This behavior is not indicative of poor performance but rather reflects feldspar’s intrinsic mineralogy. Feldspar contains alkali-bearing aluminosilicate phases, particularly Na₂O and K₂O, which can release alkali ions into the solution upon partial dissolution. These ions elevate the solution’s pH, contributing to the higher observed OH⁻ concentration. Despite this, the simultaneous reduction in Ca²⁺ concentration demonstrates that feldspar still interacts effectively with calcium hydroxide under alkaline conditions. Moreover, the high pH created by feldspar dissolution is beneficial in geopolymer systems, as it promotes the activation of other aluminosilicate precursors, thereby supporting synergistic reactivity within blended formulations. It is also important to emphasize that the methodological difference between the Frattini and lime saturation tests plays a role in these observations. In the Frattini test, the availability of Ca(OH)₂ depends on the hydration of ordinary Portland cement, whereas in the lime saturation test, the pozzolanic material is directly exposed to a fully saturated lime solution, allowing for better contact and more comprehensive reaction with available Ca²⁺ ions.

Density and fineness

The cement density was determined to be 2.84 and the fineness as determined from Blaine’s air permeability method was 3350 cm²/g. According to IS 1489 (part 1): 1991, the fineness of pozzolanic cement should not be less than 3000 cm²/g⁵⁴. A fineness of 3350 cm²/g signifies a comparatively higher surface area per unit mass of the material. The increased surface area can enhance its hydration rate when combined with water. Typically, increased fineness correlates with accelerated setting time and potentially enhanced early-age strength development in concrete. It also leads to increased water demand.

Soundness

Soundness denotes the potential of a material to sustain its integrity and shape without substantial cracking, expansion, or disintegration when subjected to environmental factors such as moisture, temperature fluctuations, or chemical exposure. Unsound cement may expand or contract post-hardening, potentially resulting in structural fissures. The test was performed in accordance with IS 4031 part 3. The test showed that the expansion of the Le Chatelier apparatus was under 10 mm for all mixtures and the results are tabulated in Table 3. All the mixes showed an expansion within the range of 1–2 mm. Neither the activator dosage nor the SS/ NH ratio was found to be having any effect on the soundness values. The M2 mixture demonstrated slightly enhanced expansion. The reduced activator dosage may have caused partial geopolymerization, leading to an amorphous or inadequately bound structure. This structure may demonstrate diminished integrity due to greater susceptibility to moisture absorption, thermal expansion, or chemical degradation. Conversely, an excessive dosage of activator may precipitate an excessively rapid setting process, resulting in internal stress that might cause shrinkage, cracking, or undesired volumetric changes, thereby weakening the integrity of the material.

Table 3.

Setting time and soundness.

Mix ID	Initial setting time (IST) (min.)	Final setting time (FST) (min.)	Soundness (mm)
M1	260	560	1
M2	230	552	2
M3	215	528	1
M4	168	515	2
M5	130	325	1
M6	108	313	1
M7	96	278	1
M8	89	246	1
M9	88	184	1
M10	83	172	1
M11	75	165	1
M12	68	150	2

Setting time

Table 3 presents the initial setting time (IST) and final setting time (FST) of the one-part GP paste prepared with varying activator dosages. The findings indicate that the setting time of the cement is greatly impacted by the amount of activator used. The IST for all mixes ranged from 68 to 260 min and FST varied from 150 to 560 min. The table shows that both IST and FST diminish as the activator dosage increases. With a rise of activator dosage from 6 to 16%, IST decreased from 260 to 68 min, while FST reduced from 560 min 150 min. This results from the improved dissolution and accelerated polymerization caused by enhanced alkalinity in the system⁵⁵. Upon increasing the activator dosage to 16%, the initial setting occurred within 68 min. An excessive amount of activator induces a very exothermic reaction, resulting in accelerated setting. As the ratio of SS/ NH increased from 1.5 to 2, the setting time of all mixtures reduced. A higher SS/NH ratio typically enhances the availability of silicates in the system, resulting in enhanced gelation and accelerated setting⁵⁶. A decreased SS/NH ratio increases the availability of OH⁻ ions, facilitating faster dissolution; nevertheless, it may impede network development due to diminished silicate ion availability⁵⁷.

Workability

The workability of one-part geopolymers is generally affected by the water-to-binder ratio, the chemical composition of precursor materials, and the inclusion of additives or activators. Figure 11 represents the flow values obtained for all the mixes. All mixes were made with a w/c ratio of 0.45. Despite the addition of sufficient water, all mixes showed reduced workability. Mix M12 exhibited the greatest workability of 33%. Superplasticizer was not used for making the mixes. M1 and M2 mixes were relatively stiff, which resulted in a flow of only around 20%. With the increase in activator dosage from 6 to 8%, the workability of the mixtures M3 and M4 improved to 24% and 22%, respectively. For M3 and M4, workability declined as the concentration of SS increased. Raising the SS/ NH ratio from 1.5 to 2 led to an approximate 9% decrease in workability. The decline in workability observed in geopolymer mixtures with increased SS content can be ascribed to multiple connected reasons. SS functions as a crucial activator in geopolymer systems, facilitating the development of alumino-silicate gels. With an increase in SS content, the viscosity of the mixture spikes due to the elevated concentration of the gel phase, which hinders the mixture’s flowability⁵⁸. The increased viscosity renders the combination more resistant to deformation and diminishes its workability. The elevated SS percentage raises the alkali concentration, resulting in an accelerated gelation rate and a diminished working time, hence impairing the handling of the mixture. Moreover, the augmented network creation among particles intensifies particle–particle interactions, resulting in a stiffer and less mobile mixture⁵⁹.

Fig. 11.

Workability of geopolymer mixes.

However, for mixes M11 and M12, an increase of SS content enhanced workability from 31 to 33%. Similar trend was reported by Tuyan et al. (2028), that the elevated alkali concentration in the GP mixes resulted in increased viscosity, however a greater flow diameter was noted. This resulted from the elevated SS concentration in mixes with a higher Na₂O %⁶⁰.

Precursor elements, along with the activator, greatly affect workability. The surface texture and particle fineness significantly influence the flowability of the mixture. Numerous investigations have shown that the incorporation of diatomite substantially diminishes the workability of geopolymers^61,62. This decrease in workability is attributed to several reasons. Diatomite is a lightweight, extremely porous substance with low density, resulting in a significant surface area and enhanced particle roughness in the geopolymer combination. The enlarged surface area necessitates additional binder to coat the diatomite particles, resulting in a more rigid mixture⁶³. Moreover, the porous characteristics of diatomite may retain some water and activators, diminishing the effective quantity available for geopolymerization and elevating the mixture’s viscosity.

Compressive strength (CS)

The compressive strength of geopolymers is a vital factor in assessing their appropriateness for structural applications, as it directly affects their load-bearing capability. Elevated CS improves the durability and lifespan of geopolymer-based materials, rendering them a viable alternative to conventional cementitious products. Figure 12 illustrates the CS of the geopolymer mortar specimens on the 3rd, 7th and 28th days. Each value is obtained by computing the mean strength of three specimens. The CS of the specimens increased as the activator dosage increased from 6 to 12%. The M1 and M2 mixtures containing 6% activator achieved 28-day CS of 28 MPa and 29 MPa, respectively. The variation in strength between these two mixes was minimal. M3 and M4, produced with an 8% activator dosage, achieved compressive strengths of 35 MPa and 34.8 MPa, accordingly. The combination of M5 and M6 with 10% activator resulted in a 28-day CS of 46 MPa and 44.88 MPa, correspondingly. Similar strength values were obtained for mixes with activator dosages of 10, and 12%. The strength of each mix was similar, with M5, which has an SS/NH ratio of 1.5 and 10% activator, having the maximum strength. In comparison to M5, there was a noticeable drop in strength when the activator dosage rose to 16%. The 28-day strengths for M11 and M12 were 38 MPa and 30 MPa, respectively.

Fig. 12.

Compressive strength result of one-part geopolymer.

The improvement of CS with elevated activator dosage is ascribed to the improved dissolution of aluminosilicate constituents and the resultant development of a broader and more resilient polymeric network⁶⁴. As more activator is added, the concentration of hydroxide ions grows, enhancing the decomposition of precursor materials and aiding in the development of more robust geopolymer gel formations. This results in a more compact microstructure, hence improving the load-bearing capacity of the material. Upon increasing the activator dosage to 16%, a marginal decrease in strength was observed. Elevated activator dosages result in an increased concentration of OH⁻ ions, hence creating a very alkaline environment. The surplus alkalinity may result in the accelerated dissolution of silica and alumina from the precursor materials, causing an uneven polymerization process, leading to the formation of a brittle and cracked microstructure. A reduction in strength was noted on the 7th day for M1 and M3 in comparison to the 3rd day. Since all other mixes exhibited a significant increase in strength over the same period, this decline suggests potential issues with the curing process or mix proportions for M1 and M3. Additional examination is needed to evaluate the exact causes of this decrease and to adjust the formulations accordingly.

The SS/ NH ratio for mixes M1, M3, M5, M7, M9 and M11 was 1.5, while the ratio for mixes M2, M4, M6, M8, M10 and M12 was 2. The graph clearly indicates that as the SS to NH ratio increases from 1.5 to 2, there is a decline in strength. This tendency indicates that lower SS/NH ratios result in a greater availability of OH⁻ ions in the system, hence increasing its alkalinity. The elevated alkalinity leads to enhanced dissolving of silica and alumina, facilitating the fast production of the polymeric chain⁶⁵. The creation of a denser and more stable gel structure develops, directly enhancing the strength of the material. Conversely, a reduction in NH dosage minimizes the alkalinity of the matrix.

Flexural strength (FS) and split tensile strength (STS)

Table 4 shows the FS and STS of the geopolymer specimens on the 3rd, 7th and 28th days. The FS of geopolymer materials is a vital characteristic that directly affects their efficacy in structural applications. This feature is crucial for assessing the resistance of the material to bending forces and deformation under load, serving as a significant indicator of its durability and compatibility for building applications. STS denotes the capacity of the material to resist cracking and failure under indirect tensile stress, as encountered in beams, slabs, or pavements. The FS on the 28th day ranged from 3.16 MPa to 7.448 MPa, whereas the STS varied between 2.66 and 4.69 MPa. Both the outcomes exhibited a similar pattern to the CS. The strength values substantially improved with an increase in activator dosage up to 10%, followed by a decline at higher dosages. A surplus of alkalis may hinder the polymerization process, resulting in lesser strength. High concentrations of alkalis led to minor efflorescence on the surface of the specimens, likely resulting from the reaction between surplus NH and atmospheric CO₂, producing carbonates. The mix with a 10% activator and a SS/ NH ratio of 1.5 achieved the maximum strength both in flexural and split tensile test. For most of the mixes, both flexural and STS diminished when the SS/ NH ratio increased from 1.5 to 2.

Table 4.

Flexural and split tensile strength result.

MIX	Flexural strength (MPa)			Split tensile strength (MPa)
	3rd day	7th day	28th day	3rd day	7th day	28th day
M1	2.98	3.12	3.16	2.1	2.3	2.66
M2	2.67	2.88	3.23	2.08	2.39	2.52
M3	3.15	3.64	5.03	3.21	3.26	3.74
M4	3.16	4.5	5.19	3.18	3.31	4.32
M5	3.42	6.09	7.448	4.13	4.22	4.69
M6	4.21	5.29	7.22	4.2	4.39	4.60
M7	3.64	5.7	6.68	4.13	4.41	4.49
M8	3.62	4.69	5.89	4.2	4.22	4.38
M9	3.16	4.64	5.86	3.5	4.08	4.19
M10	3.42	4.5	5.5	3.2	3.81	3.85
M11	3.13	3.23	4.26	3.1	3.19	3.36
M12	2.9	3.2	4.24	3.08	3.18	3.22

Ultrasonic pulse velocity (UPV)

Figure 13 illustrates the fluctuation in ultrasonic pulse velocity values of geopolymer specimens produced with varying activator dosages. The UPV increased with the increase in activator dosage. However, when the activator dosage exceeded 14%, a slight reduction was noticed. The M5 and M6 mixes with 10% activator achieved maximum velocities of 3997 m/s and 3988 m/s, respectively. The increase in UPV readings with higher activator dosage indicates improved densification and less porosity in the GP matrix, hence improving the propagation speed of ultrasonic waves through the material⁶⁶. Conversely, minimal activator dosages may cause inadequate geopolymerization, resulting in diminished UPV values due to increased porosity and weakened cohesion within the GP matrix. SEM images also confirm the formation of a less dense and less cohesive microstructure with gaps in between the particles for mixes with a very low activator dosage. The velocity with a 16% activator dosage was 3471 m/s, which was lower than that at 14%. This may result from excessive acceleration of the geopolymerization reaction, which leads to defects such as internal voids, micro-cracks, or inadequate bonding between the silica and alumina phases. These structural inhomogeneities restrict the efficient transmission of ultrasonic pulses, hence reducing UPV⁶⁷. The SEM image further confirms the development of a cracked microstructure at a 16% activator dosage.

Fig. 13.

Ultrasonic pulse velocity of geopolymer mortar.

The UPV value declined as the SS/ NH ratio increased from 1.5 to 2. An increased SS/SH ratio leads to a reduction in NH concentration relative to SS, which slows down the geopolymerization, and the activation of the alumino-silicate source material becomes less efficient. Consequently, the ultrasonic pulse experiences more attenuation and reduced propagation velocity within the material, leading to a notable fall in the UPV.

X-ray diffraction analysis (XRD)

XRD analysis of geopolymer revealed distinct mineralogical transformations as the activator dosage was adjusted. The diagram is depicted in Fig. 14. At lower activator dosages, the XRD patterns exhibited prominent peaks corresponding to unreacted phases, including quartz and cristobalite, indicating incomplete activation. As the activator dosage increased to 10% and 12%, a reduction in the crystalline phase intensity was evident, associated with the increased dissolution of raw materials and the development of an amorphous aluminosilicate gel which is the characteristic of the GP network. Additionally, the formation of calcium silicate hydrate (C–S–H) phases, indicative of the interaction between GGBS and the activators, was observed in the range of 25–30°⁶⁸. All mixtures exhibited some peaks within the 2θ range of 20–40°. It is commonly attributed to the amorphous aluminosilicate gel phase. The specific position of this peak reflects the average spacing of the silicon–oxygen (Si–O) and aluminum–oxygen (Al–O) bonds in the GP gel, which lacks the long-range order necessary for distinct, sharp diffraction peaks typically associated with crystalline phases⁶⁹. With the increase in alkaline content to 14% and 16%, the intensity of peaks in the XRD diagram increased, indicating the production of further crystalline phases. The elevated activator dosage accelerates the polymerization rate of the geopolymer matrix, thus promoting the development of crystalline byproducts, thereby enhancing peak intensity. As a result, the XRD pattern shifts from being dominated by broad, amorphous peaks to sharper, more intense peaks corresponding to these crystalline phases. As the proportion of crystalline phases increased, the structure became brittle.

Fig. 14.

XRD profile of one-part geopolymer.

Fourier transform infrared spectroscopy (FTIR)

FTIR spectroscopy is essential for examining the microstructure of geopolymers by delivering comprehensive insights into their chemical composition, bonding, and molecular interactions. Figure 15 illustrates the FTIR spectra obtained for the mixture with varying activator dosages and Table 5 shows the corresponding bonds obtained for each peak. Narrow broad bands were seen within the range of 3473–3344 cm⁻¹, indicative of OH stretching vibrations⁷⁰. Peaks occurring between 1655 and 1642 cm⁻¹ correspond to OH bending vibrations. These vibrations yield significant insights about the presence and characteristics of hydroxyl groups and water molecules within the material. The OH stretching band signifies the existence of residual water, perhaps as free water in the pores or bound water inside the polymeric matrix. OH bending vibration corresponds to water molecules involved in hydrogen bonding. The low-intensity OH stretching and bending vibrations generally signify a minimal presence of water in the material. This signifies a substantial degree of polymerization at all proportions of activator, wherein the majority of the water has been eliminated or integrated into the network in a non-labile state, potentially enhancing the strength and stability of the material⁷¹. The peak within the range of 1467 cm⁻¹ to 954 cm⁻¹ indicates the asymmetrical stretching vibration of the Si–O/Al–O bond⁶⁴. The peaks between 875 and 871 cm⁻¹ are indicative of the stretching vibration of the Si–O–Si link in quartz. These bands provide critical information regarding the aluminosilicate network and the interactions between silicon and aluminium atoms within the material⁷². The Si–O–Si stretching vibration in the geopolymer indicates the formation of a three-dimensional silicate framework structure. The Si–O/Al–O band indicates the incorporation of aluminium into the silicate framework and it also denotes the creation of amorphous aluminosilicate gel in binary systems⁷³. Aluminium plays a crucial role in the creation of the geopolymer network. The peaks between 585 and 530 cm⁻¹ correspond to the bending vibrations of the Si–O–T bonds within the aluminosilicate network, where T can be silica or alumina. These lower wavenumber bands often represent the symmetrical bending modes of oxygen atoms that connect silicon and aluminium atoms. The peak spanning from 466 to 431 cm⁻¹ was attributed to the O–Si–O bending vibrations of SiO₄ tetrahedra⁶⁴.

Fig. 15.

FTIR spectra of one-part geopolymer.

Table 5.

FTIR characteristic bands identified.

MIX	OH stretching	OH bending	Si–O/ Al-O	Si–O/ Al–O	Si–O–Si stretching	Si–O-T	Si–O–T	O–Si–O bending
M1	3452	1655	1467	991	873	584	530	466
M3	3440	1647	1423	962	871	584	538	441
M5	3457	1642	1413	970	875	585	534	445
M7	3473	1645	1412	979	875	585	539	436
M9	3430	1650	1406	954	874	585	534	435
M11	3344	1646	1408	983	872	581	536	431

As the activator dosage increased from 6 to 16%, a shifting of bands to lower wavenumbers was noticed. This signifies the variation in polymerization with varying dosages of activators. Such transitions arise from the establishment of stronger covalent bonds and a denser configuration. As silicon atoms establish stronger and more solid connections with oxygen and aluminium atoms, the vibrational frequencies of these bonds decrease, resulting in the observed shift to lower wavenumbers⁷⁴. With the increase in activator dosage, there was a greater extent of dissolution and activation of the raw materials, facilitating improved cross-linking among the aluminosilicate units. The enhanced network connectivity results in the development of more robust and denser bonds. Moreover, excessive activator dosages can also result in the overactivation of the precursors, leading to the formation of a denser and more compact network, which further contributes to the reduced vibrational frequencies. But the excessive dosage of activator can make the structure more brittle, as evidenced from the SEM image and compressive strength results.

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)

Figure 16 illustrates the microstructure of the geopolymer as examined by SEM. Each specimen is formulated with a distinct dosage of activators such as 6%, 8%, 10%, 12%, 14% and 16% respectively. The SS/ NH ratio was 1.5. All images demonstrate that gel formation has occurred at each dosage of activator. However, a significant amount of unreacted particles is evident at reduced activator dosages. Figure 16a illustrates the geopolymer composed of 6% activator. The microstructure comprises a higher concentration of unreacted particles. This indicates that the 6% concentration of activator is insufficient to fully dissolve the precursor. The image depicts unevenly dispersed phases with a restricted development of a robust geopolymer network, characterised by the absence of interconnected gel-like structures. The presence of unreacted precursor particles is more evident, signifying incomplete breakdown and polymerisation. The reduced concentration of activators hinders the development of a stable Si–O–Al framework, resulting in the noted absence of a continuous and uniform matrix. The morphological characteristics correlate with the diminished mechanical strength of the material, as the decreased activator dosage leads to slower reaction kinetics and an inadequately built geopolymer network, ultimately compromising the structural integrity of the material. However, the structure exhibited significantly greater density when the activator concentration increased to 8%. Nevertheless, it indicated the existence of fissures and cavities (Fig. 16b). 8% activator dosage also resulted in the presence of some unreacted particles. At an activator dosage of 10%, a denser and more cohesive microstructure with less unreacted particles is observed (Fig. 16c). The microstructure was nearly same for both 10% and 12% activator doses. This aligns with the compressive strength results, which indicated approximately equivalent strength for mixes containing 10% and 12% activators.

Fig. 16.

SEM and EDS micrograph of geopolymer.

When the activator dosage exceeded 12%, geopolymer gel was formed, but the structure was more brittle, resulting in the formation of more cracks, which led to lower strength. The microcracks observed on the geopolymer surface may possibly result from an overabundance of OH⁻ ions. Excess hydroxide ions can accelerate the dissolving of the precursor material, resulting in a non-uniform and occasionally brittle microstructure.

The EDS spectra demonstrated notable alterations in the atomic distribution of essential elements like silicon (Si), aluminium (Al), calcium (Ca), sodium (Na) etc. with variations in the activator dosage (Fig. 16). Table 6 presents the chemical composition of the geopolymer as determined by EDS analysis. An elevated dosage of activator generally resulted in a higher sodium content, signifying more effective dissolution and activation of aluminosilicate precursors. Moreover, the Si/Al ratio was shown to decrease with increased activator dosages, indicating a more significant development of alumino-silicate networks. The concentration of calcium was affected by the activator dosage, indicating its significance in the geopolymerization process.

Table 6.

Chemical composition of geopolymer as determined from EDS analysis.

Mix	Si	Al	Na	K	Ca	O	Si/Al
M1	10.2	2.7	3.8	0.2	5.5	46.3	3.8
M3	14.9	3.5	5.7	0.6	6.8	54.5	4.3
M5	15	5.7	7.1	0.6	9.5	56.3	2.6
M7	15	4.8	8.3	0.4	9	52.8	3.1
M9	15	5.2	5.6	0.4	11.7	51.7	2.9
M11	16.2	5.5	4.9	0.4	11.4	51.7	3.0

Summary and future scope

This study conducted experimental work to examine the impact of activator dosage on the microstructure and strength development of a one-part ternary blended geopolymer. It also examined the feasibility of using the combination mentioned in this study as a one-part geopolymer cement by verifying if it complies with the standard specifications for pozzolanic cement. Based on the experiments, the following conclusions are drawn;

• Diatomite, feldspar, and GGBS demonstrated pozzolanic properties in lime saturation and Frattini tests. However, diatomite reached only 43% of the target 28-day compressive strength, primarily due to the extra water needed for workability.

• The cement met most PPC criteria, with a density of 2.84 g/cm³ and a fineness of 3350 cm²/g. All mixes showed an expansion between 1 and 2 mm in soundness tests.

• The setting time of one-part geopolymer cement was strongly influenced by activator dosage. IST ranged from 68 to 260 min, and FST varied from 150 to 560 min. Increased activator dosage and a higher SS/ NH ratio (1.5–2) both reduced the setting time.

• The workability of all mixes was determined to be exceedingly low across all activator dosages. Rheological investigations of these cements should be conducted to determine methods for enhancing workability. The optimal workability achieved was 33% for the mixture containing 16% activator.

• Compressive strength increased to 46 MPa with a 10% rise in activator dosage, but decreased thereafter due to a brittle microstructure. Additionally, strength reduced with a rise in the SS/NH ratio.

• Flexural and split tensile strengths exhibited a similar pattern to compressive strength, with maximum values of 7.448 MPa and 4.69 MPa, respectively, achieved at a 10% activator dosage and a SS/NH ratio of 1.5.

• The maximum ultrasonic pulse velocity of 3997 m/s was achieved, with higher UPV readings indicating improved densification and reduced porosity in the geopolymer matrix due to increased activator dosage.

• Microstructural analysis confirmed the results, with XRD showing more amorphous phases, FTIR indicating effective geopolymerization, SEM revealing a denser microstructure, and EDS confirming a stronger aluminosilicate network for the optimum mix.

• The one-part geopolymer cement was determined to be a viable substitute for ordinary Portland cement. However, further investigation is required to improve the workability of the blend. Furthermore, durability assessments of the cement must be conducted to facilitate its actual application on-site.

Author contributions

ESP : Conceptualization, Investigation, Methodology, Writing-original draft, PN: Supervision, review and editing, JSK: Supervision, review and editing, BST: Supervision, review and editing.

Data availability

Data will be available from the corresponding author upon reasonable request. Email id: poojalakshmies1@gmail.com.

Declarations

Competing interests

The authors declare no competing interests.