Effect of curing in an arid climate on the properties of metakaolin-based geopolymer mortarAbstract
In this research, medium crude metakaolinic clay (MK, Algeria) was used as raw material for the synthesis of geopolymers. Geopolymer synthesis involved mixing metakaolin with alkaline activators, with silica fume incorporated in some formulations to enhance mechanical properties. Samples were subjected to control curing at ambient and solar conditions, and their compressive and flexural strengths were evaluated at 28 days curing age, with a compressive strength of 35 MPa and flexural strength of 10 MPa. Additional tests assessed by XRD and FTIR revealed that new crystalline phases were formed in the geopolymer samples stored under ambient and sun exposure conditions; the curing condition had a significant effect on the change in the strengths of the investigation. The results indicate that the performance of geopolymer mortars can be considerably improved, even in extreme conditions, by optimizing curing protocols and material compositions.
Keywords:
Moderate-grade kaolin; thermal activity; alkaline activators; silica fume; solar curing
INTROCUCTION
Geopolymer binders have been gaining considerable interest in the construction industry as more people are becoming inclined towards eco-friendly materials 1. In addition to being environmentally friendly, geopolymer binders are known for their high durability. Compared to traditional cement-based concrete, geopolymers offer several significant advantages: they often achieve higher compressive and flexural strength, exceeding that of conventional concrete. Moreover, they exhibit exceptional resistance to chemicals, abrasion, and fire, making them well-suited for harsh environments and contributing to superior long-term durability 2), (3.
Curing is believed to be one of the most important aspects in the development of a geopolymer mortar 4. Nevertheless, curing becomes rather difficult in desert regions that are arid and dry, and hot. Summer high temperatures in arid regions impair concrete quality in the construction business. It requires expensive precautions during construction and delays building 5. Geopolymer manufacture may benefit from that heat. Generally, at ambient temperatures, geopolymer concrete develops poor early strength, but higher temperatures greatly increase strength, and this limits its use in cast-in-place industries 6.
B.Bouzidi et al used Tamazert kaolinic clay to produce potassium-based geopolymers 7. Geopolymers activated using combined potassium-sodium silicate solutions had higher compressive strength than those activated with potassium silicate alone, reaching 40 MPa and 16 MPa, respectively. A study examined using Algerian Beni Saf natural pozzolan to make geopolymers as a complete Portland cement alternative. At 28 days, the best formulation had 38 MPa compressive and 12 MPa flexural strengths, proving the natural pozzolan’s feasibility in geopolymer mortar manufacture 8. In another study, which used ceramic sanitaryware waste in the production of Portland cement mortar, experimental results indicated that WSP powder has the potential to be successfully recycled into mortars as a partial replacement for Portland cement by up to 15%. PCM materials modified with 5% and 10% WSP showed better performance than the reference mortar (R-PCM) at ambient and elevated temperatures. Furthermore, WSP recovery contributes to reducing environmental pollution and carbon dioxide emissions 9. An investigation into geopolymer cement derived from sediments of the Fergoug dam. Calcined and activated sediments utilize 8M sodium hydroxide. Optimal mechanical performance was attained with a 5-hour calcination, 24-hour curing at 60 °C, and curing at ambient temperature 10.
Recent studies have shown that replacing kaolin with up to 30% fly ash enhances the physical and mechanical properties of geopolymers at all curing temperatures 11. Likewise, dolomite- or slaked lime-based mortars activated with NaOH or KOH, and enhanced with additives like kaolin, micro-silica, or BaCO3, benefit from a filler effect that reduces porosity and increases density 12. While curing is essential to geopolymer development, achieving optimal conditions can be difficult in hot and arid environments, making the choice of materials and activators even more critical.
To determine the optimum thermal activation of kaolin clay, the process began with the characterization of the raw clay. Its elemental content was determined by chemical composition analysis using X-ray fluorescence (XRF), and the presence of kaolinite and illite was confirmed by X-ray diffraction (XRD). Thermal behavior studies, including thermogravimetric analysis (TGA) and differential thermal analysis (DTA), were carried out to determine the optimal dehydroxylation temperature. Based on these results, the kaolin clay was calcined at intervals of 1, 2, 3, and 4 hours to determine the most effective activation duration. XRD and FTIR confirmed the breakdown of hydroxyl bonds, which revealed the conversion of kaolinite to amorphous metakaolinite. The surface area was also measured using BET analysis, while FTIR confirmed the chemical compositions of metakaolin exceeded ASTM C-618 requirements.
To prepare and test geopolymers, metakaolin was mixed with alkaline activators such as sodium hydroxide, potassium hydroxide, or sodium silicate solution. In some samples, we Replaced sodium silicate with silica fume (SF) in geopolymer synthesis to obtain a Si-rich geopolymer matrix 13. Eight formulations of alkaline-activated geopolymer mixtures. With constant fine aggregates to binder of 2.75 and a water to binder ratio of 0.5 for all based geopolymer mixtures. One of the parts of the metakaolin geopolymer samples was stored in the same ambient temperature (E), while the other was placed outside to achieve sun resistance conditions (S). Finally, Mechanical strength tests of the specimens were conducted at 28 days of curing age.
As previously mentioned, the double conservation tests in two distinct climatic systems required ten mortar mixes. This quantity of samples indicates that there are numerous potential applications for metakaolin-based geopolymers and setting agents in order to achieve the desired objective of an alternative geopolymer mortar to cement mortar. The laboratory treatment methodology, the environment in which the samples were stored, and the treatment materials employed encompass a diverse array of findings that will be detailed in the subsequent sections. This work is one of the new research projects within the framework of using recently discovered materials available from local sources.
MATERIALS AND METHODS
The primary aluminosilicate source material used in this study to make MK-based geopolymer mortars is moderate-grade kaolin clay from Tabelbala, south of Algeria, as shown in Figure 1. The clay was crushed with a jaw crusher, ground with a disc mill, and sieved to a particle size of less than 20 μm.
Location of Tabelbala within Algeria and kaolin deposit + kaolin and metakaolin powder (29°24′N 3°15′W).
The clay sample is first dried at 105 °C in an oven for 24 hrs. After that, Thermogravimetric analysis (TG) and Differential thermal analysis (DTA) were carried out using Labsys Evo - gas option (TG-DSC 1600 °C). The platinum crucible was packed with 30 mg of powdered kaolin clay sample and was heated at a rate of 5°C/min from room temperature (RT) to 1000 °C.
The obtained powder was calcined in a muffle furnace at 900 °C for 1, 2, 3, and 4 hours, with a heating rate of 10°Cmin-1 to produce metakaolin (Sakhri, #15). To determine chemical and mineralogical compositions, raw and calcined clay samples were analyzed using X-ray fluorescence, Fourier transform infrared (FTIR) spectra, and X-ray diffraction (XRD). Moreover, BET-specific surfaces are determined.
The chemical composition of the starting materials, Kaolin, Metakaolin, and Silica Fume (SF), and Ordinary Portland Cement (OPC), is presented in Table I.
The alkaline activator solutions include Sigma Aldrich-purchased NaOH, KOH, and Na2SiO3. NaOH powder has 99% purity and a density of 2.13 g/cm3, while KOH pellets have 85% purity and 2.04 g/cm3. In contrast, the sodium silicate solution has 10.6% SiO2 and 26.5% Na2O, 1.39 g/cm3 density, and 11.30 pH. NaOH and KOH solutions of the desired molar concentrations were prepared and cooled at room temperature Figure 2.
Eight different mixtures of geopolymer mortar proportions were examined (ME(Na+SF), MS(Na+SF), ME(K+SF), MS(K+SF), ME(Na+SS), MS(Na+SS), ME(K+SS), and MS(K+SS), Table II. With a constant fine aggregate-to-binder ratio of 2.75 for MK-based geopolymer mixtures and a ratio of three for the OPC reference mortars. The water-to-binder ratio of all mixtures was 0.5. We replaced sodium silicate with silica fume (FS), obtained from GRANITEX (Algeria region), with a specific surface area >15 m2/gr and a high percentage of amorphous silica (SiO2 = 94.57%), in geopolymer synthesis in some samples to obtain a Si-rich geopolymer matrix. This can enhance the cross-linking density of the geopolymer network, thus improving its mechanical properties. All Mortar specimens were prepared using DIN-EN 196-1 standard sand as the fine aggregate.
One of the parts of the samples was stored in the same ambient temperature (E), while the other was placed outside to achieve sun resistance conditions (S).
We conducted mechanical strength tests on specimens cured at 28 days, adhering to EN 196-1. procedures for MK-based geopolymer mortar and OPC reference mortars. The mortar prisms (40 mm × 40 mm × 160 mm) were tested for flexural strength using the three-point bending method according to standard protocols. A compression testing machine measured the mortar’s compressive strength under a uniaxial load for a precise assessment of its load-bearing capacity. At least three tests were done.
The total silica modulus (SiO2/Na2O) and other molar ratios (H2O/Na2O and H2O/SiO2) resulting from the hydroxyl ions, silica monomers, and alkali solids present in the alkaline activators and added water. Table III summarizes the molar composition of all mixes.
MK: Metakaolin, SF: Silica fume, NH: Sodium Hydroxide, KH: Potassium Hydroxide, SS: Sodium silicate solution.
Additionally, residual fragments from the mortar compression tests were ground and sieved to a particle size <45 μm. These samples were then dried at 60°C for 24 h before being subjected to X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy analyses.
RESULTS AND DISCUSSION
The variations in weight of the Tabelbala clay sample during heat treatment between 25 and 1000 °C, as determined by TGA, are shown in Figure 3. Differential thermal analysis (DTA) is also obtained and is depicted by the blue curve in Figure 3. On the basis of the observed changes in weight, the heat treatment process is split into two different temperature range stages. Stages 1 and 2 correspond to temperature ranges of 25-600 °C and 600-900 °C, respectively.
In stage I, weight loss (6.68%, ~100-600 °C) is related to the combined removal of adsorbed or intercalated water in the clay, combustion of organic matter, dehydration of gypsum, and dehydroxylation of kaolinite and illite 14), (15. This desorption process is visible in three successive endothermic peaks. These correspond to 404 °C, 465 °C, and 520 °C peaks in the DTA curve (endothermic), with corresponding TGA mass losses due to dehydroxylation.
Finally, in stage II, which corresponds to temperatures between 600 and 950 °C and a weight loss of 6.15%, the broad peak at higher temperatures (~705 °C) in the DTA curve is considered to be the stage of complete dehydroxylation of kaolinite and illite and the persistence of the meta-kaolinite and meta-illite phases, including the transformation of anatase to rutile 16, and possible decomposition of minor impurities.
The surface features and morphology of the raw and calcined Tabelbala kaolinitic clay were analyzed using Scanning Electron Microscopy (SEM), as shown in Figure 4 (a). The raw kaolin particles exhibited a more structured appearance, with a discernible texture and a less amorphous character, indicating a crystalline or semi-crystalline phase rather than an amorphous one 17. The elemental composition of raw Tabelbala kaolinitic clay, as determined by (EDX) in Figure 4 (b), shows that it is primarily composed of oxygen (44.32 wt.%), silicon (33.02 wt.%), and aluminum (15.46 wt.%), which corresponds well to the theoretical composition of kaolinite (Al2Si2O5(OH)4). Minor amounts of impurities such as potassium (1.34 wt.%) and sodium (0.37 wt.%) were also detected. The calculated Si/Al mass ratio of 2.42is significantly higher than the theoretical value of 1.18 for pure kaolinite, suggesting the presence of free silica, most likely in the form of quartz, On the other hand, Al/Si ratio (0.49) is lower than the value of 0.85 assigned for the ideal kaolin introduced by Schroeder et al. 18.
After thermal treatment at 900 °C, the kaolin underwent dehydroxylation and partial recrystallization into metakaolin (Figure 5). This process led to the separation of kaolin particles from quartz and a notable reduction in particle size. The SEM images of the resulting metakaolin revealed its characteristic platelet morphology, displaying the well-defined hexagonal faces typical of the kaolinite structure 19. Simultaneously, EDX analysis showed a slight decrease in oxygen (reflecting OH-group loss), an increase in aluminum (20.67 wt.%), and a decrease in silicon (27.71 wt.%), shifting the Si/Al ratio to 1.34. The carbon content dropped significantly (2.51 wt.%), indicating combustion of organic matter. Additionally, an increase in potassium (2.18 wt.%) suggests a concentration of thermally stable impurities during calcination.
Figure 6 shows the compressive strength and flexural strength at 28 days for different geopolymer mortar mixtures. The compressive strength of mixtures with MS(KH+SS) is the highest (30.7 MPa), followed by mixtures with MS(NH+SS) (25.6 MPa). This shows that both silica fume and potassium-based activators greatly improve mechanical properties. Flexural strength trends similarly but is lower than compressive strength. For instance, MS(KH+SS) reached 6.1 MPa, and MS(NH+SS) reached 6.2 MPa in flexural strength.
Compressive and Flexural strengths of GP and OPC mortars at 28 days, cured under ambient, solar conditions.
The lowest values are for conventional mixtures (TS: 32.8 MPa / 6.0 MPa and TE: 35.2 MPa / 6.2 MPa), while the weakest geopolymer mix was ME(Na+SF) with only 12.4 MPa in compression and 3.3 MPa in flexural strength. This demonstrates that the geopolymer mortars, especially those containing silica fume and potassium-based activators, perform best, with a significant improvement, for example, a +85% increase in compressive strength and +84% in flexural strength was observed when comparing ME(Na+SF) and MS(KH+SS). While in the study 20, it was found that the bending strength improved by 3.72-fold, the current study recorded an increase of up to 2.8-fold in flexural strength (from 3.3 MPa to 9.2 MPa in ME(Na+SS)).
Moreover, mixtures cured under solar conditions consistently outperformed those cured at ambient conditions. For instance, MS(K+SS) achieved 30.7 MPa (solar) vs. ME(K+SS) with only 18.9 MPa (ambient), reflecting a 62% increase in compressive strength due to solar curing. The flexural strength of MS(K+SS) also doubled compared to its ambient counterpart (6.1 vs. 3.1 MPa), representing a 97% increase. This supports literature findings that solar curing enhances geopolymerization through increased solubility and faster condensation of precursors, leading to denser and stronger matrices 21.
The choice of alkaline activator significantly influenced the geopolymerization process. As shown in Figure 6, mixtures activated with KOH + sodium silicate (K+SS) (e.g., MS(K+SS)) consistently outperformed those using NaOH, with a +18.3% increase in compressive strength compared to MS(Na+SS) (30.7 MPa vs. 25.6 MPa) and a +9.6% increase in flexural strength (6.1 MPa vs. 5.6 MPa). This superior performance is due to the larger ionic radius of K+, which enhances the incorporation of reactive silica into the aluminosilicate gel. This leads to a more cross-linked and stable network compared to the Na+ system, despite sodium ions being more reactive in early-stage geopolymerization 22), (23.
The infrared spectra of the raw and calcined clay for 1 h, 2 h, 3 h, and 4 h are shown in Figure 7. The first spectrum shows the presence of the kaolinite and illite characteristic bands.
The bands at 3700, 3655, 3625 cm and 3430 cm show frequency peaks with a strong OH-stretch absorption of kaolinite and illite 24. The peak at 3700 cm is assigned to inner-surface OH stretching and agrees with 3701cm reported for Alkaleri kaolin by Aroke 25. The peak at 3655 cm is attributed to inner-cage OH stretching of kaolinite and agrees with 3655 cm reported by Davarcioglu 26. The internal hydroxyl group is obtained at 3625 cm, which is typical of a large amount of Al-OH in the octahedral shell and corresponds to 3626 cm reported by Aroke 25.
The other bands at 909, 1038, 1117, and 470 cm-1 are those of the Si-O planar stretching shared with kaolinite. In addition, the band at 909 cm-1 is attributed to Al-OH deformation vibrations 6, and the bands at 757 and 533 cm-1 originate from Al(IV)-O-Si and Al-O-Si deformation vibrations 24), (27. A double band at 791 and 757 cm-1 and a single band located at 700 cm-1 are attributed to quartz vibrations 28, and this result is consistent with the XRD patterns.
Comparison of the IR spectra of the clays before and after calcination at 900 °C in the spectral range between 400 and 4000 cm-1 shows the disappearance of the O-H bands attributed to kaolinite as well as those of illite (3625 cm-1). It can also be seen that after calcination at 900°C, between 1200 and 900 cm-1, the various bands attributed to illite and kaolinite moved towards silicates Q4 to Q1, corresponding to temperature effects. We can therefore conclude that the calcination temperature of 900 °C is sufficient to convert kaolinite to metakaolinite and illite to metaillite.
Figure 8. presents the XRD patterns of the raw clay. According to the relative patterns, the samples mainly consist of kaolinite (PDF:00-026-0911) and white illite (PDF:00-029-1488). They also contain quartz (PDF:01-086-2237). In addition, the clay contains a small amount of hematite (PDF:021-0920), gypsum (PDF:00-021-0167), and anatase (PDF:01-071-1168). The proportion of each phase presented in the legend of Figure 1 was calculated by Rietveld refinement, using the free license software Maud.
The high-temperature calcination weakens the diffraction peaks of kaolinite and enhances the diffraction peaks of quartz. Due to the moderate purity of the Tabalbala deposit, the obtained metakaolin contained about 27.87% of impurities, mainly quartz (18.9%), and the remaining 72% comprised metakaolinite (Al2Si2O7) and metaillite (KAl3Si3O10) as shown in Figure 8. The above results allowed us to determine the amount of amorphous silica and aluminum in the calcined clay. The geopolymer formulation will include SiO2=38% and Al2O3=32%.
Figure 9 shows the infrared spectra of OPC reference and metakaolin alkali-activated mortars. Table IV summarizes the absorbance peaks in FTIR patterns and their descriptions.
The Hydroxyl Stretching (O-H) at ~3430-3645 cm-1 shows that structural water or calcium hydroxide Ca(OH)2 is present, which is a result of OPC hydration. The bond at ~1453 cm-1 corresponds to C-O stretching vibrations, and the one near ~870 cm-1 indicates out-of-plane bending of carbonate ions. These vibrations result from the carbonation of calcium hydroxide, which forms calcite (CaCO). Silicate Vibration (~1057 cm-1), this is the same as the Si-O stretching vibrations from the calcium silicate hydrate (C-S-H) gel that forms when OPC is watered down.
When OPC mortars are left to dry at room temperature, they keep more water, which shows in a stronger O-H stretching peak (~3430 cm-1) 31. This is because lower temperatures result in slower water evaporation.
In MGP mortars such as ME (Na+SF), ME (K+SF), and MS (K+SF), the main peak around 1057 cm-1 corresponds to Si-O-T (T = Al or Si) asymmetric stretching. This shows that the aluminosilicate network formed in geopolymers, which means that the geopolymerization reaction went well 29. Compared to OPC, this peak is broader and shifts slightly due to the incorporation of Al in the silicate network.
The Si/Al ratio changes, or the structure reorganizes, causing the peaks of MS (K+SF), ME (Na+SS), and others to shift slightly. There is a clear peak at about 454 cm-³ that is caused by Al-O-Si or Al-O vibrations in the geopolymer framework. This peak is not present in OPC mortars 30.
The differences among geopolymer mortars (e.g., ME (Na+SF), ME(K+SF), and MS(K+SF)) may reflect variations in curing conditions, mix design, or activator composition. ME (Na+SF)/ME (K+SS) vs. MS (Na+SF)/MS (K+SS), Variations in peak intensities at ~1057 and ~770 cm-1 peaks could indicate differences in the degree of geopolymerization. MS (Na+SF), ME (Na+SS), etc., these show slight shifts in peak positions due to changes in the Si/Al ratio or structural reorganization.
Mortars cured in the sun (S) exhibit higher peak levels or sharper peaks in the 1000-1100 cm-1 range compared to those cured at room temperature (E). This shows that the geopolymerization process works better when exposed to sunlight. This is probably because the higher temperature from the sun speeds up the breakdown of aluminosilicate species and their subsequent polymerization.The XRD patterns of the OPC samples are shown in Figure 10. The diffraction pattern, list of d-spacings, and relative intensities of diffraction peaks were prepared and compared with the standard peaks of compounds in the diffraction database released by the International Centre for Diffraction Data (ICDD). The main phases present in the OPC mortars are calcium hydroxide (PDF: 01-081-2041), calcium silicate hydrate (PDF: 00-029-0329), calcite (PDF: 01-086-2334), and calcium aluminum silicate hydrate (PDF: 00-020-0452). The XRD spectra of powdered mortars also revealed remnants of aggregate in the form of quartz (PDF: 01-070-3755). The diffraction peaks from calcium silicate hydrate, calcium aluminum silicate hydrate, Calcium Aluminum Silicate Hydrate, and aggregates tend to swamp the weak diffraction peaks of Portlandite (calcium hydroxide). We observed a lower intensity of the main peaks in the solar-treated mortar, explaining the lower mechanical strength compared to the ambient-treated samples.
X-ray diffraction (XRD) of OPC and Metakaolin geopolymer mortar: ▶ calcium hydroxide, ¦ calcium silicate hydrate, ♣ calcite, ◆ calcium aluminum silicate hydrate, Q Quartz, A Albite, L Leucite, Fa Faujasite-K, CK Chabazite-K.CN Chabazite-Na, Fe Ferrierite-Na.
Figure 10 shows the XRD patterns of the Metakaolin geopolymer samples prepared with various alkali activators. This set of samples revealed the presence of several mineral phases, including faujasite (PDF:00-038-0239) and chabazite-Na (PDF: 01-083-1295). These phases play a major role in the thermal response of geopolymer samples, as each has specific thermal properties, such as drying temperature, thermal expansion, and melting point 32. The presence of faujasite is likely enhanced the strength of Na-geopolymer by contributing to a more rigid structure. However, this rigid structure may shrink with heating, potentially resulting in little to no microcracking 33.
We observed a higher tendency to form chabazite K (PDF: 00-044-0250) through inter-geopolymer networks; the chabazite K is commonly identified in geopolymers using potassium-type alkali activators 34. In the case of Na-geopolymer samples, contrary to K-geopolymer samples, it appears that new peaks representative of Ferrierite-Na (PDF: 00-043-0577) and Chabazite-Na(PDF: 01-083-1295) appear. The second one is a product of a reaction between sodium hydroxide and carbonates of air: Chabazite-Na. 35.
The formation of leucite (PDF:01-076-2298) was observed in the majority of the sun-cured samples; this is demonstrated by studies on the evolution of the microstructure of metakaolin-based geopolymers at elevated temperatures, where pores close and leucite crystallizes. 36.
A comparison between XRD patterns of ME (Na+SF) and ME (Na+SS) specimens demonstrate the positive effect of SS. By adding SS to the mixture, the sharp crystalline peak associated with albite (PDF:00-002-0515) dropped substantially. The XRD pattern of the MS(K+SS) specimen indicates less-intense peaks compared with other specimens. These specimens had the highest compressive strength with the most compact structure. 30
New phases of ferrierite-Na were found in the sample MS(Na+SS). This phase could densify the microstructure and increase the bending strengths of the samples 37.
CONCLUSION
This study explored the thermal activation of Tabelbala clay to produce metakaolin for synthesizing geopolymer mortar and assessing its properties. Analytical techniques such as TGA/DTA, FTIR, and XRD confirmed that thermal treatment at 900°C successfully transformed the clay into metakaolin. Solar curing at higher temperatures enhanced the geopolymerization process, leading to improved mechanical performance. The key findings of the study are as follows: compressive strength of up to approximately 35 MPa, improved flexural strength, for the best composition of GP mortars incorporating Potassium based activators and Silica fume. Furthermore, the inclusion of silica fume boosted the cross-linking density in the geopolymer; solar curing brought out a major improvement in the mortar’s performance in mechanical properties by accelerating the geopolymerization, thus implying the significance of the curing conditions to the geopolymer characteristics; this research provides evidence for the use of metakaolin-based geopolymer mortars is more sustainable than using ordinary Portland cement (OPC) as it is less carbon intensive amongst construction materials; and TGA, DTA, FTIR, XRD, and mechanical testing results were well incorporated together to form cohesive understanding on the raw material changes, geopolymerization analysis.
DATA AVAILABILITY STATEMENT
Research data are available in the body of the document.
ACKNOWLEDGEMENT
The authors extend their sincere thanks to all staff of STG’s Adrar cement plant. In addition, the engineers of the laboratory of the university, Ahmed Draya Adrar, for their contributions.
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Publication Dates
-
Publication in this collection
17 Oct 2025 -
Date of issue
2025
History
-
Received
09 Apr 2025 -
Reviewed
11 July 2025 -
Accepted
28 July 2025
