3.1 Fresh properties
In engineering applications, workability refers to the ease with which freshly manufactured cement, mortar, and concrete can be mixed, placed, consolidated, and finished with minimal loss of homogeneity. It is a critical parameter that has a direct impact on the performance, quality, appearance, and labour costs of placement and finishing operations. To evaluate the workability of the prepared MH-based geopolymer mortars, the slump flow height and slump flow diameter were measured by varying the concentration of the prepared alkaline solution and the ratio of the alkaline solution to calcined MH powder. This was performed to investigate their effect on the slump flow test. Figures 2 and 3 highlight the relationship between the variation in the slump value (Fig. 2) and flow value (Fig. 3) with the variation in the alkaline solution/MH ratios, respectively.
As shown in Fig. 2, we noticed that the slump values ranged from 25 to 118 mm, 14 to 115 mm, and 12 to 102 mm, for the concentrations of 8, 10, and 12 M of the alkaline solution, respectively. The slump of fresh MH-based geopolymer mortars increased with an increase in the concentration (8–12 M) and alkaline content (0.6–0.8). This behaviour could be attributed to the plasticising effects of the released silicate anions, as reported in previous studies. The obtained results were in accordance with the published findings reported in the literature describing the effect of the concentration of the alkaline solution in controlling the slump behaviour [27–30]. Therefore, the viscosity changed, decreasing the flowability and explaining the observed increase.
Figure 3 shows the results obtained for the flow value as a function of the concentration of the alkaline solution with the ratio of activating solution/calcined halloysite. These values increased with increasing concentrations of the activating solution. This can be attributed to the fact that with the increase in Na2O and silicate content in the activating solution, the dissolution of calcined halloysite was enhanced, thus improving the workability. This result is in agreement with the findings of certain studies wherein the authors investigated the effect of molarity and solid/liquid ratio on the flow or slump behaviour of geopolymer products [30–34]. In accordance with this observation, the obtained results are expected to positively affect the strength development.
The setting times were assessed to determine the applicability of the geopolymer mortars. Allowing adequate time for handling and pouring is critical. If the setting time after pouring is excessively long, it must be adjusted to achieve a moderate or desired setting time when pumping is required for engineering applications.
Figure 4 illustrates the relationship between the initial setting time and the alkaline solution/MH ratio of fresh MH-based geopolymer mortars activated with 8, 10, and 12 M alkaline solutions. The setting time values obtained were between 77 and 163 min, 54 and 140 min, and 35 and 121 min for the alkaline solution to calcined halloysite ratios of 0.6, 0.7, and 0.8, respectively. The initial setting times tended to decrease when the molarity of the alkaline solution increased from 8 M to 12 M. However, lengthening the first setting time tends to increase the ratio of alkaline solution to calcined halloysite. Indeed, increasing the alkaline molarity at a constant alkaline solution-to-MH ratio leads to an increase in the alkaline solution content and concentration of Na2O. This trend favoured the dissolution of the solid precursor, leading to the release of Si and Al oligomers and the enhancement of the polymerisation rate. Consequently, the setting times were significantly reduced [8, 35–37]. The use of a lower alkaline solution/MH ratio (0.6) would induce the matrix to precipitate earlier, producing short setting times, as shown in Fig. 4. However, the increase in setting time with an increase in the alkaline solution to calcined halloysite ratio beyond 0.6 is due to the increased content of Na2O, which enriches the entire system and may delay the setting time, as reported previously by other researchers using different source aluminosilicate materials [8, 14, 28, 29, 35, 38]. Hence, this would have hindered the polymerisation or polycondensation step during the geopolymerisation reaction, even though the utilisation of a high quantity of alkaline activating solution promoted high dissolution of reactive Al2O3 and SiO2 in the solid precursor [39, 40].
3.2 Mechanical properties
Understanding the mechanical properties of hardened cementitious materials is essential for designing structural mortar or concrete. These properties are typically defined as empirical functions of compressive or flexural strength and tensile strength. The mechanical properties of hardened MH-based geopolymers cured for 7 and 28 days are shown in Figs. 5 and 6, respectively. The compressive strength and split tensile strengths were evaluated and found to increase with increasing curing age (7 and 28 days) and with increasing alkaline/MH ratios along with an increase in the concentration of the alkaline solution (Fig. 6). Increasing the alkaline solution/MH ratio (from 0.6 to 0.8) by increasing the concentration of the activating solution (8 to 12 M) increased the split tensile strength by 23–54%, 18–44%, and 29–43% and from 21–35%, 8–35%, and 10–33% at 7 and 28 days, respectively (Fig. 5). Similar observations were reported by Hosein et al. [35], where metakaolin-slag-based geopolymer mortars cured for 7 or 28 days demonstrated split tensile values higher than those of conventional OPC. This clearly shows the potential of geopolymer materials as an alternative to the widely used Portland cement around the world. This improvement in the split tensile strength is attributed to the densified matrix and the highly dense interfacial transition zone between the quartz aggregates and the geopolymer binder phase, as reported earlier by other researchers [41–43]. The highest values obtained for the geopolymer samples are likely linked to the higher strength bonding between quartz sand aggregates and the newly formed matrix [44].
3.3 Microstructure
The influence of the activator content and concentration of the activating solution on the microstructure of the selected MH-based geopolymer mortars consolidated with the concentration of the activating solution (8 and 10 M) were evaluated using SEM analysis. Figures 7 and 8 display the micrographs of selected MH-based geopolymer mortars GM8 and GM10 prepared with activator/MH ratios of 0.6 and 0.8, respectively. The micrographs of GM12 made with MH-based geopolymer mortar prepared with activator/MH ratios of 0.6 and 0.8 are presented in Figs. 9 and 10, respectively. At lower magnification (100 µm), all micrographs appear almost heterogeneous and compact, exhibiting microfissures across the matrix of specimens consolidated with activator/MH ratio of 0.6 when using alkaline solution molarity of 8 and 10 M (Figs. 7 and 11). This can be attributed to the less significant development of a geopolymer binder capable of improving the cohesion between various matrix components [46, 47]. Thus, using a lower activator/MH ratio does not favour the high dissolution of solid precursors favourable for high geopolymerisation and polycondensation, which are expected to form a dense structure. The shorter initial setting time recorded for the samples consolidated with an activator/MH ratio of 0.6 proves their poorly compact structure (Figs. 7 and 9), which agrees with the inferior mechanical performance. However, when the NaOH concentration and activator/MH ratio increased, the matrix became denser and more homogeneous with greater cohesion between the coarse particles (quartz sand) and geopolymer binder. Thus, increasing the alkaline solution content and molarity of the activating solution yields a sufficient binder phase, ensuring better cohesion among the different components in the geopolymer matrix, thereby developing a compact structure with high strength, as explained in Section 3.2. The microstructural evolution in the present work is consistent with the findings presented in the literature, where the alkaline solution content or molarity of the alkaline solution was varied, resulting in few accessible open voids and fissures within the matrix [34, 45, 48, 49]. The higher alkaline solution content and higher concentration of alkaline solution help in a higher degree of condensation and dissolution, respectively, thus reaching a compact structure with higher compressive strength.
EDS analyses conducted on the selected micro-areas based on samples prepared using activator/MH ratios of 0.6 and 0.8 using 8 and 12 M activating solution are reported in Figs. 11, 12, 13 and 14. From the different areas indicated in Figs. 11, 13, and 14, it can be observed that the typical geopolymer binder phase is an amorphous sodium aluminosilicate hydrated gel (N-A-S-H). The main elements recorded in these areas are Al, Si, O, and Na. At a ratio of 0.8, the high Na content of GM12 is compatible with the high alkalinity (12 M) employed (Fig. 14). This also explains the importance of Na in integrating the geopolymer network, suggesting that a sufficient binder is favourable for better cohesion, resulting in a highly compact matrix. The elements (Si and O) belonging to quartz used as aggregates are shown in Figs. 7 and 11. This quartz sand mainly represents the unreacted coarse particles with various forms embedded in the geopolymer binder, as seen within the geopolymer matrix, as evidenced by SEM/EDS analyses.
3.4 Water absorption, porosity, and bulk density
Figure 15 shows the water absorption, porosity, and bulk density of hardened MH-based geopolymers consolidated with 8, 10, and 12 M alkaline solutions at different alkaline solution/MH ratios. The bulk density values of MH-based geopolymer mortars prepared using alkaline solution/MH ratios of 0.6, 0.7 and 0.8 are in the range of 1705–1804 kg/m3, 1899–2002, and 1993–2135 kg/m3, respectively. The porosity values of MH-based geopolymer mortars prepared with alkaline solution/MH ratios of 0.6, 0.7, and 0.8 are 35.20–29.30%, 29.50–22.20%, and 21.02–17.03%, respectively, and water absorption values are 14.30–11.80%, 14.70–10.74% and 10.13–8.40% for MH-based geopolymer mortars prepared using alkaline solution/MH ratios of 0.6, 0.7, and 0.8, respectively. The influence of the alkaline solution/MH content on water absorption, porosity, and bulk density was evaluated. The reduction recorded in the aforementioned parameters coincides with the strength development, and the microstructure is explained in Sections 3.2 and 3.3. Thus, the increase in the combined action of the molarity of the alkaline solution and the alkaline solution/MH ratio led to the progressive formation of compact and dense structures with few accessible voids or pores in the geopolymer matrix. This ensured lower water absorption and porosity when the samples were immersed in water. This reduction is likely attributed to the sufficiently produced geopolymer binder ensuring better cohesion between different components in the entire system, which is favourable for the high bulk density achieved in geopolymer mortar samples with high alkalinity (12 M) and high alkaline solution/MH ratio (0.8). The evolution of water absorption, porosity, and bulk density matches those reported in previous studies [35, 48]. In conclusion, the mechanical properties and bulk density decrease with a reduction in water absorption and porosity.