# Low-Grade Clay as an Alkali-Activated Material
## 4 Results and Discussion
### Effect of Activator  
Initial trials on the effect of the activator dosage and modulus were undertaken on the clay pre-treatment at 120 \({}^{\circ}\)C. This was selected as the amorphous content and surface area, identified as key parameters in the performance of AAM, were both higher than the clay pre-treated at 750 \({}^{\circ}\)C. The compressive strengths of the 10% dosage and 15% dosage specimens are presented in, Figures 1 and 2, respectively, with standard deviation. Figure 1: Compressive strength vs. time, 10% Na\({}_{2}\)O dosage specimens. The results show that the strength generally increased from 7 to 14 days for both the 10% and 15% Na\({}_{2}\)O dosages, excluding the 1.75 AM, specimens. A slight decrease in strength was observed in all mortars at 28 days. The results indicate an increase in strength comparable to those observed at 14 days for the 10% dosage specimens, while the 15% dosage specimens maintained the strengths observed at 28 days. Additional testing at 40 days was undertaken to determine whether a further reduction in strength was observed. The results indicate an increase in strength comparable to those observed at 14 days for the 10% dosage specimens, while the 15% dosage specimens maintained the strengths observed at 28 days. It is hypothesized that due to the low amorphous and aluminosilicate content in the clay, small variations within the pre-cursor material in the batch can have an impact on the strength. Similar behavior has been observed in class C brown coal fly ash with low amorphous and aluminosilicate content [46]. The Na\({}_{2}\)O dosage of 15% with 7-day compressive strength was highest (20.02 MPa) for AM 1.75, whereas AM 1.5 provided the highest strength (13.45 MPa) at day 14. The 15% dosage of 1.75 AM achieved maximum strength at day 7 but decreased significantly by day 14 (11.96 MPa). The compressive strength at day 28 was similar for all specimens, with AM 1.75 achieving the highest strength (12.04 MPa). For the Na\({}_{2}\)O dosage of 10%, AM 1.75 had the optimum strength at day 7 (14.92 MPa), whereas AM 1.0 showed optimum strength at days 14 (14.5 MPa) and 28 (10.42 MPa), but similar strengths were also achieved by the AM 1.25 specimens. The compressive strengths of all 10% Na\({}_{2}\)O dosage specimens were greater than the strengths of those of the corresponding 15% Na\({}_{2}\)O dosage, except for that of AM 1.75. A Na\({}_{2}\)O dosage of 10% provided consistent strength for all tested AMs; however, all of the specimens prepared with a Na\({}_{2}\)O dosage of 15% showed a reduction with time. This behavior has been observed for the fly ash-rice husk bark ash (RHBA) geopolymer, where it was found that the compressive strength of specimens cured at a higher temperature (i.e., >90 \({}^{\circ}\)C) started to decrease after a period of time [47]. It was reported that the higher temperature destroyed the granular structure of the geopolymer, which caused dehydration of the geopolymer matrix and excessive shrinkage due to contraction of the geopolymeric gel. [PERSON] et al. observed the existence of micro-cracks at an elevated curing temperature (85 \({}^{\circ}\)C) with a short curing duration (24 h), and even at a lower curing temperature (75 \({}^{\circ}\)C) with a longer curing duration (75 h) for clay-fly ash geopolymer  
Figure 2: Compressive strength vs. time, 15% Na\({}_{2}\)O dosage specimens. bricks [48]. Micro-cracking has also been observed in alkali-activated slag geopolymers following elevated curing [49]. Micro-cracking adversely affects the compressive strength due to the breaking of interparticle bonding. This is also supported by the research of [PERSON], who reported that at lower curing temperatures, the geopolymer matrix developed slowly with lower porosity, while geopolymers formed at higher curing temperatures contained larger pores and a loosely packed structure [50]. The results indicate that the compressive strength was adversely affected by an increase in AM and dosage, which corresponds to a higher Na\({}_{2}\)O content.