# Effect of Curing Conditions on Microstructure and Pore-Structure of Brown Coal Fly Ash Geopolymers
## 5 Discussion
Thus, a combination of the Al\({}_{2}\)O\({}_{3}\)/Na\({}_{2}\)O and SiO\({}_{2}\)/Na\({}_{2}\)O ratios influence the compressive strength, and the threshold values for these ratios are required to achieve a specific strength [51]. Furthermore, the dissolution rate starts to reduce during this stage, as water is lost and dissolution ceases. The increased gel formation in this period is reflected in the compressive strength increase to 34 MPa at 10 h. In the final stage, the dissolution process is completed and the evaporation of the remaining water together with a cessation in gel formation and the reorganization step in the geopolymer formation. This corresponds with the slight rise in the temperature profile from 120 \({}^{\circ}\)C at 10 h to 123 \({}^{\circ}\)C at 14 h. During this stage, the aluminate matrix structure which is attributed as providing the compressive strength is created, with the maximum strength of 42 MPa achieved at 12 h.  
The slight rise in temperature at 14 h is indicative of the amalgamation of alumina into the silicate backbone. The results are indicative of a high rate of release and incorporation of aluminium that leads to the production of a gel with a more uniform composition and structure, as observed in Figure 7. This corresponds to the increase in strength observed to the 42 MPa maximum at 12 h. However, at 14 h, the reduction in strength and micro-cracking observed indicated that although there has been incorporation of aluminium into the matrix backbone, the micro cracking resulted in a reduction of strength. The concrete profiles, Figures 3-5 would suggest a similar mechanism is occurring in the concrete specimens, though the timeframe for the three stages is extended as the specimen size increases. This is supported by a comparison of the temperature profile and the compressive strength. In the 100 mm concrete specimen, the increase in the internal temperature observed in the mortar at 6 h is observed between 12-14 h in the concrete. This is consistent with the compressive strengths which rise from 23.0 MPa to 42.10 MPa in the mortar between 6 and 12 h. In the concrete, where no increase in internal temperature is observed, the strength only increases from 8.75 MPa to 14.25 MPa, but then rises to 30.85 MPa in the period from 12 to 24 h as the internal temperature starts to rise. In the 200 mm and 300 mm cubes, the temperature profiles would suggest that the gelation stage is still underway and that the geopolymerization process is not complete at the center even after 24 h heat curing at 120 \({}^{\circ}\)C.  
In addition, the condition of the 200 mm and 300 mm specimens observed on demoulding is indicative that geopolymerization is not complete as structural integrity was not achieved. The increase in strength from 12 to 24 h is also demonstrated by the changes in the pore structure identified by the tomography, Figure 10. During the period from 4 to 12 h, identified as corresponding to stage 1, a reduction from 6.56% to 4.12% in the porosity is observed, while in the period from 12 to 24 h the porosity falls to 1.64%. This is indicative of an increase in gel formation leading to a refinement of the pore structure. The pore-structure of mortar and concrete specimens (50 mm cubes) also correlate well with the proposed reaction mechanism, Figure 9. Macropores, ranging from 50-200 nm, are formed during the early stage of the geopolymeric reaction. As the gel continues to form, this transforms macropores into mesopores. The presence of macropores larger than 200 nm is characteristic of less reacted geopolymers. This has been reported as primarily occurring in geopolymer pore-structure when synthesized with alkali hydroxide solutions or at low curing temperatures, such as 30 \({}^{\circ}\)C, for Class F fly ash geopolymers [43]. The research on these low calcium, Class F fly ash geopolymers has shown that mesopores represent the voids between the geopolymer phases, while micropores primarily are within the gel network.