# Effect of Curing Conditions on Microstructure and Pore-Structure of Brown Coal Fly Ash Geopolymers
## 5 Discussion  
The fly ash geopolymerization is an exothermic polycondensation process which involves alkali activation by a cation in solution. The rapid alkali-fly ash reaction does not allow time for the development of well-structured crystalline phases, and thus hardened fly ash geopolymer concrete is similar to the zeolitic precursors, producing an amorphous, semi-crystalline aluminosilicate gel [44,45]. The microstructure, Figure 7, correlates well with the temperature profile observed for the brown coal fly ash geopolymer mortar, Figure 2. The first stage, 2 to 6 h, can be correlated to the period when the dissolution and speciation equilibrium steps occur as proposed by [PERSON] et al. (2007) in the conceptual model of geopolymerization. This first stage can also be related to the nucleation and growth stages identified by [PERSON] et al. (2007). Nucleation is the dissolution of the aluminosilicate material and the formation of polymeric species which are highly dependent on thermodynamic and kinetic parameters. The stage after nucleation is growth. Growth is the condensation-crystallization phase when the nuclei reach a critical size and crystals begin to develop. This process is a structural reorganization which forms the microstructure of the material and the nano-pore distribution which is critical in determining the physical and durability properties of the geopolymer [46,47]. Gelation is  
Figure 10: Pore interconnectivity and pore volume distribution of 100 mm geopolymer concrete specimen, 4–24 h.  
also identified as commencing in this first stage. Dissolution consumes water which is then released during the formation of the gel, with some residual water remaining within pores in the gel. The gel comprises a complex mixture of silicate and aluminate species together with sodium and calcium oxides and is followed by (sodium/calcium) aluminosilicate being released by dissolution into the aqueous phase. This correlates with the temperature profile during this period displaying a slight increase from 105 \({}^{\circ}\)C to 108 \({}^{\circ}\)C as the nucleation utilizes the energy from the heat curing. The fall in temperature observed, Figure 2, is hypothesised as relating to the energy required during this phase. Moreover, during this stage, the geopolymeric gel matrix begins to form, which corresponds to the strength increase observed from approximately 5 MPa at 4 h to 23 MPa at 6 h.  
In the second stage, the temperature profile rose significantly from 108 \({}^{\circ}\)C at 6 h to 120 \({}^{\circ}\)C at 10 h. The rapid rise of temperature observed in the second stage between 6 to 10 h curing is identified as corresponding to the on-going geopolymerization process, with reduced dissolution, on-going gel formation and growth, and re-organization within the geopolymer matrix. This is reflected in the variation in the Si/Al ratio observed in the specimens. A ratio of 2.73 was noted at 8 h, this decreased to 2.69 at 10 h and further decreased to 2.24 at 12 h. It is hypothesised that this corresponds with gel formation continuing while reorganising occurs in the gel matrix. The Na\({}^{+}\) ions act as a structure forming element. The structure of the sodium-aluminosilicate gel contains Si and Al tetrahedrons randomly distributed along the polymeric chains that are cross-linked to provide cavities of sufficient size to accommodate the charge balancing hydrated Na\({}^{+}\) ions. It has been noted that geopolymers with higher strengths are more likely to be produced by fly ash having higher Al\({}_{2}\)O\({}_{3}\)/Na\({}_{2}\)O and SiO\({}_{2}\)/Na\({}_{2}\)O ratios [48; 49]. The higher SiO\({}_{2}\)/Na\({}_{2}\)O weight ratios retard zeolite formation, primarily due to the higher degree of polymerization of soluble silica in the system and which results in the production of more geopolymeric gel [50]. Thus, it is believed that the higher Al\({}_{2}\)O\({}_{3}\)/Na\({}_{2}\)O ratio would add sufficient alumina to react with soluble silica in the system.