The effect of silica availability on the mechanism of geopolymerisation

The effect of silica availability on geopolymer binder formation is investigated in the geothermal silica-sodium aluminate-water system, using sodium silicate solution as an additional, highly available silica source. Time-resolved and spatially-resolved FTIR data are combined to provide a mechanistic understanding of the role of silica availability in controlling geopolymer nucleation and gel growth behaviour. A higher degree of alumina contribution to geopolymer gels and newly formed crystal phases is observed in systems with higher silica availability. Gel nucleation is observed to take place in the region immediately surrounding the solid silica source particles when no dissolved silica is initially supplied. Conversely, mixes which initially contain dissolved silica show nucleation in bulk regions, and involving more of the Al which is rapidly released from the sodium aluminate precursor. These differences in nucleation lead to a more chemically heterogeneous binder in the case where silica is released more gradually. (C) 2011 Elsevier Ltd. All rights reserved.