The effect of the presence of Ca2+ on geopolymerisation has recently been the subject of a number of detailed investiga- tions [33,34], with segregation of high- and low-calcium regions within the product as was noted earlier. The amount of Ca2+ added and the form in which it is added both play a significant role in determining the physical properties of the final geopoly- mer. The level of dissolved silicate in the activating solution also plays a highly significant role in determining the effects of calcium on the final reaction product by controlling the pH of the activating solution and therefore influencing the relative stabilities of the different calcium-containing precipitates [34]. In particular, the addition of highly alkaline activating solutions to a fly ash containing any calcium leads to rapid dissolution of calcium from the ash followed by precipitation of Ca(OH)2. This has the net effect of lowering the pH of the activating solution due to the removal of the OH− ions, which will then significantly affect the rate of further dissolution/precipitation processes. Lee and van Deventer [24] observed that the addition of even a small amount of calcium as a soluble salt to a Class F fly ash-based geopolymer formulation drastically increases its solidification rate and early yield stress.