Effect of internal alkali activation on pozzolanic reaction of low-calcium fly ash cement paste

Abstract

Fly ash is often added to concrete as a filler and to improve strength and durability. The pozzolanic reaction between the silica and alumina in low-calcium fly ash and the Ca(OH)2 from hydrated cement proceeds slowly after an initial rapid reaction so unreacted fly ash particles remain in the paste even after aging. This limits the improvement in the strength of the concrete and it is therefore necessary to accelerate the pozzolanic reaction of low-calcium fly ash cement paste to maximize strength and durability. The present study suggested an internal alkali activation at late age (i.e. at 3 months), considering that an alkali activation can affect the cement hydration adversely at the early age (i.e. at 1 month). In the experimental study, the effects on the pozzolanic reaction of a saturated Ca(OH)2 activation applied 3 months after casting were investigated, as indicated by Ca(OH)2 content, pore size distribution and microstructure of cement paste with 40 mass% replacement with low-calcium fly ash and a water to binder ratio of 0.30. This was compared with water injection, and with untreated control samples. Water injection into fly ash cement paste 3 months after casting increased the Ca(OH)2 content after the injection for 1 and 3 months, but decreased the Ca(OH)2 content slightly in fly ash cement paste after the injection for 7 and 9 months. Meanwhile, saturated Ca(OH)2 solution injected 3 months after casting slightly decreased the Ca(OH)2 content in fly ash cement paste after the activation for 1 month and later, and reduced both total pore volume and macropore volume. As a result, internal alkali activation was more effective in accelerating the pozzolanic reaction of low-calcium fly ash cement paste than water injection. These findings were supported by the observation of differences in the microstructure between the untreated control sample and the sample applying alkali or water injection.