Mechanical and microstructural characterization of alkali sulfate activated high volume fly ash binders

This paper presents a detailed characterization of cementitious blends containing high volumes of fly ash, activated using pH-neutral alkali sulfates. It is shown that this methodology, while resulting in a clinker factor reduction of 70%, provides requisite early-age strengths while compromising the 28-day strengths by only 30-40% as compared to plain OPC mixtures. The early age heat release for blends containing Class F fly ash is reduced by about 50% as compared to the straight OPC mixture. The overall pore volume increases with sulfate addition for the Class C fly ash based binder while it decreases when Class F fly ash is used, indicating the beneficial effect of the sulfate activation process in conjunction with a low calcium fly ash. The differences in reaction product constitution are brought out using thermal analysis and FTIR spectroscopy. Si-29 NMR spectroscopy coupled with Gaussian spectral deconvolution on Class F fly ash-OPC blends provides valuable information on the changes in Q(n)(mAl) structures with addition of sodium sulfate, indicating the changes in the reaction products. From a durability perspective, Class F fly ash-based binders are found to be less susceptible to external or internal forms of sulfate attack as compared to plain OPC or the corresponding unactivated mixtures. (C) 2017 Elsevier Ltd. All rights reserved.