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

• Published in: Materials & design Vol. 122; pp. 236 - 246
• Main Authors: Dakhane, Akash, Tweedley, Shannon, Kailas, Siva, Marzke, Robert, Neithalath, Narayanan
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.05.2017
• Subjects: Alkali sulfate; Characterization; Fly ash; Porosity; Spectroscopy; Characterization; Fly ash; Spectroscopy; Alkali sulfate; Porosity
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2017.03.021

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. 29Si NMR spectroscopy coupled with Gaussian spectral deconvolution on Class F fly ash-OPC blends provides valuable information on the changes in Qn(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. [Display omitted] •70% reduction in clinker factor results in only a 30% reduction in strength.•Neutral pH activation, avoids the drawbacks of caustic activators.•Pore structure refinement through neutral pH activation.•Hybrid reaction products quantified through spectroscopic techniques.•Volumetric expansion lower than conventional binders.