Sodium silicate activated slag-fly ash binders: Part I - Processing, microstructure, and mechanical properties

Alkali silicate activated slag and class F fly ash-based binders are ambient curing, structural materials that are feasible replacements for ordinary Portland cement (OPC). They exhibit advantageous mechanical properties and less environmental impact than OPC. In this work, five sodium silicate activated slag-fly ash binder mixtures were developed and their compressive and flexural strengths were studied as a function of curing temperature and time. It was found that the strongest mixture sets at ambient temperature and had a Weibull average flexural strength of 5.7 +/- 1.5MPa and Weibull average compressive strength of 60 +/- 8MPa at 28days. While increasing the slag/fly ash ratio accelerated the strength development, the cure time was decreased due to the formation of calcium silicate hydrate (C-S-H), calcium aluminum silicate hydrate (C-A-S-H), and (Ca,Na) based geopolymer. The density, microstructure, and phase evolution of ambient-cured, heat-cured, and heat-treated binders were studied using pycnometry, scanning electron microscopy, energy dispersive X-ray spectroscopy (SEM-EDS), and X-ray diffraction (XRD). Heat-cured binders were more dense than ambient-cured binder. No new crystalline phases evolved through 28days in ambient- or heat-cured binders.