Nanostructural Evolution of Calcium-Containing Alkali-Activated Metakaolin During High Temperature Exposure

Abstract

Alkali-activated metakaolin (AAMK) is a known low CO 2 alternative material to ordinary Portland cement, where CO 2 emissions can be lowered even further via the use of reduced alkali concentration activators and moderate water-to-metakaolin ratios. We have previously shown that a relatively small amount of calcium hydroxide can be used (10 wt % replacement for metakaolin) to help offset the loss of ambient temperature performance associated with lowering the alkali concentration of the activator, specifically for silicate-based activators, without adversely compromising desired CO 2 emissions reductions. Here, the impacts of activator type (sodium silicate and sodium hydroxide), alkali concentration, and inclusion of calcium hydroxide on the high temperature phase transformations (up to ∼900 °C) occurring in AAMK are investigated using in situ synchrotron-based high-resolution X-ray diffraction and pair distribution function (PDF) analysis. High-resolution reciprocal space data reveal important insight on the evolution of minor crystalline phases, including those containing calcium. PDF analysis of the short-range order of the AAMKs shows temperature-induced changes to the sodium and calcium local bonding environments. After removal of crystalline contributions from the PDF data at high temperature, the residual PDFs provide new insight on the amorphous atomic arrangements present in the material and, together with identification of the calcium-containing crystalline phases, reveal the most likely location of the calcium atoms.