Impact of calcium hydroxide, reduced activator concentration, and flash-calcined metakaolin on alkali-activated metakaolin at ambient and high temperatures

Abstract

Limitations must be placed on anthropogenic CO2 emissions due to their direct correlation with climate change. Currently, the cement industry accounts for 5-8% of global CO2 emissions, with expectations to nearly double by 2050. Alkali-activated materials (AAMs) are a class of alternative cementitious materials with the potential to reduce CO2 emissions compared with cement. Although significant research exists regarding the viability of AAMs, there are still many gaps regarding their complex reactions, atomic structure, and durability. The overall goal of this thesis is to understand the reaction kinetics, product formation and high temperature behavior up to 1000°C of alkali-activated metakaolin (AAMK) with reduced activator concentrations and the addition of calcium hydroxide. The initial segment of this thesis focuses on understanding the impact of calcium hydroxide on the formation mechanism of AAMK, and to assess if this is a viable route for reducing activator concentration. A range of experimental characterization techniques are used to uncover this mechanistic insight, showing the favorable impact of calcium hydroxide in silicate-activated metakaolin. The second part of this thesis is focused on the high temperature performance of calcium-containing AAMK, since existing studies demonstrate that AAMK possesses high thermal stability yet the influence of calcium remains unknown. Ex situ characterization techniques reveal that calcium promotes the formation of nepheline on heating, while in situ X-ray pair distribution function analysis indicates the incorporation of calcium into both crystalline and amorphous phases, including as an amorphous calcium aluminosilicate glassy phase. Due to the high water demand of rotary-calcined metakaolin, flash-calcined metakaolin, which possesses a lower water demand, is the focus of the latter part of this thesis. Both ambient temperature properties and high temperature behavior of alkali-activated flash-calcined metakaolin are investigated, with and without particle and fiber reinforcement, revealing the suitability of flash-calcined metakaolin as an AAM precursor.