Alkali-Activated Metakaolin: Mechanical Properties and the Effects of Calcium Hydroxide

Abstract

Ordinary Portland Cement (OPC) concrete is the most abundantly used construction material in the world, and its manufacturing accounts for over 5% of global CO2 emissions. This negative environmental impact paired with an increasing global demand for the building material emphasizes the importance of finding a suitable alternative.

Given their high strength and low associated CO2 emissions, alkali-activated materials (AAMs) could serve as an alternative to OPC. Alkali-activated metakaolin has the potential to substantially impact the concrete emissions of the cement industry; however, it has inferior set times and early-strength development compared to OPC. The present work focuses on the mechanical properties of alkali-activated metakaolin with a specific objective of establishing the effect that small amounts of calcium hydroxide have on improving these undesirable properties.

Data were collected by testing the mechanical strength of eight different mix designs via INSTRON machine. The setting of the materials was also tested by Vicat apparatus and Pundit ultrasonic instrument.

The results obtained indicated a strong correlation between activating solution type and mechanical properties. Na2SiO3 activated samples had significantly higher strengths and lower set times than NaOH activated samples. Additionally, the increase in activator molarity from 5 to 10M dramatically increased strength and decreased set time. At 5 wt% replacement of metakaolin, calcium hydroxide had varying effects on the samples, with the most prominent being that it lessened the impact that altering other factors had on the samples. The optimal mix was found to be one made with 10M Na3SiO3 and 0% calcium hydroxide.