# Mitigation of Volume Changes in Alkali-Activated Slag by Using Metakaolin

## TL;DR

This study explores using metakaolin to reduce volume changes in alkali-activated slag, affecting strength and expansion depending on the activator used.

## Contribution

The study introduces metakaolin as a partial slag substitute to control volume changes in alkali-activated systems.

## Key findings

- Metakaolin increased compressive strength with NaOH but decreased it with sodium silicate.
- Shrinkage was mitigated with sodium silicate activation but increased with higher NaOH substitution.
- A third reaction peak appeared with higher metakaolin substitution using 8 M NaOH.

## Abstract

This research investigates whether metakaolin can be used as a partial substitution for slag to mitigate significant volume changes in alkali-activated slags. Its effect on compressive strength and workability (as well as on isothermal calorimetry, autogenous strain, and coefficient of thermal expansion (CTE)) were found to depend on both the type and concentration of the alkaline activator. When using 8 M and 10 M sodium hydroxide (NaOH), increasing the substitution rate increased the compressive strength. With sodium silicate (Na2SiO3), compressive strength decreased as the substitution increased. Isothermal calorimetry revealed metakaolin’s dilution effect at 10% substitution. With 8 M NaOH, a third reaction peak appeared, whose magnitude increased with the substitution rate, while the second peak decreased. The swelling was increased at 10% substitution, followed by constant shrinkage in case of NaOH-activation. Shrinkage was mitigated with Na2SiO3-activation. Higher substitutions with 8 M NaOH resulted in a significant increase in the shrinkage rate and CTE, occurring when the third reaction peak appeared. A 10% substitution delayed the CTE increase but resulted in higher later-age values (dilution effect). The 20% substitution led to a similar final CTE value at 300 h, while 30% substitution resulted in a decrease in CTE after the initial increase.