Optimization of the shrinkage assessment of metakaolin based alkali activated materials

Alkali-activated materials (AAMs) are considered a more sustainable alternative to ordinary Portland cement because they produce less carbon dioxide and can incorporate industrial by-products. Metakaolin-based AAMs exhibit good mechanical strength; however, they often suffer from high shrinkage, which can negatively affect durability and long-term performance. Therefore, accurate understanding and measurement of shrinkage are essential. This thesis investigates the strength and shrinkage behavior of metakaolin-based alkali-activated materials. Different mixtures were prepared by varying the type of metakaolin, slag content, water-to-solid ratio, aggregate type, activator type, mixing procedure, and curing conditions. Compressive and flexural strength tests were conducted, and shrinkage was measured using a Demec gauge under controlled temperature and humidity conditions. The results indicate that curing conditions have a strong influence on material performance. Short-term heat curing at 70 °C produced the highest strength, while prolonged heat curing and storage under ambient conditions led to strength loss due to moisture loss and increased shrinkage. Feldspar aggregates resulted in higher strength compared to sand. Shrinkage measurements revealed early swelling during high-temperature curing, followed by continuous shrinkage after transfer to ambient conditions (20 °C, 65% RH). Mixtures containing slag exhibited higher shrinkage than those composed solely of metakaolin. The study also demonstrates that frequent manual shrinkage measurements can disturb curing conditions and introduce measurement errors due to the high sensitivity of the Demec gauge. Overall, the results highlight the strong relationship between curing conditions, moisture loss, strength development, and shrinkage, and provide practical guidance for improving shrinkage assessment and mixture design of alkali-activated materials for sustainable construction.