Autogenous shrinkage of alkali-activated slag and fly ash materials: From mechanism to mitigating strategies

Alkali-activated materials (AAMs), as eco-friendly alternatives to Ordinary Portland cement (OPC), have attracted increasing attention of researchers in the past decades. Unlike cement, which requires calcination of limestone, AAMs can be made from industrial by-products, or even wastes, with the use of alkali-activator. The production of AAMs consumes 40% less energy and emits 25-50% less CO2 compared to the production of OPC.
Despite the eco-friendly nature of AAMs, doubts about these materials as an essential ingredient of concrete exist, regarding, for example, their volume stability. One possible volume change concerns autogenous shrinkage. Autogenous shrinkage is the reduction in volume caused by the material itself without substance or heat exchange with the environment. If the autogenous shrinkage of a binder material is too large, cracking might happen, which will seriously impair the durability of concrete. According to the literature, AAMs can show higher autogenous shrinkage than OPC-based materials. However, the mechanism behind the high autogenous shrinkage of AAMs is still unclear. Existing shrinkage-mitigating strategies for OPC are not necessarily applicable for AAMs. There is also a lack of new strategies particularly designed for AAMs. Moreover, the cracking sensitivity of AAMs-based concrete induced by restrained autogenous shrinkage has not been investigated yet.
The aim of this study is, therefore, set to understand and mitigate the autogenous shrinkage and the cracking tendency of AAMs.