Abstract

Alkali-Activated Binders (AABs) are promoted as sustainable alternatives to ordinary Portland cement (OPC) due to their promising characteristics and ability to reduce the global greenhouse emissions of construction materials. Despite many years of research, volumetric stability (i.e. shrinkage) of AABs, which has been scarcely investigated, is hindering their wider implementation in the construction sector. Hence, the initial attempt of the current study is to fully characterize and understand the shrinkage behaviour of AABs considering different factors. These factors included precursor materials, activator types and concentrations, and curing temperatures. Moreover, the effectiveness of shrinkage reducing admixtures (SRA) as a shrinkage mitigation technique and its interaction with AABs were evaluated. Two preparation methods for AABs, namely one-part (i.e. powder binder-powder activator) and two-part (i.e. powder binder-liquid activator), were also tested for comparison. Fresh properties including workability, setting behaviour, and heat evolution along with hardened properties (i.e. compressive strength) were explored. Results revealed that shrinkage for fly ash-based alkali-activated mortars was the lowest followed by blended fly ash/ slag, while alkali-activated slag-based mortars exhibited the highest values. Regardless of the preparation method, the higher the activator concentration, the higher the shrinkage. There is a strong link between AAB’s matrix rigidity and shrinkage behaviour. For both preparation methods, SRA was more effective in reducing shrinkage strains for slag-based alkali-activated mortars at different temperatures. Similar to OPC, SRA addition had induced a reduction in the compressive strength for all tested ABBs mixtures. However, the reduction in AABs mixtures strength was significantly lower than that for OPC mixtures.