Shrinkage characteristics of alkali-activated slag cements

Recent interest in creating green construction materials has sparked the development of portland cement-free binders. Alkaliactivated slag (AAS) concrete has a low embodied energy and comparable or superior strengths to ordinary portland cement (OPC) concrete. However, one factor limiting AAS usage is its durability performance, specifically its susceptibility to shrinkage. Before declaring AAS a marketable product, the mechanisms behind its volumetric instability need to be understood. This paper presents a preliminary study of the shrinkage deformations of various AAS mixtures, wherein four unique AAS mortars were designed and tested for autogenous, chemical, and drying shrinkage; time of setting; and compressive strength. All results were compared to those obtained for a control OPC mortar. Alkaliactivated slag mixtures with comparable strength to OPC show a higher autogenous and drying shrinkage. A lower elastic stiffness, higher degree of saturation, and potentially higher chemical shrinkage contribute to the high autogenous shrinkage of AAS. A lower elastic stiffness also leads to a large drying shrinkage, although other mechanisms, such as reduced pore size and carbonation shrinkage, are likely to also contribute to the high drying shrinkage measured for AAS.