Drying shrinkage in alkali-activated binders – A critical review

Abstract Alkali-activated binders have been proposed and have emerged as an alternative to ordinary Portland cement (OPC) binders with acceptable mechanical and durability performances in addition to positive environmental impacts. These alternative binders use a wide range of aluminosilicate precursors, with differing availabilities, reactivities, costs and CO2 emissions. Despite all the studies published on improving the characteristics of these materials in the last decades, some aspects of alkali-activated binders require further investigations. For instance, shrinkage in alkali-activated binders due to the different chemical processes being involved is more complicated than OPC composites and it plays a key role in long-term performance. However, information on the shrinkage behavior of these binder systems is scattered and fragmented. Therefore, this review critically surveys the drying shrinkage behavior of alkali-activated binders with slag, fly ash and metakaolin. The mechanisms of the drying shrinkage and the factors that affect shrinkage are discussed. Different solutions to limit the shrinkage are also presented. Then, an analysis is conducted based on the literature, and the main chemical elements affecting the drying shrinkage of alkali-activated materials are proposed. Finally, the feasibility of using the developed empirical models for predicting the drying shrinkage of OPC concretes is also investigated for alkali activated binders. This review shows that alkali-activated slag binders generally have the higher rate of the drying shrinkage compared to other alkali activated binders, albeit, reinforcing the plain compositions with fibers, using expanding admixtures, curing at elevated temperatures, using a combination of slag with fly ash or metakaolin reduces successfully the drying shrinkage. Moreover, utilizing ternary diagrams, it is revealed that the molar ratio of Ca/Si has the greatest impact on the rate of drying shrinkage in the alkali-activated binders.