Effect of alkalis on cementitious materials: Understanding the relationship between composition, structure, and volume change mechanism

Previous studies demonstrated that alkalis can considerably affect the volumetric properties of hardened cementitious binders, including shrinkage, creep, and micro-cracking. The objective of this paper is to reexamine the effect of alkalis on the composition, nanostructure, phase stability, and morphology of calcium-silicate-hydrate (C-S-H) in cementitious materials, and to further propose a conceptual model bridging the C-S-H characters with its volume change mechanisms in alkali-enriched systems. The proposed microstructural model is an extension of the colloidal model of C-S-H considering the effect of alkalis. It is suggested that the presence of alkalis makes C-S-H more thermodynamically unstable and structurally disrupted, and easier to reorganize and redistribute upon drying-induced internal stresses or external loading. In addition, new experimental results regarding the influence of lithium on shrinkage and micro-cracking of alkali-Activated blast-furnace slag are discussed. It shows that lithium addition can dramatically improve the cracking resistance and volumetric stability of alkali-Activated slag.