Effect of Alkalis on Cementitious Materials:Understanding the Relationship between Composition, Structure, and Volume Change Mechanism

Abstract

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.