Alkali-Silica Reaction (ASR) Expansion: The Role of C-S-H Degradation in the Presence of Alkalis

Abstract

Sodium (Na) and potassium (K), both facilitate alkali-silica reaction in concrete causing expansions. However, the swelling mechanism remains unclear. The interchangeability of Na and K is questionable. This study adopted both alkalis respectively to examine their differences in microstructure and macroscopic expansions. The effect of alkalis on macro-µ-mechanical properties and the structural alterations of C-S-H are investigated. Results demonstrate that Na and K induce comparable aggregate cracking and produce amorphous Ca-deficient Na/K-rich gels within aggregate veins.

The expansion mechanism results from electrostatic repulsion between negatively charged silicate species. Na generates 2–3 times greater expansions. Na exposure significantly impairs the compressive strength and elastic modulus of hardened paste according to macroscopic and nanoindentation results. Mechanistic analyses suggest that Na facilitates greater C-S-H precipitation but causes shorter silicate chains and increases interlayer water content, as Na penetrates C-S-H interlayers. This difference explains the severer mechanical deterioration and greater expansion in Na-dominated samples.

Keywords: alkali-silica reaction (ASR), calcium silicate hydrate (C-S-H), sodium, potassium, mechanical degradation, expansion mechanism, cement hydration