Role of the alkalis of supplementary cementing materials in controlling pore solution chemistry and alkali-silica reaction

Abstract

Declining supplies of quality fly ash in many parts of the world necessitates a search for new sources of supplementary cementing materials (SCMs) to meet the growing demand for sustainable materials to partially substitute for portland cement in concrete. In recent years finely-ground waste glass, landfilled/ponded coal ashes and natural pozzolans have (re)drawn the attention of researchers and the industry to fill this gap. In this study a wide range of SCMs were characterized for water-soluble and available alkalis using, respectively, ASTM C114 and ASTM C311. Pastes containing portland cement and SCM were prepared at a 25 % replacement level and cured in airtight containers at 23 °C. The pore solution of these pastes at 28 and 91 days was analyzed for the concentration of alkali (Na+ and K+) and hydroxyl (OH−) ions. The efficiency of SCMs in mitigating alkali-silica reaction (ASR) was determined using the Pyrex mortar bar test, ASTM C441. The results indicate that all alkali oxides of SCMs are neither water-soluble nor “available.” Many natural pozzolans contain a large portion of insoluble alkalis. Except for high-alkali (Type GS) ground glass and Class C fly ash, the use of all other reactive SCMs considerably decreases the presence of alkali ions in the pore solution at 91 days. Compared to available alkalis in C311, pore solution concentration provides a better indication of the efficacy of SCMs in mitigating ASR.