Title:
A New Test for Measuring Soluble Alkalis in SCMs and Predicting Their Influence on Concrete Pore Solution
Author(s):
Rajabipour
Publication:
Web Session
Volume:
Issue:
Appears on pages(s):
Keywords:
DOI:
Date:
3/30/2025
Abstract:
Supplementary cementitious materials (SCMs) are commonly used in concrete to reduce alkali-silica reaction (ASR), primarily by diluting and binding alkalis. However, high-alkali or high-calcium SCMs (e.g., volcanic ash, ground glass, class C ash) may be less effective or ineffective against ASR depending on how they impact the composition of concrete pore solution. This presentation introduces a new test method to measure the soluble fraction of SCM alkalis at pH values relevant to concrete. The test was applied to 14 SCMs (volcanic ashes, calcined clays, coal ashes, and ground glass), with alkali release monitored over six months and compared with the total and ASTM C311 Available alkali contents of each SCM. Cement pastes containing these SCMs were prepared, and their pore solution was extracted and analyzed over a one-year period. Thermodynamic modeling and statistical analyses were used to predict the pore solution composition of pastes as a function of mixture proportions and the properties of the cement and SCM. The results show that a significant portion of the SCM alkalis is soluble. However, the impact of SCM on concrete’s pore solution, and the dosage needed for ASR mitigation, is a function of the SCM bulk chemistry (CaO, SiO2, Al2O3 contents) and pozzolanic reactivity, in addition to its soluble alkali content.