Predicting the Alkali Contribution of SCMs to Concrete Pore Solution

Supplementary cementitious materials (SCMs) are often used in concrete to reduce the risk of alkali-silica reaction (ASR). This is primarily through dilution of the alkalis contributed by Portland cement and binding of alkalis as a result of pozzolanic reaction. Some SCMs, however, contain a high level of total alkalis, and these may be soluble, leading to increased pore solution pH and reduced efficiency in mitigating ASR. This may be especially problematic for many non-conventional and rapidly growing SCMs, such as volcanic ashes, marginal coal ashes, and ground glass, having total Na2Oeq > 3.0%wt. This paper presents a new soluble alkali test to quantify the soluble fraction of alkalis in SCMs. The test was applied to 14 SCMs, including coal ashes, volcanic ashes, calcined clays, and ground glass, and their alkali release was monitored for 90 days. The results are compared with ASTM C311’s “available alkali test.” Additionally, the pore solution of cement pastes made using 11 of these SCMs were extracted and analyzed over 90 days. The findings indicate that a considerable fraction of the total alkalis in SCMs is soluble in concrete’s pore solution. However, the pozzolanic reaction can bind most of the dissolved alkalis, resulting in a net alkali sink/reduction for the majority of the tested SCMs. A regression analysis of the results revealed that the pore solution [OH⁻] reduction by an SCM can be estimated based on the SCM’s pozzolanic reactivity, soluble alkali content, and Ca/(Si + Al).