A New Soluble Alkali Test for Supplementary Cementitious Materials

Supplementary cementitious materials (SCMs) are often used in concrete to reduce the risk of ASR, primarily through alkali dilution and binding. However, some SCMs also contain a high level of alkalis, and these may be soluble in the pore solution of concrete, leading to increased OH^- concentration and ASR risk. This may be especially problematic for many non-conventional and rapidly growing SCMs, such as volcanic ashes, marginal coal ashes, and ground glass, with Na₂O_eq > 3.0%wt. This paper presents a new soluble alkali test to quantify the soluble fraction of alkalis in an SCM. The test was applied to 14 SCMs, including volcanic ashes, calcined clays, coal ashes, and ground glass, and their alkali release was monitored for six months. The results are compared with ASTM C311’s Available Alkali Test method. The pore solution compositions of cement pastes containing the SCMs were also analyzed over one year. 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 for a majority of the tested SCMs. Additionally, regression analysis revealed that the reduction in [OH^- ] of the pore solution of concrete by using SCM is significantly dependent on the SCM’s soluble alkalis, pozzolanic reactivity, and Ca/(Si + Al) oxide ratio.