Towards a test method for measuring soluble alkalis in supplementary cementitious materials

Abstract Supplementary cementitious materials (SCMs) are often used in concrete to reduce the risk of alkali-silica reaction (ASR), primarily through alkali dilution and binding. However, the effectiveness of SCMs with high alkali content (e.g., Na 2 O eq  > 3.0 wt.%) in ASR mitigation is of concern, as these alkalis may dissolve in concrete’s pore solution and elevate its pH. This study proposes and preliminarily evaluates a test method to quantify the soluble alkalis in SCMs. The method was applied to 14 SCMs, including natural pozzolans, coal ashes, and ground glass, to monitor their alkali release over a 180-day period. The test involves measuring soluble sodium in 1 M KOH host solution and soluble potassium in 1 M NaOH solution. Thermodynamic modeling confirmed that most released alkalis remain in solution, with minimal solid precipitation and alkali binding. Findings revealed that a significant portion of alkalis in SCMs is soluble, often exceeding the level identified by ASTM C311 Available Alkali Test.