Accelerated Alkali–Carbonate Reaction in Aggregates

This study develops and evaluates an accelerated testing method to assess alkali-carbonate reaction (ACR) activity in dolomitic aggregates, a critical step in preventing long-term concrete damage. The research involved a comprehensive analysis of the ACR and potential alkali-silica reaction (ASR) activity of six different types of dolomitic limestone aggregates sourced from quarries in Egypt. This was performed using a novel protocol and compared against established standards, including RILEM AAR-2, RILEM AAR-5, and ASTM C1105. Aggregate samples were prepared in two size fractions, 2.5–5 mm and 5–10 mm, and were subsequently cured in a 1 mol/L tetramethylammonium hydroxide (TMAH) solution at elevated temperatures of 60°C and 80°C. The use of a TMAH solution was a key methodological choice, as it has been reported to react with dolomite while remaining inert with the reactive silica phases common in ASR, thereby isolating the effects of ACR-induced expansion.The study systematically investigated the impact of aggregate particle size and curing temperature on the expansion behavior of concrete microbar specimens to assess their alkali-carbonate reactivity. The results indicated that the specimens fabricated from larger 5–10 mm aggregates and cured at the higher temperature of 80°C exhibited the most significant and rapid expansion, exceeding a preliminary threshold of 0.1% after 42 days. While this value requires further validation, it is proposed as a potential indicator for rapidly identifying ACR activity in aggregates.To substantiate these findings, microstructural analysis was conducted using a scanning electron microscope (SEM) combined with energy-dispersive X-ray spectroscopy (EDS). The analysis of the reaction products confirmed that dolomite crystals within the aggregates reacted with the TMAH solution, resulting in the formation of calcite (CaCO₃) and brucite (Mg (OH)₂), the characteristic products of the dedolomitization process. No evidence of ASR gel was found, confirming the selectivity of the test method. These findings support the viability of the accelerated TMAH method as a rapid and specific tool for evaluating the ACR potential of carbonate aggregates.