Influence of aggregate type on conversion and strength in calcium aluminate cement concrete

Calcium aluminate cement (CAC) is gaining popularity in North America as a rapid repair material due to its ability to quickly gain strength, even at low curing temperatures. Use of CAC has been limited due to a lack of understanding of the process of conversion and the role of aggregates in CAC concrete. Conversion, which occurs only in 100% CAC systems, is a chemical process in which metastable hydrates convert into denser, stable hydrates. Presented is an examination of aggregate source impacts on this conversion process and converted CAC concrete strengths. Nine different concrete systems with fifteen varying aggregate sources were examined. Results indicated that carbonate limestone and siliceous limestone aggregate systems had significantly less strength reduction due to conversion compared to siliceous aggregate systems. Microstructural analysis of systems suggested that the carbonate limestone system had less porosity and better-formed aggregate/paste interfacial transition zones compared to the siliceous systems. Chemical analysis of the concrete pore solution indicated that the carbonate limestone system's pH and ionic concentrations of aluminum, sodium, and potassium were significantly higher than that of a siliceous system, indicating more dissolution of unhydrated cement in the carbonate limestone system. These studies are presented along with a proposed theory explaining the cause of the significant converted strength differences in CAC concrete systems made with limestone aggregates compared to siliceous aggregates.