Understanding the Carbonation Performance of Cements Containing Calcined Clay

Low-purity calcined clays are becoming increasingly popular as supplementary cementitious materials (SCMs) due to their wide availability, and potential ability to reduce the carbon footprint associated with concrete production. To ensure the longevity of concrete structures, it is crucial to understand the mechanisms governing long-term durability when using new SCM-containing cement formulations. Understanding of the carbonation resistance of cements containing calcined clay is limited, and this remains a concern. This research is part of the collaborative USA-UK project “Response to CO2 exposure of concrete with natural supplementary cementitious materials” (RENACEM), aiming to understand the connections among the properties of natural clays, activation treatments to enhance their chemical reactivity, and the response to CO2 exposure of cements, mortars and concretes produced with them. The current study presents the carbonation resistance results of binary and ternary materials containing calcined clays upon exposure to natural CO2 concentrations under controlled relative humidity (57% RH), resembling in-service conditions. Four cement compositions were studied, including a CEM I (OPC), CEM I with 30% limestone substitution (L30), CEM I with 30% calcined clay substitution (CCF30) and CEM I with 30% calcined clay + 15% limestone (CCF30L15). The carbonation performance of these binders was monitored using pH indicators on cement paste and mortar specimens. Scanning electron microscopy coupled with EDX was used to identify the carbonation front. The results demonstrate that chemical alterations identified using analytical techniques can be used to characterise the reaction front of these materials, offering more insights into the effect of carbonation beyond potential changes in alkalinity of these systems.