Hydration, Soundness, and Strength of Low Carbon LC3 Mortar Using Waste Brick Powder as a Source of Calcined Clay

The construction industry is responsible for 39% of global CO2 emissions related to energy use, with cement responsible for 5–8% of it. Limestone calcined clay cement (LC3), a ternary blended binder system, offers a low-carbon alternative by partially substituting clinker with calcined clay and limestone. This study investigated the use of waste clay brick powder (WBP), a waste material, as a source of calcined clay in LC3 formulations, addressing both environmental concerns and SCM scarcity. Two LC3 mixtures containing 15% limestone, 5% gypsum, and either 15% or 30% WBP, corresponding to clinker contents of 65% (LC3-65) or 50% (LC3-50), were evaluated against general purpose (GP) cement mortar. Tests included setting time, flowability, soundness, compressive and flexural strengths, drying shrinkage, isothermal calorimetry, and scanning electron microscopy (SEM). Isothermal calorimetry showed peak heat flow reductions of 26% and 49% for LC3-65 and LC3-50, respectively, indicating a slower reactivity of LC3. The initial and final setting times of the LC3 mixtures were 10–30 min and 30–60 min longer, respectively, due to the slower hydration kinetics caused by the reduced clinker content. Flowability increased in LC3-50, which is attributed to the lower clinker content and higher water availability. At 7 days, LC3-65 retained 98% of the control’s compressive strength, while LC3-50 showed a 47% reduction. At 28 days, the compressive strengths of mixtures LC3-65 and LC3-50 were 7% and 46% lower than the control, with flexural strength reductions being 8% and 40%, respectively. The porosity calculated from the SEM images was found to be 7%, 11%, and 15% in the control, LC3-65, and LC3-50, respectively. Thus, the reduction in strength is attributed to the slower reaction rate and increased porosity associated with the reduced clinker content in LC3 mixtures. However, the results indicate that the performance of LC3-65 was close to that of the control mix, supporting the viability of WBP as a low-carbon partial replacement of clinker in LC3.