In-situ 4D synchrotron tomography of cementitious systems incorporating calcined clays: Microstructural insights

This study investigates the microstructural evolution of calcined clay-blended cement pastes using time-resolved Synchrotron X-ray tomography (Syn-CT), supported by XRD, R 3 reactivity tests, and compressive strength measurements. Pure kaolinite and pure montmorillonite, together with natural kaolinite and montmorillonite from Australia, were evaluated as supplementary cementitious materials (SCMs). High-resolution Syn-CT enabled realistic tracking of microstructural changes at the same sample location at 1, 7, and 28 days. Despite low Al₂O₃ content (2.15%) and low R 3 reactivity, natural montmorillonite reached ∼56 MPa strength due to the formation of hemicarboaluminate, resulting in a 70% reduction in interconnected porosity. Natural kaolinite achieved ∼54 MPa with 6.7% clay dissolution and 3.8% porosity. Pure montmorillonite had the lowest strength, correlating with minimal clay dissolution (∼3%). Natural montmorillonite exhibited a fine pore structure, with only 0.2% pores > 10 µm. Tortuosity-based analysis showed 70–75% reduction in interconnected pores across all mixes at 28 days, with the lowest in pure kaolinite due to a finer pore distribution around unreacted clinker. This study demonstrates that Syn-CT can provide repetitive scans at the same location (up to 28 days), accommodating synchrotron beam time constraints. It also highlights the viability of low-grade natural clays with low alumina content as SCMs.