Calcined clays for climate neutral (“net zero”) cements: shear-dependent rheological behavior and application performance

The performance of low-carbon cements prepared from different neat calcined clays (CCs) can be very inconsistent due to substantial variations in the composition of the CC samples. To gain a better understanding and further promote the practical application of such low-carbon cements incorporating CCs, the influence of calcined clays possessing different mineralogical compositions on the rheological behavior of the blended cements was investigated. First, the shear-dependent rheology of pastes preparing from four types of CCs (1:1 and 1:2 type) blended with OPC at 70:30 and 30:70 wt./wt. ratios was elucidated. As it is well established that mortar tests are more representative for the behavior of actual concrete than cement paste, the rheological properties of OPC/CC mortars were evaluated via spread flow, flow line and V-funnel empty time tests. An industrial precast type HPEG (methallyl ether) PCE and a non-ionic co-dispersant were employed as dispersants. It was found that incorporation of CC greatly influences packing density and water film thickness of the composite cements. Moreover, performance of the blended cements significantly depends on the physio-chemical properties of the individual CCs, e.g. their particle size distribution, morphology and surface chemistry. To elucidate the underlying mechanism, PCE adsorption and surface charge (zeta potential) of the CC composite cements were determined, and a correlation between effectiveness of the PCE and its adsorbed amount per BET specific surface area of the binder was revealed.