The influence of metakaolin-to-vaterite ratio on the properties of vaterite-calcined clay cement: fresh mortar rheology, microstructure, and mechanical performance

This study investigates the influence of the metakaolin-to-vaterite (MK/V) ratio on the fresh properties, mechanical performance, and microstructural development of vaterite-calcined clay cement (VC 3 ) mortars. Eight mixtures were produced with MK/V ratios between 1.0 and 3.0, alongside a limestone-calcined clay cement (LC 3 ) reference. Fresh- and hardened-state behaviors were assessed using flow-table testing, rheological characterization, setting-time measurements, isothermal calorimetry, compressive strength testing, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. Replacing calcite with vaterite enhanced flowability and reduced both static and dynamic yield stresses due to the spherical morphology of vaterite. Increasing metakaolin content, however, progressively elevated yield stresses and thixotropy. Vaterite addition slightly prolonged setting time relative to LC 3 due to the high surface-related adsorption capacity of vaterite for calcium and sulfate ions, although higher MK/V ratios accelerated hydration and offset this delay. Early-age compressive strengths were comparable to LC 3 , while all VC 3 mixtures exceeded LC 3 performance after 7 days. Strength increased with MK/V ratio up to an optimum range of 2.0-2.5, where a maximum improvement of 42% at 91 days was recorded. Thermogravimetric and XRD analyses showed efficient portlandite consumption and the formation of a multiphase assemblage dominated by C-(A)-S-H and AFm-type carboaluminate phases. SEM-EDS confirmed the development of a dense, chemically complex matrix underpinning the superior strength performance. Overall, the MK/V ratio was found to regulate the interplay between rheology, hydration, and microstructure, identifying 2.0-2.5 as the optimal range for balanced performance. Given that vaterite can be produced via carbon-negative synthesis routes that permanently store captured CO 2 , ternary samples including metakaolin emerge as a technically robust and environmentally beneficial alternative for next-generation low-clinker cements.