Comparative study of the rheological behaviour and structural build-up of Portland cement, LC3 and natural hydraulic lime pastes

Alternative cementitious binders based on supplementary cementitious materials and natural pozzolans offer reduced clinker contents and, consequently, lower CO 2 emissions per unit mass of binder compared to ordinary Portland cement (CEM-I). Among these materials, limestone–calcined clay cement (LC3) typically exhibits low clinker factors, whereas natural hydraulic lime (NHL-5) is produced at kiln temperatures significantly lower than those required for Portland clinker production, resulting in substantially reduced embodied carbon emissions. In this study, the rheological behaviour of CEM-I, LC3 and NHL-5 pastes was systematically investigated using shear-rate-controlled rheometric protocols. Particular emphasis was placed on lime-based systems, which remain comparatively underexplored despite their relevance for repair mortars and heritage-compatible construction. Continuous ramp and ramp–step sequences were applied to characterize both steady-state flow behaviour and time-dependent structural reconstruction. The descending branches of the flow curves were analysed using the Herschel–Bulkley model. The results reveal distinct rheological signatures among the investigated binders. CEM-I pastes behave as weakly structured suspensions with limited thixotropy, whereas LC3 binders exhibit an intermediate response characterized by enhanced structural stability while maintaining measurable flowability under shear. In contrast, NHL-5 systems display strong interparticle interactions and pronounced structural build-up. These findings provide quantitative rheological data that can support mix-design strategies for sustainable cementitious systems at the paste scale, although extrapolation to mortar or concrete behaviour requires further investigation.