The Influence of Mobility Parameters on the Rheological Behaviour and Mechanical Properties of Low-Carbon Mortar Mixtures

Environmental targets towards net-zero carbon concrete are increasing the demand for eco-efficiency in concrete production. Promising measures to increase sustainability include the combination of high levels of limestone fillers (LFs) and the use of advanced mix-design techniques, such as particle packing models (PPMs). However, there is still a limited understanding of the fresh and hardened state properties of eco-efficient mixtures; the literature suggests that mobility parameters (MPs; interparticle separation distance—IPS; maximum paste thickness—MPT) can help explain the fresh behaviour of concrete mixtures. Yet, the impact of MP values on fresh properties is still not fully understood. To address this gap, this study evaluates a reduced-complexity system comprising twelve concrete mortar fractions developed with distinct MP ranges and high LF contents (up to 52%). The use of mortar mixtures was intended to reduce the number of variables in the system and provide a clearer assessment of the role of mobility parameters. Time-dependent rheological behaviour (flow behaviour factor, torque, and viscosity) is analyzed and correlated with MP ranges to identify governing fresh state mechanisms. In addition, the relationships of IPS and MPT with compressive strength and porosity are evaluated to examine their relevance to the hardened state behaviour of low-carbon mixtures with reduced cement content. Results indicate that MPT and IPS can be used as practical indicators of rheological behaviour, with MPT showing the strongest influence on rheological response across all mixtures. Based on compressive strength and porosity measurements, empirical models are proposed to describe the effect of mobility parameter-based spacing concepts on hardened properties. Finally, the environmental performance of the optimized mixtures is assessed, confirming the potential of LF-rich, MP-tailored mixtures to contribute to low-carbon, net-zero concrete production.