Packing density of limestone calcined clay binder

The partial substitution of Portland cement by a mixture of calcined clays and limestone fillers offers a promising way to reduce the environmental impact of concrete while improving its long-term mechanical performance and durability properties. However, the literature shows that calcined clays can display a higher water demand than Portland cement, which implies the use of additional mixing water or superplasticizer to maintain appropriate workability. This study aims to determine the optimal proportion of ternary mixes, i.e. cement, calcined clays and limestone fillers, in order to obtain the highest possible packing density influencing the water demand. The associated experiments were conducted with one CEM I, two metakaolins (calcined clay) and three limestone fillers. Optimization was performed using the Compressible Packing Model of granular mixtures, which notably considers the grain size distribution and virtual packing densities (βi) of the various materials. In conjunction with this model, the tests carried out on pastes of normal consistency controlled by the Vicat apparatus or the spread of mortars show that the type and amount of metakaolin used have a high impact on compactness given their particularly low virtual packing densities (βi). Moreover, this study demonstrates that the addition of limestone fillers improves packing density, but the grain size distribution proves not to be the key parameter, especially in mortars. Lastly, when the water demand of the metakaolin is high, a large limestone proportion (30% vol.) allows significantly enhancing the spread and manufacturing mortars with a compressive strength of close to 32.5 MPa at 28 days.