Influence of low carbon cement and recycled aggregates on mortar fresh state and early hydration

Calcined clay and limestone are promising clinker substitutes for production of new low carbon cements. Limestone calcined clay cement (or LC3) can reduce carbon dioxide emissions by up to 40% and are made of raw materials which are abundant and globally available. Many studies have already examined the hydration behavior of calcined clay-based cements and linked it to early- and late-age strength development. However, very few works were dedicated to the analysis of mortars prepared with this type of binder and recycled sand. The introduction of construction and demolition waste as aggregate in combination with low-carbon cement would further decrease the environmental impact of building materials. In this paper, the fresh state of mortar containing recycled sand and calcined clay in variable amounts was analyzed by means of flow table test. The pore size distribution of fresh mortar was identified using 1H Time Domain Nuclear Magnetic Resonance (TD-NMR), that is a low field TD-NMR approach. The compressive strength of samples was also measured. Results show that mortar containing calcined clay immediately consumes part of the available capillary water with drastic consequences on its rheological behavior. Moreover, the recycled sand influences fresh properties and mechanical compressive strength. However, proper mix designs obtained by substituting the natural sand with recycled sand according to the occupied volume proved that mortar composed by low-carbon cement and recycled aggregate is still suitable for many applications contributing to the sustainability of construction materials.