Improving the sustainability of the construction sector is now a major issue. In order to reach more eco-friendly buildings and structures, the use of cements which incorporate additions as clinker replacement has reached a great development. Furthermore, the use of lightweight aggregates has a great interest regarding the sustainability of construction industry. Among other advantages, they lead to a lessening in the density of cement-based materials, increasing their thermal resistance and producing a reduction of dead loads in the construction elements.

In this research, it has been studied the effects at early hardening ages of combining a sustainable cement with high content of ground granulated blast furnace slag (type CEM III/B) and different lightweight aggregates in the microstructure and durability of mortars. In these mortars, the 50% of the volume of the sand has been replaced by lightweight aggregates, such as natural cork, expanded cork and expanded clay. Reference mortars with an ordinary Portland cement (type CEM I) has also been prepared, as well as mortars without lightweight aggregates. The samples were kept submerged in water until the testing age. The microstructure has been studied using mercury intrusion porosimetry and scanning electron microscopy. Regarding the durability, the ingress of water is one of the main ways through which the aggressive substances can go into the materials. Therefore, in this work several parameters related to water absorption have been also determined. According to the results obtained, the incorporation of slag improved the performance of the mortars with the studied lightweight aggregates.