Formulation of eco-friendly rendering mortars based on new alternative raw materials: performance and microstructure

The cement-based construction materials industry needs to adapt its formulations and production processes to the new paradigm characterized by sustainability, circularity and decarbonization. In this context, one of the strategies proposed by the prepared mortars industry is to seek new alternative recycled raw materials derived from industrial by-products. This paper analyses the feasibility of using mixed recycled sand (0/2 mm) derived from construction and demolition waste as a 100 % replacement for natural sand in the formulation of prepared rendering mortars. It additionally explores its concurrent use with mixed recycled ultra-fine fractions (<0.075 mm) of construction and demolition waste and/or biomass bottom ash collected from power plants. The by-products were characterized in terms of their physical, chemical and mineralogical properties. The formulations of the new eco-friendly mortars (Mix O, Mixes A–D) were then adjusted, and their properties were assessed in their powdered, fresh, and hardened states. The results indicate that incorporating these new raw materials does not adversely affect their consistency (162–166 mm) or fresh-state density (634–678 kg/m 3 ). In addition, the new eco-friendly mortars (Mixes A–D) exhibit greater compressive and flexural strength than the conventional mortar (Mix O), presenting increases in these values at 90 d ranging from 8.6 to 23.7 % and 2.4–26.2 %, respectively, versus Mix O. The formulated mortars meet the criteria of category CS I mortar (0.4 MPa ≤ f cm ≤ 2.5 MPa), as per standard EN 998-1. Additionally, the pore structure of the eco-friendly mortars differs from that of Mix O: the volume of pores with Ø ≥ 50 μm increases when only CDW m is added and decreases when both mixed recycled sand and ultra-fine fractions are incorporated. Based on the results obtained, it can be concluded that all the formulated eco-friendly mortars would, a priori, be suitable for use as prepared rendering mortars in construction and civil engineering. Highlights • Use of by-products (CDWm, CDWmf and BWf) in rendering mortar manufacture. • Concurrent use of recycled fractions optimizes performance and microstructure. • Eco-friendly rendering mortars are classified in category CS I. • The incorporation of ultra-fine recycled fractions decreases of pores Ø≥ 50 μm. • The CDWm integrates into the cementitious matrix more effectively than the ANm.