The Effects of Biomass Combustion Ash and Lignin on the Properties of Cement Mortars and Their Environmental Impact

Combustion and hydrolysis of lignocellulosic biomass generate renewable energy and biofuels, but also yield by-products, such as biomass combustion ash (BCA) and waste lignin (WL). This study investigates the reuse of these by-products in cement mortars, promoting circular economy principles and sustainable construction practices. The addition of BCA at 1–10% improved mortar consistency, homogeneity, and adhesion—most notably, formulations with 5–10% BCA increased adhesion to EPS by up to 4.3%, and compressive strength remained above the 20 MPa threshold. WL additions of 0.5–1% enhanced viscosity and adhesion to both mineral and EPS substrates, with a 0.2% WL dosage improving adhesion to EPS by 9.4% compared to the control sample. Life Cycle Assessment (LCA) confirmed a reduction in the carbon footprint by up to 14% (from 1509.5 to 1297.5 Mg CO2/year), while VOC emissions remained within acceptable limits. Leachability tests confirmed safe environmental performance. The results validate BCA and WL as functional and eco-efficient additives in cementitious composites suitable for thermal retrofitting.