Mechanical characterization of cement mortars and concrete with recycled aggregates from Coal Mining Wastes Geomaterials (CMWGs)

This paper presents the results of an extensive experimental campaign related to the use of Coal Mining Waste Geomaterials (CMWGs) as recycled constituents (fine and coarse aggregates) in Ordinary Portland Cement mortars and concretes. To this purpose, a reference mix and other mixes with different percentages of replacement of natural aggregates by CMWGs, up to 40% by volume, were investigated. CMWGs came from different providers: Central Mining Institute (GIG), POLTEGOR, both in Poland, and SUBTERRA in Spain and tests were performed at two different laboratories working on similar, but not identical, compositions. This represents a novelty in the literature, generally focusing on one single-source waste and single-lab results. The physical and mechanical properties of all the mixes were evaluated and correlated with respect to the percentage of replacement of natural aggregates by CMWGs. While the presence of CMWGs, likely because of their grain size distribution, reduced the porosity of mortars (decrease of 9.5 and 20.4% for 10 and 20% of replacement respectively) and concretes (70% reduction for concretes with 10% of fines and 30% of coarse aggregates replaced by CMWGs), the mechanical properties decreased when natural aggregates were replaced with CMWGs, likely because of the reduced strength of the CMWGs aggregates. This decrease was found to be roughly proportional to the percentage of replacement of aggregates (for instance, a 12–23% reduction of flexural strength in mortars with 25% replacement of sand and, a decrease of 25% in concretes for a 25% replacement of fine and coarse aggregates); nonetheless the concrete performance remained in the range of applicability for several civil engineering applications without affecting their functionality. In conclusion, the replacement of natural aggregates by CMWGs has resulted an interesting option for real applications providing an added value to the implementation of circular economy concepts in the management and up-cycling of coal mine tailings and CMWGs.