Sustainable concrete manufactured with mixed recycled aggregate and biomass bottom ash, replacing aggregate and cement to a high percentage

Abstract

Given the current problem of CO2 emissions and the overexploitation of natural resources, it is essential to make a change in the materials typically used for concrete manufacturing. The aim of this research is to establish the scientific basis for the manufacture of concrete with sustainable materials, achieving strengths and durability results comparable to conventional concrete. Alternative materials include biomass bottom ash (BBA) and mixed recycled aggregate (MRA), which replace both aggregate in different percentages and cement. Eleven concrete mixes with different degrees of substitution were manufactured, dividing to set into two series. Series 1 included CEM II/A-L 42.5 concrete mixes with different proportions of both natural aggregates and recycled aggregates. Series 2, which included cement and aggregate substitutions, compared with a CEM II/B-L 32.5 control mix, as the substitution of cement by recycled powders is 25 %. Physico-chemical characterisation, mineralogy and leaching studies were carried out to determine the suitability of the materials to be included in the cementitious matrices. For the technical evaluation of the manufactured concrete mixtures, mechanical strength studies (compression, flexural, indirect tensile and modulus of elasticity), durability studies (density, water absorption, porosity, dimensional changes, freeze/thawing) and microstructure studies (ITZ and SEM maps) were carried out. The mixes containing the Eco-Hybrid alternative cement (EcHy), composed of 75 % clinker, 19 % MRA powder (pMRA) and 6 % BBA powder (pBBA), achieved strengths results of 97 % at 28 days of the control mix with CEM II/B-L and natural aggregate and, at 90 days, the mechanical strength of the alternative concrete containing the EcHy cement exceeded the strength of the conventional concrete mix. The replacement of natural aggregate by recycled byproducts also showed good results with respect to mechanical strength. Substitutions of 50 % natural aggregate achieved strength results of 86 % of the strength of the control concrete mix. In the long term (strengths measured at 360 days) the mixes that included BBA in both cement and aggregate substitution showed a considerable increase in mechanical performance, increasing their strength at 360 days by between 20 % and 25 % of the strength achieved at 28 days.