Influence of selected mineral additives on properties of recycled aggregate concrete (RAC) considering eco-efficiency coefficients

The paper is focused on the eco-efficiency calculations of recycled aggregate concrete (RAC) consisted of ordinary Portland and blast-furnace slag cements with three fine-grained wastes: quartz powder (Q), limestone powder (L) and fly ash (F). The characteristics of concrete mixes (slump and air content) and hardened concrete properties: 28- and 90-day compressive strength, water absorption as well as the depth of the carbonation (after the accelerated carbonation in the chamber) were examined. In order to parameterize the third feature of contemporary concrete, which is (next to the strength and durability) its sustainability potential, the binder intensity (bi) and carbon intensity (ci) indexes were calculated, taking into account CO2 emissions from cement production and technological processes related to preparation for use of other ingredients. The comparison of the effect of limestone powder and fly ash showed some advantages of the fly ash (taking into account the results of strength tests). However, limestone powder turned out to be more favorable in influencing other properties (especially resistance to carbonation). Nevertheless, the use of fly ash proved to be the most effective in terms of eco-efficiency factors. The pozzolanic properties of F increased the strength of the concrete, reducing both bi and ci values. Furthermore, the presence of F canceled out the negative effect of quartz powder. The use of F together with cement containing ground-granulated blast-furnace slag (GGBS) was most beneficial. Although quartz powder due to technological reasons occurred to be the worst option, its use may be beneficial when CO2 sequestration by concrete carbonation is taken into account. Lower strength and resistance to carbonation of concrete with Q allow CO2 to penetrate deeper into the structure, especially in concrete made of GGBS cement. In this way, the later uptake of CO2 from the atmosphere allows for a partial reduction of the ci value, making RAC more eco-efficient.