Synergic effect of recycled aggregate, fly ash, and hydrated lime in concrete production

The use of construction and demolition waste (CDW) as recycled aggregate (RA) and fly ash (FA) in the composition of cementitious materials can make the resulting concrete more sustainable. However, RA is more heterogeneous, porous, and less dense than the natural aggregate (NA), influencing the mechanical strength. On the other hand, the reaction of FA with hydration products of Portland cement is slow, delaying the strength development at an early age. An alternative to increase the strength of concrete with FA at an early age is using the hydrated lime (HL). This research intends to evaluate the efficiency of adding HL in concrete with RA and FA. In this way, this research intends to evaluate the efficiency of adding HL in concretes with RA and FA and comprises two stages. First, a w/c ratio of 0.55 and a replacement of 50% natural coarse aggregate by concrete recycled aggregate (CRA) and mixed concrete aggregate (MRA) was established, considering similar compressive strengths. In the second stage, the properties of concretes with 20% FA in partial replacement of Portland Cement (PC), adding HL at 5% and 10% (by mass) were evaluated. The results showed that concretes with CRA present performance better than concretes with MRA, and the effect of HL was significant for all properties investi-gated. Concretes with CRA, FA, and 10% HL improved the compressive strength by 12.47%, reduced water absorption and porosity by 9.87% and 23.82%, respectively, and increased apparent density by 8.77% compared to the mixture without HL. The SEM analysis showed the beneficial effect of HL in concretes with RA, due to the high availability of calcium hydroxide. For concretes with RCA, a reduction in the mean equivalent diameter of 62.29% was verified; for concretes with MRA, this reduction was 23.25%. A reduction of carbonation depth was observed when concretes with MRA, FA and 10% HL showed a carbonation coefficient 44.5% lower than concrete without HL after 35 days of exposure to CO2, while concretes with CRA presented a carbonation coefficient of 87.7% lower.