Improving the engineering properties of sustainable recycled aggregate concrete modified with metakaolin

This study investigates the potential of metakaolin as a mineral admixture to enhance the performance of concrete mixtures with varying proportions of recycled aggregates (RA). Sixteen distinct concrete combinations were formulated by substituting natural aggregates (NA) with RA at 0 %, 50 %, 75 %, and 100 % and incorporating metakaolin at 0 %, 5 %, 10 %, and 15 % as a replacement for cement. Experimental investigations were conducted to evaluate the effects of metakaolin (as a cement replacement) and RA (as a replacement for NA) on key concrete characteristics, including density, water absorption, compressive and tensile strength, and acid resistance. The results revealed that an increase in RA reduced mechanical properties, such as compressive strength, tensile strength, and dry density. Additionally, RA-based concrete mixtures exhibited higher water absorption and void volume than those with natural aggregates. However, including metakaolin in RA-based concrete significantly reduced the void volume and water absorption. The optimal proportion of metakaolin substitution was 15 %, resulting in the best overall performance for RA-based concrete mixtures. Notably, the highest compressive and splitting tensile strength (41.6 MPa and 4.3 MPa, respectively) were observed at 90 days in mixtures with 0 % RA and 15 % metakaolin. Similarly, the highest hardened density and lowest water absorption (2494.7 kg/m3 and 4.7 %, respectively) were recorded in samples with 0 % RA and 15 % metakaolin. Furthermore, concrete mixes containing 15 % metakaolin and 50 % RA demonstrated comparable qualities to conventional concrete, indicating that sustainable concrete can be produced by utilizing a significant quantity of waste concrete (15 % metakaolin and 50 % RA) without compromising strength criteria. This study sheds light on the potential of incorporating metakaolin and recycled aggregates as environmentally friendly alternatives in the construction industry, contributing to sustainable and eco-conscious practices.