Effective utilisation of ultrafine slag to improve mechanical and durability properties of recycled aggregates geopolymer concrete

Eco-friendly materials like recycled aggregates (RA) and geopolymer binders play a key role in the path towards sustainable construction. However, the literature suggests that the properties of recycled aggregates geopolymer concrete (RGPC) are inferior to normal concrete. The primary objective of this study is to improve the characteristics of recycled aggregates geopolymer concrete (RGPC) cured at ambient temperature by using low-emission binding agents and ultrafine slag (UFS). A total of twenty-four mixes were prepared in two series, in which natural aggregates (NA) were replaced by recycled concrete aggregates (RCA) at various replacement levels up to 100%, and the UFS inclusion varied from 0 to 30%. For Series I, UFS was used as a replacement to fly ash, whereas UFS was used as an addition to fly ash in Series II. The mechanical properties of all the mixes were satisfactory. Mix a-R100S30 with 100% RCA and 30% UFS addition showed the highest compressive strength of 46.24 MPa among all the mixes. The water absorption, chloride ion penetration, and electrical resistivity were also investigated. Mix r-R100S15 with 100% RCA in Series I exhibited the highest water absorption of 5.61%. In general, increasing the UFS content resulted in an overall improvement in the RGPC's characteristics, whereas increasing RCA% resulted in a degradation in the RGPC's performance. RCA formed a weaker zone in the matrix, whereas UFS served as a filler, increasing the availability of nucleation sites at the interfacial transition zone (ITZ) and resulting in a denser matrix structure. The results showed that a high percentage (up to 100%) of RCA with 30% of UFS can be incorporated into RGPC to provide sufficient strength for practical applications. Overall, with the UFS incorporation, the mechanical and durability properties of RGPC improved while its susceptibility to water and chloride attacks reduced.