Synergistic Effect of Metakaolin Waste and Nano-Silica on the Properties of Ultra-High-Performance Concrete

This article investigates the synergistic effect of metakaolin waste (MW) derived from the production of expanded glass granules and nano-silica (NS) on the hydration and other properties of ultra-high-performance concrete (UHPC) reinforced with steel fibres. The study focusses on cases where 5%, 10%, or 20% of cement is replaced with MW and 1% of NS is added. Various properties are evaluated, including the exothermic temperature, mineral composition (XRD), relative main compound quantities according to their decomposition (TG and DTG), shrinkage, density, and flexural and compressive strengths after 7 days, 28 days, and 2 years. In addition, changes in the concrete microstructure are analysed after 28 days and 2 years. The results demonstrate that the combined addition of MW and NS accelerates hydration by about 3 h compared to the control sample. The TG results confirmed a lower portlandite content due to the dilution effect of cement replacement. However, when both additives were used simultaneously, the portlandite content decreased further because of the intensified pozzolanic reaction, while the amount of C–S–H increased. Using MW and NS together significantly enhanced the long-term strength of concrete: after 2 years, the compressive strength of the mix with 5% of cement replaced by MW and 1% of NS was 182 MPa, compared to 146 MPa for the control sample.