Design optimization of a recycled concrete waste-based brick through alkali activation using Box- Behnken design methodology

Construction and demolition waste (CDW) generation is a major concern for the present decade owing to improperly managed disposal and re-utilization brought on by the rapid increase in the global infrastructure segment. The present study focuses on the utilization of recycled concrete waste powder as a precursor in geopolymer to design eco-friendly bricks. Three-factor and threelevels (33) designs were considered. Molarity (M) of NaOH solution, alkaline mix ratio (AMR) and curing temperature (CT) were the independent variables and compressive strength, water absorption, flexural strength, drying shrinkage, and efflorescence as response target values. The response surface method (RSM)- Box Behnken design (BBD) was used to test and develop a quadratic polynomial regression model, which suits the accuracy of the factual data. Multi-objective response optimiser and desirability analysis were carried out to identify the optimum levels. The optimum levels were 12 M, 2.32AMR, and 50 & DEG; C CT. Analysis of variance (ANOVA) reveals the molarity of NaOH solution was a significant parameter and contributed 50-70% in property performance enhancement. In addition, Scanning Electron Microscopy (SEM)- Energy dispersive spectroscopy (EDS), X-Ray diffraction (XRD), Fourier transforms infrared spectroscopy, and Thermal gravimetric analysis (TGA) was carried out to study morphology, mineralogy, bonding types, and thermal resistance of the brick mixes. Microstructural investigation divulges the presence of zeolites and calcium-silicate-hydrate gels in the brick mixes, which makes a hybrid geopolymer and boosts the alkali activation in the recycled concrete waste mix.