•Taguchi L16 matrix was used to design the experiments with five factors and four levels.•TOPSIS method was used to optimize alkali-activated ladle slag mortar constituents.•Four quality characteristics were used in the multi-criteria optimization method.•Taguchi-TOPSIS integration can optimize alkali-activated ladle slag mortar mix design.•Optimum mix compressive strength, workability, and initial setting time were 17.9 MPa, 177.5 mm, and 13 min. This study examines the effect of various parameters on the properties of alkali-activated composites made with unprocessed ladle furnace slag. Taguchi method was used to design the experiments. A total of five factors, each with four levels, were considered, including ladle slag content (LS), alkaline-activator solution-to-binder ratio (AAS/B), sodium silicate-to-sodium hydroxide ratio (SS/SH), sodium hydroxide molarity (M), and crushed sand replacement by dune sand (CSR). A total of 16 alkali-activated ladle slag mixtures were designed, cast, and tested. The performance responses were the workability, setting time, and compressive strength. To assess the influence of the factors on the responses, Taguchi analysis and ANOVA were employed while determining the signal-to-noise (S/N) ratios. Further, TOPSIS analysis was carried out to optimize the mixture proportions of alkali-activated ladle slag composites. The optimum mix entailed 650 kg/m3 of ladle slag, AAS/B ratio of 0.45, SS/SH of 2, and CSR of 25% to maximize strength and workability. Meanwhile, the optimum mix to maximize workability and setting time included 650 kg/m3 of ladle slag content, AAS/B ratio of 0.6, SS/SH of 2.5, and CSR of 50%. Anticipated results of the optimum mixes were experimentally verified. Microstructure analysis of the optimum mixes (isothermal calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy) highlighted the accelerated rate of the activation reaction, the amorphous morphology, and the formation of calcium aluminosilicate hydrate gel.