Preparation and performance optimization of fly ash- slag- red mud based geopolymer mortar: Simplex-centroid experimental design method

Abstract

Cement production emits a large amount of CO2 into the atmosphere and consumes significant mineral resources, necessitating urgent green transformation in this traditional industry. To address carbon emissions in the building materials sector and effectively utilize industrial solid waste, this study presents the preparation of geopolymer mortar using a ternary mixture of fly ash (FA), slag (GGBS) and red mud (RM), making full use of the advantages of each component to optimize the mix proportion. The mix proportions are determined using the simplex-centroid experimental design method, with NaOH concentration (8 M, 10 M, 12 M) as the independent variable, aiming to meet multi-objective performance requirements including fluidity, setting time, flexural strength, and compressive strength. The microstructure and composition of hydration products are examined through scanning electron microscopy (SEM) and X-ray diffraction (XRD). Finally, the optimal mix proportion range is proposed. Results indicate that: (1) When the concentration of NaOH increases, the setting time is shortened, and the mechanical properties first increase and then decrease. The optimal alkali concentration is 10 mol/L. (2) RM and FA exhibit similar effects on mechanical properties, suggesting RM can replace FA in geopolymer material preparation. Besides, RM significantly reduces fluidity. (3) GGBS enhances mechanical properties and shortens setting time. The increase of GGBS content provides more CaO for the matrix and generates more compact C-S-H gel, and thus, the flexural strength and compressive strength of the geopolymer mortar are improved. When GGBS blending amount is 50 % and NaOH is 10 M, the flexural strength and compressive strength of mortar can reach 15.66 MPa and 69.06 MPa, respectively. (4) The simplex-centroid experimental design method provides a mix proportion design method that can meet the requirements of multiple properties at the same time.