The effects of hardening accelerator on microstructure and mechanical performance of fly ash-cement-based materials using response surface methodology

To enhance the engineering application range of fly ash-cement-based materials (FCM), the combined effects of hardening accelerator on FCM were investigated by using response surface methodology (RSM). Based on a three-factor three-level Box-Behnken design (BBD), the linear, quadratic and interaction relationships between the dosages of calcium chloride (CaCl 2 ), aluminum sulfate (Al 2 (SO 4 ) 3 ) and triethanolamine (TEA) on the primary properties (setting time, mechanical properties and shrinkage rate) of FCM were discussed. The prediction model was established to determine the optimal ratio of the composite system . The microscopic performance and the mineral fraction of the paste were explored by using X-ray diffraction (XRD), thermogravimetric differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). Results indicated that the paste performance was affected by the interaction of multiple factors, and the interaction between CaCl 2 and Al 2 (SO 4 ) 3 was the most significant. The optimum ratio of paste was 2.81 % CaCl 2 , 0.75 % Al 2 (SO 4 ) 3 and 0.12 % TEA, achieving a 28 d compressive strength of 40.48 MPa, closely matching the predicted value. The dominant hydration products of the composite cementitious system were C–S–H gel, AFt and Friedel's salt, with changes observed in the morphology of CH. With the prolong of curing age, the hydration products and unhydrated fly ash particles were bonded to each other, so that the slurry maintains a certain strength performance.