Mechanical properties and micro mechanism of alkali-activated tannery sludge/fly ash composite cement-based recycled concrete

In order to alleviate the environmental degradation and resource waste caused by cement production and construction waste disposal, some cement is replaced by tannery sludge and fly ash, and natural aggregate is replaced by recycled coarse aggregate. The preparation of alkali activated tannery sludge/fly ash composite cement-based recycled concrete (AA-SFRC) is of great significance for the safe disposal and utilization of solid waste. In order to study the mechanism of the effect of alkali-activator on the mechanical properties and microstructure of AA-SFRC, six kinds of AA-SFRC with different alkali equivalents (AE) were designed, and the strength tests, nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were carried out. On this basis, the influence of AA-SFRC pore structure parameters on strength performance was discussed by using grey entropy analysis method, and a grey model GM (1, 5) prediction model of 28d strength was established. The results demonstrated that with the increase of AE, the AA-SFRC strength increased at first and then decreased, and the tension-compression ratio (Ts/Cs) remained in the normal range, between 0.065 and 0.081. When the AE was 9%, the mineral composition of AA-SFRC reached equilibrium, and the hydration reaction was the most sufficient. The compressive strength and splitting tensile strength reached the maximum. Meanwhile, the pore structure showed that the porosity and the proportion of macro pores were the smallest. The 28d strength of AA-SFRC was greatly affected by its 7d strength, first peak ratio, fractal dimension Dmax and porosity. The average relative errors between the predicted value of GM (1, 5) model and the experimental value were less than 0.03%. This study can provide a reference for the resource utilization of tannery sludge, fly ash and construction waste.