Enhancement of sintered sludge ash-modified cement paste with CaSO4 and CaCl2

For the combination of ecological environment governance with cement industry upgrading, this paper seeks innovation by proposing a low-carbon cementitious material, in which CaSO4 and CaCl2 are used in combination with cement to solidify sintered sludge ash (SSA). Additionally, XRD, FTIR, TGA and SEM are used to explored systematically the mechanism of CaSO4 and CaCl2 enhancing SSA-modified cement paste from three perspectives: hydrate assemblage, hydration degree and micromorphology. The sludge activation system of "dryinggrinding-calcining at 850 CC-grinding" is adopted. High-temperature calcination makes little difference to the layered structure of the sludge, but the compactness of its surface improves and the content of amorphous Al2O3 increases significantly. Compared with the cement paste containing 50 % SSA, the addition of 1 % CaCl2 and 5 % CaSO4 shortens the initial setting time by 39.4 % and the final setting time by 31.5 %. Besides, the 28d-compressive strength increases by 32.6 % and the holes clearly observed are repaired effectively. Due to the competition between SO42- and Cl- for reaction with Al2O3, the number of Ettringite, Kuzel's salt and Friedel's salt is redistributed and Hydrotalcite-SO4 is generated in large amounts. The incorporation of CaSO4 and CaCl2 performs better in improving the early hydration degree of SSA-modified cement paste, with an increase of 30.1 % and 16.0 % in 7 and 28 days, respectively. The microstructure is transformed from porous matrix with microcracks to the dense grid formed by overlapping C-S-H, resulting from pozzolanic reaction of SSA, Ettringite expansion and aluminate filling in combination. Additionally, the enhanced SSA-modified cement paste demonstrates a lower embodied CO2 index and a lower integrated cost index than ordinary cement.