Influence of high-volume water treatment plant sludge powder on strength and microstructure of concrete

The calcined water treatment plant sludge powder (CWTS) was used to replace part of the cement to prepare high-volume sludge concrete, and the effect of the high-volume CWTS on the mechanical properties and microstructure of concrete was studied. The results show that although the high-volume CWTS is detrimental to the development of the 28-days compressive strength of concrete, 20wt% and 40wt% of the CWTS can significantly improve the 90-days compressive strength of concrete. The CWTS has pozzolanic reactivity and filling effect, which makes the 90-days pore structure of concrete mixed with 20wt%‒40wt%CWTS significantly refined, and the harmful pore volume (>1 µm) is significantly reduced. Meanwhile, it is observed from the nanoscale characteristics that adding 20wt%CWTS can significantly reduce the content of pore phase and unhydrated phase in the matrix, and increase the volume fraction of C—S—H phase, especially the high-density C—S—H phase. In addition, the addition of 20wt%CWTS can reduce the width of the interface transition zone (ITZ) by 20%, the addition of 40wt%CWTS and the control group (0wt%CWTS) have similar ITZ width. Therefore, using a large amount (20wt%‒40wt%) of CWTS to replace cement to prepare concrete not only has better economic and environmental benefits, but also benefits the improvement of its 90-days mechanical properties and microstructure.