Recycling hazardous water treatment sludge in cement-based construction materials: Mechanical properties, drying shrinkage, and nano-scale characteristics

Abstract

Water treatment sludge (WTS) management is a growing global problem, which is often disposed as a hazardous material in landfill space. This paper aims to use modified WTS as a novel supplementary cementitious material, by determining the effects of calcined WTS addition on the composition and performance of Portland cement composites. The effect of WTS to replace cement partially by weight on the mechanical properties and drying shrinkage of specimens has experimentally been studied, and the internal microstructure is also determined by scanning electron microscope and nanoindentation techniques. The results showed that mortar with 10% modified WTS presented higher 90-day compressive strength although specimens with 20% and 30% modified WTS showed 10.54% and 16.20% reduction compared to the control group. Meanwhile, the modified WTS has pozzolanic activity and can react with cement hydration products to increase the compactness of the microstructure, thereby reducing the drying shrinkage of the pastes (10% modified WTS). When 10% of the cement was replaced by modified WTS, the volume fraction of unhydrated clinker and pore phase in the matrix decreased significantly, while the volume fraction of C–S–H phase increased. In addition, the interface transition zone (ITZ) width of the specimen (10% WTS) is the smallest, and as the replacement rate of modified WTS increases, the ITZ width of the specimen increases significantly. This paper proves that the modified WTS, as a supplementary cementitious material, can be considered a feasible and sustainable alternative for use in the building materials.