Hydration and mechanical properties of cement/sludge/anhydrite gypsum system

Setting and hardening of fresh blends of ordinary Portland cement (OPC), fired drinking water treatment sludge (DWTS) and calcined anhydrite gypsum were investigated. Sludge or Portland cement was substituted with 5 wt%, 10 wt% or 15 wt% of anhydrite gypsum. These systems lead to the preparation of supersulfated cement. These blends were shown to possess both the advantages of gypsum (early hardening, high early strength and enhanced workability) and Portland cement (improved durability in moist conditions). The initial and final setting time, bulk density and compressive strength of cement pastes were measured. Differential scanning calorimetry, X-ray diffraction and Fourier transform infrared spectroscopy techniques were used to investigate the hydration products. Substitution of DWTS with 15% anhydrite accelerates the final setting time and increases the bulk density as well as the compressive strength. Substitution of OPC with 5% anhydrite gives the same results but extends the final setting time. It can be concluded that supersulfated cement can be prepared from samples with 10-15 wt% anhydrite substituted DWTS or from the sample with 5 wt% anhydrite substituted OPC.