Performance and microstructure of sintered sludge ash modified cement paste

Sintered sludge ash (SSA) has garnered attention for its potential to mitigate carbon emissions by replacing cement. This study investigates the consequential changes in the macro and micro properties of cement paste following the addition of SSA. Fluidity tests unveiled that the high specific surface area of SSA was the primary factor contributing to the reduction in fluidity and the delayed setting time. In terms of strength, SSA affected the cement paste to a greater extent at 3 days (3 d) than at 28 days (28 d). X-ray diffraction (XRD) analyses indicated that hydration-inert components such as quartz and feldspar in SSA may be responsible for the observed early hydration retardation. For 28 d strength, thermogravimetric and differential scanning calorimetry (TG/DSC) were employed to quantify Ca(OH)2 content and the degree of cement hydration. It was found that the participation of SSA in the pozzolanic reaction could additionally produce calcite, which promoted hydration and improved the strength of cement paste at 28 d. Scanning electron microscopy (SEM) revealed that amorphous hydration products, including C-S-H and Ca(OH)2 crystals covering the SSA surface, formed a robust structure conducive to 28 d strength development. Results from mercury intrusion porosimetry (MIP) were found to be in good contrast with SEM.