Recycling incinerated sewage sludge ash (ISSA) as a cementitious binder by lime activation

Incineration is used to manage sewage sludge in many countries and regions including Hong Kong. This generates a huge amount of incinerated sewage sludge ash, the disposal of which is an environmental issue. In this research, an eco-friendly cementitious binder was developed by adding 10, 20, and 30 wt % of lime into sludge ash to obtain different lime/sewage sludge ash ratios. The same amounts of ordinary Portland cement were also added to the equivalent batches of sewage sludge ash for comparing the two systems. Paste samples were characterised for heat of hydration, mechanical properties and thermogravimetric analysis. Microstructural analysis using X-ray diffraction and scanning electron microscopy and the factors controlling strength development are reported. The results show that sewage sludge ash accelerated the hydration rate of cement, and the lime pastes with sewage sludge ash showed larger amounts of heat and higher reactivity than the cement pastes with sewage sludge ash. Considering the lime-based binder was mainly proposed for the production of controlled low strength materials, within the lime system the optimum mechanical properties (compressive strength) were achieved by the 30% lime with sewage sludge ash mix and the strength value showed remarkable improvement from 28 to 90 days of curing. The crystalline phases responsible for the strength development in the lime-based system were mainly brushite and calcium phosphate hydrates. The lime with sewage sludge ash mix has potential to be used for the development of new controlled low-strength materials.