Action Of Sewage Sludge Ash On Cement System And Its Mechanism

The rapid growth of urbanization construction increases rapidly the amount of sewage sludge for treatment and disposal.Incineration is one of the most common and effective methods for sludge disposal,which can remove organic matter and harmful substances effectively and reduce the sludge volume greatly.However,the disposal and utilization of sludge incineration ash is becoming an urgent issue and the relevant research foundation about sludge ash is still lacking.This dissertation carried out a research on sewage sludge ash（SSA）and studied the physicochemical properties and pozzolanic activity of sewage sludge ash during calcination.The hydration mechanism of SSA in cement system was explored.Furthermore,the activation mechanism and cementitious property of calcium-enhancing SSA were investigated.This dissertation aims to provide technical support for the application of sludge ash as supplementary cementitious material,realize the large-scale resource utilization of SSA and promote the development of low-carbon cement-based materials as well.The main innovative results of this dissertation were summarized as follows:（1）The analysis of the transformation of main mineral components in sludge during calcination were discussed.The pozzolanic activity sources of SSA mainly came from the active Al₂O₃ and Si O₂ formed by thermal decomposition of minerals such as kaolinite and mica.The influence of calcination process on physichemical properties and pozzolanic activity of SSA were obtained.The 28d strength activity index of mortar with 30%SSA calcined at 800℃exceeded 85%.（2）The hydration mechanism of SSA and its influence on the early hydration kinetics,hydration phase and microstructure of cement were revealed.The dissolution of active Al₂O₃ in SSA promoted the formation of ettringite and aluminum-bearing hydration products but inhibited the dissolution of C₃S.The early hydration performance of cement paste mixed with SSA can be optimized by adding gypsum.（3）SSA-limestone-cement ternary composite system was developed.The reaction of active Al₂O₃ in SSA and CO₃^2-in limestone promoted the formation of ettringite and hydrated carbonoaluminate phases.Compared with the control group,the composite mortar containing 15%SSA,7.5%limestone and 77.5%cement can save 23.13%of Portland cement and reduce CO_2-eq emissions by 13.52%for the same compressive strength level.（4）The activity of SSA can be effectively improved with the addition of calcium during calcination and the 28d strength activity index of composite mortar with modified SSA was up to 97%.The addition of calcium promoted the structure depolymerization of the silica tetrahedron and aluminum-oxide polyhedral of kaolinite,mica and other minerals in the sludge during the calcination process.It increased the content of active Al₂O₃ and Si O₂ resulting in the activity improvement of modified sludge ash.（5）The addition of calcium-modified sludge ash optimized the capillary pore structure of the SSA-cement composite mortar and reduced its dry shrinkage and capillary water absorption,contributing to the improvement of durability such as chloride penetration resistance and impermeability.