| dc.description.abstract | Abstract
MSW incinerator fly ash, comprisedof mixed cyclone ash and lime-reactant ash is characterized by its high melting point and is thus difficult to melt effectively and economically, without modification of its components. This study investigated the effects of the modification of the fly ash’’s basicity on the pozzolanic characteristics of the resultant melts. The modification was made by adding SiO2 to bring the basicity closer to unity; subsequent additions of Al2O3 and P2O5 to the previous modifications were also evaluated. These modifications resulted in three types of fly ash, referred to as the S-, A-, and P- modified series, respectively. All the resultant slag samples were pulverized to prepare cement pastes with various portions of the cement being replaced. The pozzolanic characteristics were evaluated at various curing ages by examining the pozzolanic activity, hydration degree,the hydration products and their microstructure, as well as the engineering properties of the pastes.
The results indicate that all the slag samples resulted from the modified fly ash contained CaO(9~35w/w%), SiO2(36~45w/w%), Al2O3(9~20w/w%), and P2O5(2~13w/w%), respectively; all met the specification of ASTM C989 for blast furnace slag, and showed properties that would delay the setting time of the resultant mortars. The pozzolanic activity for all the tested slag samples ranged from 78% to 106%.The P-modified series slag showed a greater activity (98~106%) than did the A-modified series slag(81~95%), whereas the activity developed in the S-modified series samples was less distinctive(79~80%), as compared to that of OPC.As to the compressive strength at 90-days, the A-modified series samples ,with a 10% and 20% cement-replacement, outperformed the OPC samples by 1~14 MPa, whereas the P-modified series exhibited compressive strength development close to that of OPC, by less than 5 MPa.
XRD, DTA, and FTIR speciation analyses indicated that the main hydrated products of all tested samples included calcium hydroxide(CH), calcium silicate hydrate (C-S-H), and calcium aluminate hydrates(C-A-H). Moreover, the NMR results indicate that the number of Si linear polysilicate anions in the C-S-H gel increased with increasing age, and outperformed that of OPC at 90 days, suggesting that pozzolanic reactions in the slag contributed to the later formation of calcium silicate hydrate (C-S-H). This was also indicated by the SEM analysis, from the formation of hexagonal sheets of CH hydrates, round and needle-surfaced C-S-H gels, ettringite (Ca6Al2(SO4)3.32H2O), and monosulfate. These products grew to intersecting to form a network structure, which increased the distribution of the finer pores, eliminated the porosity, and thus increased the compressive strength of the pastes.The results of this study demonstrated that MSW fly ash could be melted at a lower temperature when its basicity was modified, thus producing the targeted type of slag, with pozzolanic characteristics close to or outperforming those of OPC. | en_US |