| dc.description.abstract | The melting of municipal solid waste (MSW) fly ash is currently being practiced for recycling purposes under the sustainable waste management policy. Due to increasing concerns regarding the energy-intensivity of the process, in practice the basicity of MSW fly ash has been modified by the addition of sewage sludge ash (SSA),which lowers the melting temperature. However, it is essential that engineers and operators have a better understanding of how the basicity of the starting mixture affects the pouring point and the characteristics of the slag product produced by the co-melting process. Thus this study investigates the effects of the basicity on the pouring point of the fly ash-SSA mixture by using four kinds of SSAs as modifiers, which were collected from several primary and secondary sewage treatment plants (STPs) and were produced by different processes and sludge conditioning alternatives.The effects on the co-melted slags were determined by studying the slag’’s pozzolanic activity and its reactivity as a pozzolans in the slag-blended-cement (SBC) pastes.
The results indicate that the pouring point of the mixture increased with increasing basicity, within the range from 0.65-1.90. As defined by Murakami Basicity=(MgO+CaO+Fe2O3+K2O+Na2O)/(SiO2+Al2O3). The pouring point is affected by the contents of the mixtures (CaO, SiO2, Al2O3 and the flux). It is suggested that an increase in the CaO content tended to increase the pouring point; whereas an increase in the SiO2 and/or the Al2O3 contents reacted adversely.
The mineral compositions of the co-melted slags were determined by XRF analysis. Results indicate that the main components of the composition, CaO, SiO2, Al2O3 and P2O5 were close to those of the blast furnace slag and the class C fly ash. The co-melted slags also showed a high pozzolanic activity ranging from 87.9-103.2%, so could be classified as latent hydraulic materials. In addition, TCLP
testing for the targete heavy metals indicate that all the slag samples in this study met the US EPA’’s regulatory thresholds.
The pozzolanic reactivity of slag is determined by the compressive strength development of the SBC pastes and the product of the calcium silicate hydrate (C-S-H) in the pastes. SBC pastes with a replacement of cement by up to 20% showed a compressive strength at 90 days similar to or surpassing that of ordinary portland cement (OPC) paste. In particular, the SSA from Neihu STP,which was characterized by a high CaO content, due to the conditioning of the sludge by lime, outperformed the OPC paste in terms of compressive strength development. On the other hand, calcium hydroxide (CH), C-S-H, tobermorite, hydroganet and gismodine were confirmed by the XRD techniques,to be the main hydration products in the SBC pastes. NMR analyses also indicate that the formation of C-S-H in the SBC pastes increased with age, so that the degree of hydration and the growing length of the C-S-H at 90 days outperformed that of the OPC paste, as indicated by pozzolanic reactions in the slag at a later age. The pozzolanic reactions were further confirmed by SEM observation. A dense network of C-S-H which interpenetrated other hydrates, such as calcium aluminate hydrate (C-A-H) and calcium aluminate silicate hydrate (C-A-S-H) formed as the pozzolanic reactions proceeded.
From the results of this study concluded that the modification of basicity of MSW fly ash by the addition of SSA to lower the pouring point leading to a energy-efficient melting process is feasible, and the SBC which incorporated the co-melted slag has a comparable engineering performance to that at the OPC pastes. | en_US |