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Toxic waste Solidification/Stabilization (S/S) involves adding cements to toxic waste (normally as heavy metal hydroxides) before landfilling, to reduce pollution of the environment. Ordinary Portland cement (OPC) and pulverized fuel ash (PFA) are conventionally used as S/S cements. Ground granulated blastfurnace slag (GGBS) has rarely been used, which is surprising considering the widespread use of OPC + GGBS in concrete, where it is recognised as enhancing binding, durability and resistance to ion diffusion. Furthermore GGBS hydration is activated by alkalis or sulphates, which are contained in most waste streams, suggesting no need for an additional activator. This thesis is concerned with the use of GGBS in toxic waste S/S.
A comprehensive literature search and laboratory trials on about 50 different mixes were undertaken, to identify suitable alkali activators. As a result, five alkali activated slag (AAS) formulations were selected for more detailed investigation. These five formulations and four conventional binders were applied to two real wastes using strength, water permeability, sorptivity, leachability and pore solution composition as the basis for evaluation. This work showed no general benefits from AAS-based systems over OPC & OPC + PFA, apart from reducing the leachability of chromium (Cr) and they increased the leachability of nickel, molybdenum and iron.
Also, this work highlighted the problems associated with characterizing S/S wastes using
such tests. No obvious correlation between physical and chemical properties was
observed, showing a need for a method for evaluating the individual effects of physical
retardation and chemical immobilization on the leaching of toxic species. Consequently,
a new electrochemical diffusion method (EDM) has been developed; it is based on pore
solution analysis and impedance measurements. The EDM was validated successfully
using cement pastes contaminated with chlorides and when applied to S/S wastes showed
that chemical immobilization is the dominant effect in reducing heavy metal leaching.
Complementary tools such as scanning electron microscopy (SEM), X-ray diffraction
analysis (XRDA) and energy dispersive analysis of X-rays (EDAX) were used to
investigate the binders concerned. It also showed that initially immobilized hydroxides
in the raw waste are dissolved in the highly alkaline pore solution, suggesting that S/S
may not be as effective as widely claimed.
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