Immobilisation of heavy metal in cement-based solidification/stabilisation: A review
Introduction
Industrial activities in the production of materials and chemicals give rise to very large quantities of heavy metal-bearing wastes each year. Heavy metals may exist in a variety of forms, for example, chloride, sulphate, and fluoride. Most of them are toxic, mutagenic, and carcinogenic. As the concentration of heavy metals in wastes varies in a wide range and may exceed the acceptance limit of the environment, heavy metal-bearing wastes pose a serious threat to human and animal health and need treatment (Bozkurt et al., 2000).
Solidification/stabilisation (s/s) of heavy metal-bearing sludge, industrial residues and contaminated soil is an attractive technology to reduce their toxicity and facilitate handling prior to landfill (Valls and Vazguez, 2000, Collier et al., 2006). In terminology, stabilisation is a process of converting a toxic waste to a physically and chemically more stable form, that is, alters hazardous waste chemically to produce a less toxic or less mobile form. It involves chemical interactions between waste and the binding agent. By comparison, solidification converts liquid waste, semi-solid sludge or a powder into a monolithic form or granular material that will allow relatively easy handling and transportation to landfill sites (Conner, 1990, Glasser, 1997, Poon et al., 2004). It does not necessarily imply any form of chemical reaction has occurred.
The objectives of solidification/stabilisation are to achieve and maintain the desired physical properties and to chemically stabilise or permanently bind contaminants. The high strength, low permeability and relatively high durability of hydraulic cement make it a good binder for this waste management technique (Conner, 1990). Cement-based solidification/stabilisation has been widely used in the world for about 50 years (Alunno and Medici, 1995, Conner and Hoeffner, 1998, Malviya and Chaudhary, 2006), which improves the handling characteristics and lowers the leaching rates of wastes by a combination of solidification and stabilisation.
The overall process of cement hydration includes a combination of solution processes, interfacial phenomena and solid-state reactions. It is extremely complex, especially in the presence of heavy metals. The selection of cements and operating parameters depends upon an understanding of the chemistry of the solidification/stabilisation process. This paper examines interactions between cement phases and heavy metals, mechanisms of heavy metal immobilisation and factors controlling the effectiveness of cement-based solidification/stabilisation.
Section snippets
Alite
Tricalcium silicate, C3S or Ca3SiO5, is pseudo trigonal/triclinic and a solid solution of CaO in dicalcium silicate proposing the formula (Ca2SiO4 · CaO). In cement, alite is tricalcium silicate containing Mg2+, Al3+, Fe3+ and other ions, and may appear as sub-hexagonal (pseudomorphic remnants of high temperature trigonal) structure (Taylor, 1997). In alite, Ca2+ coordination number is 6 (lower than normal), and Al3+ or Mg2+ ions distort the structure. These structures readily allow water
Aluminate
Tricalcium aluminate, C3A or Ca3Al2O6, is the only cement compound not to show polymorphic transformations with temperature. In C3A, Ca2+ coordination numbers are not all equal (specifically, 6 and 12). In Portland cement tricalcium aluminate is substantially modified in structure and composition by the incorporation of ions, particularly, Si4+, Fe3+, Mg2+, Ti4+, Na+, and K+. They can constitute as much as 10% by weight in this phase (Lea, 1970). Lea estimated the probable iron content in this
Cement hydration and heavy metal immobilisation
Portland and other types of cement, for example, calcium aluminate cement, slag cement, pozzolanic cement, are often used as binders in the solidification and stabilisation of wastes (Zhou et al., 2006). Portland cement clinker is made by heating a mixture of limestone and clay to a temperature of about 1450 °C. Fluxes, notably aluminium and iron oxides, are used, although their effect on the final clinker composition has to be carefully controlled. The clinker is mixed with a few percent of
Evaluation method of solidified waste
There are many test methods applied to the solidified product evaluation. Physical evaluation methods, for example, measurements of bulk density, porosity, and moisture content, are used to determine the engineering properties and the volume change factor for a solidification process. Chemical evaluation methods, including leaching tests and measurements of acid neutralisation capacity, address the solubility and reactivity of contaminants when exposed to different reagents and environments (
Summary and conclusions
The hydration of cement phases and cement clinker has been extensively investigated and the considerable divergences of opinions have been discussed in this paper. Hydration of cement proceeds through three successive reactions: (1) dissolution of phases, (2) precipitation of C–S–H, portlandite and other reaction products, and (3) equilibriums of hydration products. A series of self-limiting reactions occurring in cement hydration are probably largely controlled by the permeability of
Acknowledgements
We warmly thank Drs. P. Purnell, M. Derron and G. Manby for their informative comments, which greatly improved the quality of the manuscript.
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