Cement binding mechanisms

In general terms, the binding of heavy metals and metalloid ions in the cement matrix is quite well understood. There appear to be three basic types of binding mechanism (e.g., Cocke Mollah 1993 Cougar et al. 1996 Glasser 1997). A metal ion may be ... 

Binding mechanisms of ions to specific cement minerals... 

Ludwig C, Ziegler F, Johnson CA (1997) Heavy metal binding mechanisms in cement based waste materials. In Goumans JJJM.Senden Y,Van der Sloot HA (eds) Waste materials in construction - putting theory into practice. Studies in environmental science 71, Elsevier Science, pp 355-364... 

W. E. Shively, The Chemistry and Binding Mechanisms Involved with Leaching Tests of Heavy Metals Solidified and Stabilized with Portland Cement, M. S. Thesis, Univ. of New Hampshire, Durham, NH (1984). 

Because of the complexity of hydrated cement pastes and the variety of possibilities for binding, a study of the binding of metal and metalloid ions to specific cement minerals is advantageous. The binding to single components of the hydrated cement paste can be compared to the teachability of an ion bound in a hydrated cement paste and deductions made as to the dominant mechanism that limits the solubility in the porewater. 

Ziegler, F. 2000. Heavy Metal Binding in Cement-Based Waste Materials An Investigation of the Mechanism of 7n Sorption to Calcium Silicate... 

Little is known about the mechanisms causing metal binding in the cement matrix, but it is believed to be due to a combination of entrapment of insoluble metal precipitates in pores in the cement and within the cement matrix, an to adsorption to cement paste surface [7 ]. The free alkalinity present in the cement paste may be a major factor in the determination of the metal leachability of fixed wastes because high pH values generallj" tend to decrease heavy metal solubility and increase adsorptivity. 

The mechanical strength of the product can be improved by adding hardeners or binders. Various inorganic cements, including Portland cement, can be used to bind the cancrinite crystals to make a stronger product. Organic polymers can also be used. Binders appear to be most effective when the cancrinite is made from fired clays in which the clay structure has been destroyed. Various alternatives for the process are presented these depend on whether or not a binder is necessary and at what point in the process a binder is added. 

The external surface of the acetabular cup is grooved to allow bone cement (ceramic-filled acrylic resin) to key mechanically to the cup. This cement also binds to the excavated part of the pelvis. Further design variables are the cup wall thickness and whether it has a metal backing. For a... 

In an effort to enhance the osteoconductivity of PMMA bone cement, Rhee and Choi [ 16,46 ] incorporated silica nanoparticles into the polymer. This composite was synthesized by sol-gel processing with the goal of improving binding at the bone-implant interface. The authors observed high mechanical properties in addition to crystaUine apatite formation on implants in simulated body fluid. 

Si MAS NMR was used by Cappelletto et al to examine calcium silicate hydrate gel (C-S-H) present in Portland cement and is the complex phase mostly responsible for the binding properties and the mechanical resistance of the material. The effects of different comb-shaped superplasticizers on the silicate structure are investigate. The analysis of Si MAS NMR spectra shows that the additives increase the degree of polymerization and hence the average length of the silicate chains in C-S-H. ... 

Solidification may involve chemical reaction of the waste with the solidification agent, mechanical isolation in a protective binding matrix, or a eombination of chemical and physical processes. It can be accomplished by evaporation of water from aqueous wastes or sludges, sorption onto solid material, reaction with cement, reaction with silicates, encapsulation, or embedding in polymers or thermoplastic materials. 

The biomimetic processes can be used in the preparation of apatite-protein composites at low temperature, in aqueous suspension. The proteins used generally have the property of binding strongly with calcium phosphates (casein, albumin etc.). Ciystals of calcium phosphates involved are nanociystalUne and of apatitic stracture. The evaporation of water from the suspension results in solid materials with generally reduced porosity and having mechanical properties close to those of polymer cements but still relatively different from those of natural tissues [SAR 99]. 

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