POLYMER CEMENT

Sulfur Polymer Cement. SPC has been proven effective in reducing leach rates of reactive heavy metals to the extent that some wastes can be managed solely as low level waste (LLW). When SPC is combined with mercury and lead oxides (both toxic metals), it interacts chemically to form mercury sulfide, HgS, and lead sulfide, PbS, both of which are insoluble in water. A dried sulfur residue from petroleum refining that contained 600-ppm vanadium (a carcinogen) was chemically modified using dicyclopentadiene and oligomer of cyclopentadiene and used to make SC (58). This material was examined by the California Department of Health Services (Cal EPA) and the leachable level of vanadium had been reduced to 8.3 ppm, well below the soluble threshold limit concentration of 24 ppm (59). 

Sulfur polymer cement shows promise as an encapsulation and stabilization agent for use with low level radioactive and mixed wastes. Use of SPC allows accommodation of larger percentages of waste than PCC. As of this writing (1997), SPC-treated waste forms have met requirements of both the Nuclear Regulatory Commission (NRC) and the Environmental Protection Agency (EPA). 

The solvent was recovered from the polymer cement by steam stripping, followed by the separation of the organic layer. It was then purified in a small batch still, incorporating a packed column and dried by passage through a bed of molecular sieve, before being returned to the storage vessels. 

Hornsby, P. R. (1977). A study of the formation and properties of ionic polymer cements. Thesis for PhD, Brunei University, Middlesex, England. 

The last stage in the development of the EBA cement is represented by the polymer cements. Brauer Stansbury (1984b), taking advantage of the fact that the EBA-HV liquid does not inhibit vinyl polymerization, included methacrylates into the cement composition. The object was to produce a material that set after mixing, both by polymerization and by salt or chelate formation. 

The liquids used were 1 1 mixtures of EBA-HV and liquid methacrylate which also contained dihydroxyethyl-p-toluidine as the accelerator. Both mono- and di-methacrylates were used. The benzoyl peroxide initiator was included in the EBA zinc oxide/silanized (1 1) glass powder. These polymer cements set 5 to 10 minutes after mixing. Since there is a substantial amount of monomer in the liquid (50%) the contribution of the polymer to the strength of the cement must be considerable. Brauer Stansbury (1984b) suggested that the two matrices, the polymer matrix and the salt matrix, may be interpenetrating but separation of the two phases is likely. 

Resin cements or polymer cements have been reviewed by Chandra, Justnes, and Ohama [343]. Polymer cements are materials made by replacing the cement at least partly with polymers. Cements can be modified by latex, dispersions, polymer powders, water-soluble polymers, liquid resins, and monomers. 

Properties of Polymer Cement in Comparison with Conventional Cement... 

Freeze-thaw resistance + Pure polymer cements absorb essentially no water [82]... 

The properties of a latex depend on the nature of polymers in the latex, particularly the monomer ratio in copolymers and the type and amount of plasticizers. The monomer ratio affects the strengths of the latex modified mortars to the same extent as the polymer-cement ratio [87, 92]. Mechanical and chemical stability, bubbling and coalescence on drying all depend on the type and amount of surfactants and antifoamers and the size of dispersed polymer particles. It is important that the use of selected antifoamers and surfactants as stabilizers or emulsifiers produces no adverse effect on cement hydration. 

Fig. 6.14 Simplified model of formation of polymer-cement comatrix (Ohama [87]).

Such effects increase with an increase in the polymer content or the polymer-cement ratio (the weight ratio of total solids in a polymer latex to the amount of cement in a latex-modified mortar or concrete mixture). However, at levels exceeding 20% by weight of the cement in the mixture, excessive air entrainment and discontinuities form in the monolithic network structure, resulting in a reduction of compressive strength and modulus [87, 94, 95]. 

Although the mix design of latex-modified mortar and concrete is done in much the same way as that of ordinary mortar and concrete, properties such as workability, strength, extendibihty, adhesion, watertightness and chemical resistance are controlled by the polymer-cement ratio rather... 

The presence of the cement hydrate/polymer comatrix in LMM and LMC confers superior properties, such as high tensile and flexural strengths, excellent adhesion, high waterproofhess, high abrasion resistance and good chemical resistance, when compared to ordinary cement mortar and concrete. The degree of these improvements however depends on polymer type, polymer-cement ratio, water-cement ratio, air content and curing conditions. Some of the properties affected by these factors are discussed below [87, 88, 93-95]. 

The water content, required to produce a given slump, can be significantly reduced when a latex admixture is used and the extent of the reduction increases with an increase in the polymer-cement ratio. This effect is due to... 

The nature of the polymer latex is determined by the monomer ratio in the copolymer and this property of the latex affects strength values in manner similar to that obtained with the polymer-cement ratio. The effects of polymer-cement ratio on strength are presented in Table 6.14 [87]. 

Most latex-modified mortars and concretes have good adhesion to most substrates (tile, stone, brick, steel and aged concrete) compared to conventional mortar and concrete. In general, bond strength in tension and flexure increases with an increase in the polymer-cement ratio,... 

Table 6.14 Effect of polymer-cement ratio on mortar properties (Ohama et al.)...

The significant differences in the moduli of latexes and the cement hydrates (elastic moduli 0.001-10 GPa and 10-30 GPa respectively) causes most LMMs and LMCs to have a higher deformability and elasticity than ordinary cement mortar and concrete. Depending on the polymer type and polymer-cement ratio, the deformability and elastic modulus tends to initially increase with an increase in the polymer-cement ratio and subsequently decrease at higher ratios. Poisson s ratio however is only marginally affected [87, 94, 98]. 

The large pores (ranging from 0.01 pm to 0.1 pm) are sealed by the continuous polymer film formed in the comatrix of LMM and LMC. Consequently, they show reduced water absorption, water permeability and water vapor transmission over ordinary cement mortar and concrete and this effect increases with an increase in polymer content and polymer-cement ratio (Fig. 6.15). The improved water permeability also improves the resistance to chloride ion entry and hence corrosion mitigation [87]. 

The reduction in porosity, decreased water content, and air entrainment that results when latexes are used in mortar and concrete mixes make them much more resistant to freezing and thawing conditions than conventional mortar and concrete. Figure 6.17 presents the freeze-thaw durability in water (-18 to 4°C) of combined water-and dry-cured SBR-, PAE- and EVA-modified mortars [98], The frost resistance of mortars made with these latexes is markedly improved even at polymer-cement ratios of 5%. However, an increase in the polymer-cement ratio does not necessarily produce further improvement in freeze-thaw resistance. EMM and EMC, when exposed to outdoor conditions involving freeze-thaw, UV radiation and carbonation show better weatherability when compared with conventional mortar and concrete. 

Latex-modified mortars and concretes have become promising materials for preventing chloride-induced corrosion and for repairing damaged reinforced concrete structures. In Japan and the USA, latex-modified mortar is widely used as a construction material in bridge deck overlays and patching compounds, and for finishing and repairs [99]. Polymer-cement hydrate-... 

LMC is used in underwater concrete for both new construction and repair. The important requirements to obtain antiwashout capability, such as segregation resistance, flowability, self-leveling characteristics and lower bleeding are provided by the addition of viscosity-enhancing polymeric admixtures at polymer-cement ratios of 0.2-2.0%. These admixtures are water-soluble polymers, and classified under two groups, viz., cellulose types such as methyl cellulose and hydroxy ethyl cellulose and polyacrylamide types such as polyacrylamide and polyacrylamide-sodium acrylate [101]. 

BNL claims the following advantages of sulfur polymer cement ... 

Treatment Standards of Liquid Redox Waste in California, State of California Department of Health Services, Toxic Substances Control Program, Alternative Technology Division, June 1990 Sulphur Polymer Cement Concrete, Design and Construction Manual, The Sulphur Institute, Washington, D.C., 1994. 

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