Latex-modified mortar

Of the several types of the polymer-modified mortars and concretes used for various construction applications, latex-modified mortar and concrete are by far the most widely used materials. Latex-modified mortar and concrete are prepared by mixing a latex, either in a dispersed liquid or as a redispersible powder form with fresh cement mortar and concrete mixtures. The polymers are usually added to the mixing water just as other chemical admixtures, at a dosage of 5-20% by weight of cement. Polymer latexes are stable dispersions of very small (0.05-5 pm in diameter) polymer particles in water and are produced by emulsion polymerization. Natural rubber latex and epoxy latex are exceptions in that the former is tapped from rubber trees and the latter is produced by emulsifying an epoxy resin in water by the use of surfactants [87]. 

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. 

Table 6.11 Effect of glass transition temperature Tg on the properties of acrylic-latex-modified mortar (Ma and Brown)...

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... 

Wet curing conditions such as water immersion or moist curing applicable to ordinary cement mortar and concrete is detrimental to latex-modified mortar and concrete. Optimum strengths are obtained by providing a... 

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,... 

Fig. 6.16 Carbonation depth of latex-modified mortars after 10-year outdoor and indoor exposure (Soroushian and TIili [91]).

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-... 

Fig. 6.17 Number of cycles of freezing and thawing vs relative dynamic modulus of elasticity of latex-modified mortars (Ohama [87]).

The molecular weight, glass transition temperature (T) and size of dispersed polymer particles in the latexes can affect the strength and c loride ion permeability of latex-modified mortar and concrete to a certain extent [87,93] (Tables 6.11 and 6.12). SBR latexes with smaller particle size appear to initially provide lower chloride ion permeability to the mortars, but a difference in the permeability between the smaller and larger particle sizes eventually becomes insignificant as the concrete ages. The initial decrease in the permeability observed with smaller particles is attributed to the fact that smaller particle size coalesce into films faster than the larger particle sizes. 

Ohama, Y. (1987), Principle of Latex Modification and Some Typical Properties of Latex-Modified Mortars and Concretes, ACI Materials Jounal, November-December, pp. 511-518. 

In compressive strength develojxnent, the latex modified mortars always have higher compressive strengths than the unmodified mortars, in dry cure. Additions of latex to a ratio greater than 0.20 reduces compressive strength and is expensive. 

It has already been indicated that SBR latices are stable and are unaffected by the addition of large amounts of calcium chloride. This is an indication that calcium chloride may be used with an SBR latex modified mortar in the same way that it is used with an unmodified cement mortar. In general, the addition of 2% calcium chloride, based on the Portland cement, improves the early compressive strength development, from 24 hours to 7 days. 

The water vapor transmission characteristics of an SBR latex modified mortar are twice as good as those of an unmodified mortar. The water vapor transmission rate, as measured in grains/square foot/hour/inches mercury is as follows ... 

Several types of polymer-modified mortars and concretes, i.e., latex-redispersible polymer powder-, water-soluble polymer-, liquid resin-, and monomer-modified mortars and concretes, are produced by using the polymers and monomers shown in Fig. 2.1. Of th, the latex-modified mortar and concrete are by far the most widely used cement modifiers. 

Third Step. Ultimately, with water withdrawal by cement hydration, the close-packed polymer particles on the cement hydrates coalesce into continuous films or membranes, and the films or membranes bind the cement hydrates together to form a monolithic network in which the polymer phase interpenetrates throughout the cement hydrate phase. Such a structure acts as a matrix phase for latex-modified mortar and concrete, and the aggregates are bound by the matrix phase to the hardened mortar and concrete. 

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