Widespread international use is made of these materials, e.g. as grouts and mortar patches for finish and repair work and concretes for bridge deck overlays. Basically, unreactive polymer latex is added to the water of a fairly conventional cementitious mix, which is then cured. The cement begins to hydrate, whilst the polymer particles from the latex coalesce to give a film which then binds the hydrated cement phases and aggregate. There are numerous proprietary polymer emulsions and redispersible powders available. Polymer type and composition vary widely and all dispersions include other constituents. The manufacturers of these systems are often reluctant to disclose their complete make-up and characteristics for commercial reasons. Specific properties of the hardened material depend very much on the formulation of the polymer dispersion, mortar mix design, and curing regime used. Unfortunately, in much of the considerable published information, the exact nature of the systems and the mixing and curing procedures used are not very well documented, making an understanding of their behaviour and a comparison between different systems difficult.
The most commonly used latexes are aqueous suspensions of styrene- butadiene-rubber (SBR) and various acrylics (Ac) containing 45±50% polymer solids. In SBR, the ratio of styrene to butadiene governs the properties of the polymer, with 60±65% styrene giving a good balance. Higher styrene contents would improve compressive and tensile strengths but reduce adhesion and raise the minimum film-forming temperature (MFT). Usually ~1% carboxylic acid is chemically bound onto the polymer particle surface. These groups ionise in the high pH environment of the fresh cement, Fig. 10.1(a) (Chandra, 1987) and this generally results in improved stability of the latex and adhesion of the PMC to existing substrates (Dennis, 1985). Similarly there are a wide variety of co- and ter-acrylic polymers with some of them being available in redispersible powder form. Ethylene-vinyl acetate (EVA) was one of first redispersible powders on the market. Powders which can be pre-bagged with the cement and aggregate are preferable from a practical and environmental view, compared to latexes which are supplied in plastic containers and require batching to be carried out on site.
In addition to the polymer, a sufficiency of surfactant is added which is then adsorbed onto the surfaces of the polymer particles and helps maintain their dispersion. This dispersion should be preserved during mixing and transfer operations in the high pH environment of the cement and over a practical range of temperatures. They may be anionic, cationic or non-ionic and can affect latex- latex or latex-cement interactions. Any free surfactant in the mix water tends to stabilise air bubbles and so a de-foaming agent may be added to counter this. Other additions may also be present, e.g. anti-oxidants and bactericides.
Little detailed information is available on mix procedures although a typical mix design consists of a sand:cement ratio in the range of 3:1 to 2:1 with a polymer solids: cement ratio (p/c) of 0.10±0.20 by weight of cement and a w/c of about 0.3 compared to around 0.5 for the unmodified mortar (Dennis, 1985).
Whilst there is a wide range of latex systems available, they all tend to influence behaviour in the same manner in that: (a) when added to the fresh mix, the polymer particles improve workability to such an extent that, for a given workability, the w/c can be significantly reduced, which in turn reduces porosity and leads to improvements in compressive strength and durability, and (b) after curing, the polymer must be in the form of a film which leads to improved flexural strength, toughness and adhesion.
