Polymer-modified cement mixes

Polymer-modified cement mixes are produced by mixing together an inorganic cement (most commordy Portland cement), water, and a dispersion of an organic polymer. If concrete aggregate or sand is also added, polymer-modified concrete or mortar mixes are obtained. Compared with similar polymer-free mixes, the addition of water may be reduced or omitted entirely, as the polymeric dispersion supphes part or all of the water needed for hydration and for obtaining adequate rheology of the mix. 

In polymer-modified cement systems, there occurs air entrapment resulting from the generation of foam while mixing. Air has an adverse effect on the mechanical strength properties of cement mortars as expressed by Feret s relationship ... 

Abstract. The presence of water-soluble polymers affects the microstructure of polymer-modified cement mortar. Such effects are studied by means of SEM investigation. Polyvinyl alcohol-acetate (PVAA), Methylcellulose (MC) and Hydroxyethylcellulose (HEC) are applied in a 1 % polymer-cement ratio. The polymers provide an improved dispersion of the cement particles in the mixing water. The tendency of certain water-soluble polymers to retard the flocculation of the cement particles minimizes the formation of a water-rich layer around the aggregate surfaces. They also provide a more uniform distribution of unhydrated cement particles in the matrix, without significant depletion near aggregate surfaces. Both effects enable to reduce the interfacial transition zone (ITZ). The polymers also provide a more cohesive microstructure, with a reduced amount of microcracks. 

Epoxy resin has superior properties such as high adhesion and anticorrosion, and has widely been used as adhesives and anticorrosives in the construction industry in the world. Provided the incorporation of the epoxy resin into cement mortar can give its superior properties to the mortar, it is possible to produce a highly polymer-modified cement mortar. The first patent of an epoxy-modified cement system was taken by Donnelly in 1965 [1]. Since the patent, 30 or more papers on the epoxy resin modification of the cement mortar and concrete have already been published [2]. Most epoxy resin-based cement modifiers dealt with in the papers are specially compounded by the manufacturers, and the procedures for mixing them to fresh mortar and... 

Polymer-modified cementitious floor toppings are now widely used instead of separately laid granolithic toppings. The polymers used are normally supplied as milky white dispersions in water and are used to gauge a carefully selected sand/aggregate/cement mix as a whole or partial replacement of the gauging mortar. They must always be mixed in a forced-action mixer. 

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]. 

Polymer-modified mortar and concrete are prepared by mixing either a polymer or monomer in a dispersed, powdery, or liquid form with fresh cement mortar and concrete mbctures, and subs uently curing, and if necessary, the monomer contained in the mortar or conoete is polymerized in situ. The polymers and monomers used as cement modifiers are shown in Fig. 2.1. 

In the modification with water-soluble polymers such as cellulose derivatives and polyvinyl alcohol, small amounts of the polymers are added as powders or aqueous solutions to cement mortar and concrete during mixing. Such a modification mainly improves their workability because of the surface activity of the water-soluble polymers, and prevents the dryout phenomena (explained in Ch. 4, Sec. 3.1, Water Retention). The prevention of the dry-out is interpreted in terms of an increase in the viscosity of the water phase in the modified cement mortar and concrete and a sealing effect due to the formation of very thin and water-impervious film in them. In general, the water-soluble polymers hardly contribute to an improvement in the strength of the modified systems. 

The fabrication procedure of monomer-modified systems is similar to that of liquid resin-modified systems, except monomers are used instead of liquid resins. These systems are prepared by directly mixing the monomers with cement, aggregate, and water, followed by thermal-catalytic or radiation polymerization process. The polymerization occurs during and/or after the setting or hardening of the cement systems. Finally, the polymerization process converts the monomer-modified systems to polymer-modified systems. 

When considering a cementitious mortar mix for use in the manufacturing of an ultra-lightweight thin film product, the addition of a suitable polymer modifier can enhance both the fresh and hardened properties of such lightweight material. The addition of styrene butadiene rubber latex (SBR) latex yielded the best overall results in terms of workability, formability, mouldability, flowability, compressive and flexural strengths. The improvements offered by the addition of the acrylic polymer were less impressive and the use of an acrylic modifier is thus not recommended for use in ultra-lightweight thin film products. It is also confirmed that small adjustments to the water cement ratio alter the workability of such mixes and may be used to tweak mix designs to suit specific applications and uses. Thus with the addition of either a SBR latex or acrylic polymer and... 

The design of polymer modified concretes is based on similar reasoning as for Portland cement concretes, but there are no published programs. In PCC, the binder is composed with two phases Portland cement and polymer phase that may be introduced to the mix as polymer latexes, and water-soluble polymers or liquid polymers. There is a continuous relation between PCC and PC when the polymer fraction (starting from 10% mass) is increased... 

Epoxy-modified mortars which are prepared by mixing most popular bisphenol A-type epoxy resin with commercial modified polyamide-amine hardener, polyalkyl aryl sulfonate-type water-reducing agent, polyoxyethylene nonylphenol ether-type nonionic surfactant and silicone emulsion-type antifoamer into cement mortar have excellent properties comparable to ordinary polymer-modified mortars using latex-type cement modifiers[5]. The optimum mix proportions of the epoxy-modified mortars are shown in Table 2. Their disadvantage is a need of much higher polymer-cement ratio than the ordinary polymer-modified mortars. Therefore, the development of low-cost, effective dispersants is expected in the near future. 

Accordingto JIS A 1171 (Method of Making Test Sample of Polymer-Modified Mortar in the Laboratory), polymer-modified mortars containing silica fume were mixed using the mix proportions given in Table 3. The polymer-cement ratio [P/(C+S)I was calculated on the basis of the total solids of the polymer dispersions (P), the content of the ordinary portland cement (C) and the content of silica fume (S). Mortar specimens having the desired shapes and sizes were molded, and then given a 2-day-20 C-80%R.H.-moist, 5-day-20 C-water and 21-day-20 C-50%R.H.-dry cure. 

Combinations with polymers can be obtained by impregnation of the hardened composite (polymer impregnated concrete - PIC) or with a polymer latex which is added during mixing (latex modified cement - LMC) [137]. Both types of polymer incorporation would be expected to increase the matrix strength and the fibre-matrix bond. However, their influence on the other matrix properties is quite different. In PIC, the matrix has a higher modulus of elasticity and becomes more brittle, while in LMC the matrix will be more ductile with a lower modulus of elasticity. 

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 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. 

The principle of modification of cement mortar and concrete with redispersible polymer powders is almost the same as that of latex modification except that it involves the addition of redispersible polymer powders. Mostly the redispersible polymer powders are used by dry mixing with the cement and aggregate premixtures, followed by wet mixing them with water. During the wet mixing, the redispersible polymer powders are reemulsified in the modified mortar and concrete, and behave in the same manner as the latexes for cement modifiers. 

The process technolo of latex-modifled mortar and concrete is almost the same as fliat of conventional cement mortar and concrete. Most polymers such as latexes are in a dispersed form, and are added to the cement mortar or concrete during mixing. The polymers are used in very large amounts in comparison with air-entraining agents or water-reducing agents which are normally employed in ordinary cement mortar and concrete. The latex-modified mortar and concrete with proper mix proportions arc mixed and placed like the ordinary cement mortar and concrete, and then cured under optimum conditions. 

Latex-modified mortar and concrete mix design should recognize its improved properties such as tensile and flexural strengths, extensibility, adhesion, and durability over conventional mortar and concrete. These properties are controlled by the polymer-cement ratio rather than the water-cement ratio. Therefore, the polymer-cement ratio should be determined to meet desirable requirements. The polymer-cement ratio is defined as the weight ratio of the amount of total solids in a polymer latex to the amount of cement in a latex-modified mortar or concrete mixture. 

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