Properties of latex-modified mortar and concrete

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. 

Y. Ohama Principle of latex modification and some typical properties of latex-modified mortars and concretes. ACI Materials Journal Vol.84, No.6 (1987), pp.511-518. 

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... 

Effects of Control Factors for Mix Proportions. The binder of latex-modified mortar and concrete consists of polymer latex and inorganic cement, and their strength is developed as a result of an interaction between them. The polymer-cement ratio has a more pronounced effect on the strength properties than the water-cement ratio. However, this effect depends on polymer t3rpe, air content, curing conditions, etc. The relation between the strength properties and polymer-cement ratio has been discussed in a number of papers.P l 1 1 A general trend which summarizes the results obtained in these papers is presented in Fig. 4.19. 

Figure 4.19 Relation between strength properties and pol5Tner-cement ratio of latex-modified mortars and concretes.

Although polymers and monomers in any form such as latexes, water-soluble polymers, liquid resins, and monomers are used in cement composites such as mortar and concrete, it is very important that both cement hydration and polymer phase formation (coalescence of polymer particles and the polymerization of monomers) proceed well to yield a monolithic matrix phase with a network structure in which the hydrated cement phase and polymer phase interpenetrate. In the polymer-modified mortar and concrete structures, aggregates are bound by such a co-matrix phase, resulting in the siq)erior properties of polymer-modified mortar and concrete compared to conventional. 

A very useful aspect of latex-modified mortars and concretes is their improved adhesion or bond strength to various substrates compared to conventional mortar and concrete. The development of adhesion is attributed to the high adhesion of polymers. The adhesion is usually affected by polymer-cement ratio and the properties of substrates used. The data on adhesion often show considerable scatter, and may vary depending on the testing methods, service conditions or porosity of substrates. 

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. 

Latex-modified mortar and concrete are made by using a composite binder of inorganic cements and organic polymer latexes, and have a network structure which consists of cement gels and microfilms of polymers. Consequently, the properties of the latex-modified mortar and concrete are markedly improved over conventional cement mortar and concrete. The properties of the fresh and hardened mortar and concrete are affected by a multiplicity of factors such as polymer type, polymer-cement ratio, water-cement ratio, air content, and curing conditions. 

Latex-modified mortar and concrete have a markedly improved water retention over ordinary cement mortar and concrete. The water retention is dependent on the polymer-cement ratio. The reasons for this can probably be explained in terms of the hydrophilic colloidal properties of latexes themselves and the inhibited water evaporation due to the filling and sealing effects of impermeable polymer films formed. Accordingly, a sufficient amount of water required for cement hydration is held in the mortar and concrete and, for most latex-modified systems, dry cure is preferable to wet or water cure. This is also examined in Sec. 2.1. 

In general, latex-modified mortar and concrete show a noticeable increase in the tensile and flexural strengths but no improvement in the compressive strength as compared to ordinary cement mortar and concrete. This is interpreted in terms of the contribution of high tensile strength by the polymer itself and an overall improvement in cement-aggregate bond. The strength properties of the latex-modified mortar and concrete are influenced by various factors which tend to interact with each other. The main factors... 

Effects of Curii Conditioiis. Favorable curing condition requirements for latex-modified mortar and concrete differ from those for ordirtary cement mortar and concrete, because their binder consists of two phases of latex and hydraulic cement widi different properties. Optimum strength in the cement phase is developed under wet conditions such as water immersion and high humidities, where strength development in the latex phase is attained under dry conditions. Figures 4.24 and 4.25f lf l show the effect of the curing conditions on the strength of the latex-modified mortars and concretes respectively. 

The fabrication technique of redispersible polymer powder-modified mortar and concrete is about the same as that for latex-modified mortar and concrete. The materials and mix prc rtions used in this nnodified mortar and concrete are the same as those used in the latex-modified systems except that the addition of the redispersible polymer powders is involved. At present, commercially available redispersible polymer powders as cement modifiers are classified in Fig. 5.2. Table 5.2 gives the properties of typical redispersible polymer powders. The rediqretsible polymer powders are usually free-flowing powders, and have ash contents of 5 to 15%, whidi primarily come from the anti-blockirig aids. When the polymer powders are placed in water under agitation, they redtsperse or re-emulsify easily, and provide the polymer latexes with polymer particle sizes of 1 to 10 pm. 

Of various polymer-modified mortar and concrete, latex-modified mortar and concrete have superior properties, such as high tensile and flexural strengths, excellent adhesion, high waterproofhess, high abrasion resistance, and good chemical resistance, compared to ordinary cement mortar and concrete. Accordingly, they are widely used in many specialized applications in which the ordinary cement mortar and concrete have been employed to a lesser extent till now. In these applications, the latex-modified mortar is widely used rather than the latex-modified concrete from the viewpoint of a balance between their performance and cost Typical applications of the latex-modified mortar and concrete are listed in Table 9.1. 

Similar to latex-modified systems, the properties of redispersible polymer powder-modified systems are improved in comparison with ordinary cement mortar and concrete, and these depend on the nature of polymer and polymer-cement ratio. Figs. 5.3 to 5.5i l represent the strengths, adhesion to cement mortar, water resistance, and water absorption of the redispersible polymer powder-modified mortars. The properties are improved with an increase in the polymer-cement ratio. This tendency is very similar to that of the latex-modified systems. In general, the redispersible polymer powder-modified mortars are inferior to SBR-modified mortar (control) in certain properties. VAA eoVa powder-modified mortars show tetter properties than EVA powder-modified mortars as seen in Fig. 5.5. The film formation characteristics of recent redispersible polymer powders for cement modifiers are improved, and continuous polymer films can be found in the redispersible polymer powder-modified systems as seen in Fig. 5.6. This contributes greatly to improvements in their properties. 

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