For many years, polymers have been added to cement mortar to improve such properties as brittleness and water absorbency, which are common defects of plain cement mortar. The resin film formed by polymer in hardened cement mortar provides distinctive properties. To obtain a high level of performance, it is important to ensure the uniform dispersion of such components as cement, sand, water and polymer emulsion throughout the cement dispersion formulation.
In general, smaller particles of polymer emulsion tend to easily coagulate in the cement dispersion formulation due to the high electrolyte density and high pH conditions. As a result, the matrix formed in hardened cement is heterogeneous. This, in turn, affects negatively compression strength, an essential characteristic of cement materials.
In order to impart dispersibility to the polymer emulsion, we developed polymer particles with a soft core/hard shell-type double-layered structure (see Fig 2), aiming for a "ball bearing effect" to achieve effective dispersibility.
To determine the optimal composition of the soft core/hard shell-type double-layered structure, we examined the shell hardness (glass transition temperature: Tg) and core/shell ratios. We prepared various polymers and evaluated their dispersion properties using a 1:2 mortar (see Fig 3). When the shell layer's Tg, which indicates shell hardness, is at 10°C to 20°C or greater, such polymer emulsion displays favorable dispersive properties. However, if the Tg is too high, the formation of resin film is unsatisfactory and thus reduces the performance of the cement mortar. As a result, the optimal shell layer's Tg has been set at between 20°C and 30°C. We extended our examination of core/shell ratios using the same evaluation method (see Fig 4).
We have determined that dispersion properties are favorable when the volume of the shell layer is greater than a core/shell ratio of 7:3, provided that the shell layer's Tg is fixed at 20°C. In this case, however, the formation of resin film tends to be faulty if the ratio of shell layer with a high Tg increases. Therefore, we have determined the optimal core/shell ratio from 6/4 to 8/2. In addition, the surfaces of the polymer particle have been chemically modified to stably disperse polymers in high electrolyte density and high pH conditions.
Regarding base polymers composed of polymer particles, we used acrylic acid esters to maintain mechanical strength and durability. Particularly for applications that require mechanical strength, we made the polymer particle's core layer incorporate a cross-linked structure (3-D structure) to improve the performance of the resulting resin film.
Moreover, a specialized emulsion polymerization method is used to manufacture an ultra-fine particle core/shell double-layered polymer emulsion with a particle size of just 100nm.
The temperature-elastic modulus curve (see Fig. 5) was measured with regard to resin film derived from the aforementioned polymer emulsion through drying process. Since the two peaks in the curve that are attributable to the double-layered structure are displayed, the result suggests that a polymer emulsion with the desired particle structure has been manufactured as planned.
In addition, the tensile strength is higher than that of regular polymer emulsion and enables a mechanically superior resin film, which, in turn, will improve concrete's strength as a whole .
This is attributable to not only a core cross-linked structure but also a resin film formation process through the close-packing of the particles (see Fig 6).
Given the abovementioned evaluation results of the properties of resin films, the cement mortar (modified by mixing in the core/shell type double-layered polymer emulsion) is anticipated to greatly improve its brittleness and water absorbency, which are common defects of regular cement mortar.
We evaluated the performance of cement mortar that was modified through the addition of a polymer emulsion developed using advanced particle design (see Table 1).
This evaluation was carried out using 1:2 mortar (cement/sand = 1/2). Cement, sand, and polymer emulsions, which comprise 15% of the volume of the cement, were mixed with an appropriate amount of water using mortar mixer. The amount of water was adjusted to enable a constant level of mortar fluidity (table flow value: 180±10mm). The result indicates that the cement mortar modified through the abovementioned addition of specially designed polymer requires a minimal amount of water to maintain a certain degree of fluidity compared with other common products (acrylic, ethylene vinyl acetate and SBR types). These differences in performance are attributable to the dispersibility of cement, which was largely improved by the polymer emulsion.
This process also results in favorable hardening performance. The cement mortar modified by the abovementioned polymer emulsion displays 10N/mm2 or greater flexural strength, a measurement that indicates the level of brittleness, which is a common defect of cement mortar. In addition, the compressive strength of the polymer emulsion mixtures remains on a level equal to that of additive-free cement, in contrast to the general tendency toward a decline in such strength when polymer has been mixed with cement.
The water absorption ratio and volume of water penetration under pressure are low, indicating greater water resistance. Moreover, test results with regard to the carbonation acceleration rate indicate favorable durability. We have verified that this product demonstrates superior effectiveness compared with additive-free and other commonly available polymer emulsions.
|Designed polymer emulsion||Common polymer emulsion||Additive-free|
|Acrylic type||Ethylene/vinyl acetate type||SBR types|
|Weight per unit volume(kg/l)||2.16||2.16||2.16||2.06||2.12||1.99||2.15|
|Table flow value(mm)||188||184||181||171||183||188||188|
|Water absorption rate(%)||0.6||1.1||3.2||3.6||2||1.9||6.9|
|The amount of permeable water under pressure(g)||5||6||7||18||15||9||25|
|Carbonation acceleration rate(%)||21||24||36||41||26||35||58|