Applying RDP into mortar is actually the modification of ordinary mortar, also known as polymer modified cement mortar. Through polymer modification, the brittleness, high elastic modulus and other weaknesses of traditional cement mortar are improved; the cement mortar is endowed with better flexibility and tensile bond strength to resist or delay the generation of cement mortar cracks. Because polymer and cement mortar form an interpenetrating network structure, forming a continuous polymer film in the pores, strengthening the bond between aggregates, blocking part of the pores in the mortar, the properties of hardened polymer mortar are much better than ordinary cement mortar. Its main features are as follows:
Generally speaking, the tensile strength and flexural strength of polymer modified cement mortar are significantly higher than that of ordinary cement mortar, while the compressive strength is not significantly improved or even decreased. The improvement of tensile strength and flexural strength is mainly due to the higher tensile strength of polymer itself and the improvement of cohesion between cement hydration products and aggregates.
The cracking of materials is mainly caused by tensile failure. When the tensile stress exceeds its own tensile strength value, cracking will occur. Therefore, it is necessary to have high tensile strength value to resist crack.
The results show that the tensile strength of polymer modified mortar increases first and then decreases with the increase of the ratio of polymer to cement, which indicates that there is an optimal mixing range, and the reason for the decrease is that too many bubbles are introduced due to the addition of excessive RDP, resulting in the decrease of compressive strength. In addition, because the elastic modulus of polymer is lower than that of cement mortar, and the polymer chain segment is more flexible, the composite can not play a rigid supporting role when it is compressed, resulting in the compressive strength of the modified mortar decreases with the increase of the polymer cement ratio. Therefore, the main way to improve the compressive strength is to adjust the ratio of lime to sand, water to cement, aggregate gradation and aggregate type. By adding a certain amount of latex powder which can replace part of cement, the cost can be reduced, and the urban strength and flexural strength can be improved.
The elongation and toughness of the mortar with Redispersible Polymer Powder(RDP) are much better than that of the ordinary cement mortar. The fracture energy is more than two parts of the ordinary cement mortar, and the impact toughness increases with the increase of the aggregate cement ratio. With the increase of the content of RDP, the flexible buffering effect of polymer can inhibit or delay the development of cracks, and has a better stress dispersion effect. When the base deformation produces stress, transfer or disperse stress to avoid cracking due to stress concentration.
In addition, it should be noted that there is a certain matching relationship between the tensile strength and elongation of the material. If the content of flexible RDP is high, the elongation will increase, and the polymer will be continuous phase. In the whole hardened cement mortar, the polymer will play a leading role, showing the decrease of tensile strength and water resistance. However, in the early stage of hydration heat reaction, if the tensile strength is low, making the tensile strength value close to the ultimate tensile strength value quickly, the material is easy to produce fatigue and fracture, so while ensuring sufficient elongation at break, the tensile strength value should be as high as possible
For external thermal insulation system, all systems should be in a flexible environment. For polystyrene board adhesive and plastering mortar, flexible RDP with low glass transition temperature should be selected first, so as to release all kinds of deformation stress in time.
The results show that the content of flexible RDP is about 3%, the content of anti crack plaster mortar is between 4% and 5%, and the content of exterior wall tile adhesive is about 3%. When the compression ratio is less than 3, the expansion, hollowing and cracking caused by deformation can be effectively controlled.
(3) Bond properties
The adhesive ability of the cement mortar with RDP is much better than that of the ordinary one, which is mainly due to the adhesive ability shown by the properties of the polymer. The adhesive strength increases with the increase of the dosage, but there is an optimal dosage range and cost consideration.
High bond strength can restrain shrinkage to some extent and the stress produced by deformation is easy to disperse and release, so bond strength is very important to improve crack resistance. The results show that the synergistic effect of methyl cellulose ether and RDP is beneficial to improve the bond strength of cement mortar. The adhesion mechanism of polymer depends more on the adsorption and diffusion of macromolecules on the bonding surface. At the same time the polymer has certain permeability. Together with methyl cellulose ether it can fully infiltrate the surface of base material and the adhesive mortar have the function of sealing the pores on the surface of base material. Especially for those porous base material surfaces, it can improve the waterproof resistance of base layer and freeze and thaw.
(4) Modulus of elasticity
The elastic modulus of RDS is lower in the range of 0.001-10 GPA, while that of cement mortar is higher in the rage of 1030 GPA. Therefore, the elastic modulus of cement mortar modified by RDP is smaller than that of ordinary cement mortar. The type and amount of Redispersible Polymer Powder(RDP) also have an effect on the elastic modulus. Generally whith the increase of the aggregate cement ratio, the elastic modulus decreases and the deformation ability increases.
(5) Wear resistance
The wear resistance of modified mortar is related to the type of RDP and the ratio of aggregate to cement. Generally speaking, the increase of the polymer cement ratio and the improvement of the wear resistance are mainly due to the existence of a certain amount of surface polymers on the worn surface which play a binding role. The mesh structure of formed by the polymers can pass through the holes and cracks in the cement mortar, which improves the bonding between the aggregate and the cement hydration products, so that wear resistance is improved.
In the polymer modified mortar, the mesh membrane structure formed by polymer filling closed the holes and cracks in the cement mortar, reduced the porosity in the hardened body, thus improving the impermeability, water resistance and frost resistance of the cement mortar. The water resistance of VAC modified mortar is poor. Therefore, in addition to the occasions without water resistance requirements, VAC may also be used. At present VAC is basically not used for mortar modification. On the other side, VAC is easy to saponify and lose the bond strength when it is in strong alkali, so it is not suitable to be used with cement mortar. VAC is usually used in gypsum based products and putty powder without cement.
(7) Air permeability
The water vapor permeability of building materials is of great significance. Building materials that can be waterproof and water vapor permeable are expected in many occasions. The water absorption of polymer modified mortar is low and its water resistance is improved but it still has a certain water vapor permeability which will decrease with the increase of polymer content. At the same time some hydrophobic materials can be compounded to improve its comprehensive performance.