What effects does redispersible polymer powder have on the mortar strength?

After redispersible polymer powder is added into cement mortar, the tensile strength and flexural strength of mortar will be significantly improved, while the compressive strength won’t get significantly improved, even declined. The reason is that the toughening effect of redispersible polymer powder has enhanced the internal tensile strength of mortar and the bonding tensile strength of interface, and greatly improved the bonding tensile strength between mortar and base material.

We know that the cracking of brittle materials is mainly due to tensile failure. When the tensile stress exceeds its own tensile strength value, cracking will occur. Therefore, it is a necessary condition to have a high tensile strength value to resist cracking.

Studies show that with the increase of polymer-cement ratio, the tensile strength of polymer modified cement mortar generally increases first and then decreases, indicating that there exists an optimal range of addition amount. The reason for the decrease is generally that the addition of excess redispersible polymer powder leads to the introduction of too many air bubbles and results in a downward trend in the compressive strength. Therefore, it needs to improve the compressive strength by adjusting the cement-sand ratio, water-cement ratio, aggregate gradations and aggregate types. The improvement of tensile strength, flexural strength, flexibility, crack resistance and hydrophobic property can be achieved by adding redispersible polymer powder, but the amount added is not the more the better. When the addition amount of polymer powder is too low, it will only have some plasticizing effects but not obvious strength-increasing effects; when the amount added is too great, the strength will decrease. Only when the amount added is moderate, can it not only increase the deformation resistance, tensile strength and bond strength, but also improve the impermeability and crack resistance. The cement/sand ratio, water/cement ratio, aggregate gradation and type, and aggregate properties will ultimately affect the overall performance of product.