Polymers have been used as additives in cement mortars and concrete since the 1920s when natural rubber latex was added to road paving materials. Since then there has been considerable development of polymer modification for cement and concrete. Polymer modification of cement mortar and concrete noticeably improves application and performance characteristics. These property advancements include easier handling, better finishing, higher strength and adhesion, and increased durability.
Mechanism of Action
The properties of polymer modified concrete depend significantly on the type of polymer utilized and the polymer-to-cement ratio, which is defined as the mass ratio of the amount of polymer solids to the amount of cement. The polymer-to-cement ratio can vary from 0 to 1/3 depending on the type of polymer used in the properties required for the application.
There are two theories for why the properties of concrete are improved with the addition of polymers. In the first theory, there is no chemical interaction between the polymer and the cement. During the hydration of the cement, the hydrophillic part of the polymer is oriented toward the water phase, whereas the hydrophobic part is directed toward the air phase (pores and capillaries that are not filled with water). On drying, the water is removed and the hydrophobic particles coalesce together and forma film.
The second theory is that the polymer interacts with the components of the Portland cement hydration products and forms new complexes. This creates a type of reinforcement in the concrete and produces semipermeable membranes. Chemical reactions have been noticed to take place between the particle surface of reactive polymers such as polyacrylic esters and calcium ions in the hydrating cement. Such reactions can improve the bond between the cement hydrates and aggregates and improve the final properties of the modified cement.
In actual practice both theories are likely to be applicable depending on the type and chemistry of the polymeric ingredients and the conditions present during hydration of the cement. The situation is highly complex since some low molecular weight chemicals (e.g., amine accelerators) that are present in the commercial polymer admixtures can also influence the reaction of Portland cement with water.
It is very important that both cement hydration and polymer film formation proceed well to yield a monolithic matrix in which the cement phase and the polymer film formation proceed well to yield a monolithic matrix in which the cement phase and polymer phase interpenetrate. The cement hydration process generally will occur faster than the polymer film forming process. Aggregates are then bound by this matrix.
The properties of both fresh and hardened cement are affected by parameters such as polymer type, polymer-to-cement ratio, air content and environmental conditions during the hardening of the mixture. Superior physical properties result compared with conventional unmodified cement mixtures. It is possible that the polymer films in cement act as crack-stoppers for any micro-cracks that can develop under stress. This will also lead to increased tensile strength and fracture toughness. Such effects as improved tensile strength, fracture toughness and waterproof generally increases with increasing polymer-to-cement rations up to certain limits.
Duraamen produces several types of polymer modified overlays. Please contact us to discuss your project requirements.