Effect of Rubber Powder on Cement Hydration and Paste Microstructure

Redispersible latex powder is a commonly used organic cementitious material. It is produced by spray-drying polymer emulsions stabilized with protective colloids such as polyvinyl alcohol, resulting in a powdered form that, upon contact with water, can be uniformly redispersed to reform an emulsion. The addition of redispersible latex powder can enhance the water retention of fresh cement mortar and improve the hardened mortar’s bond strength, flexibility, impermeability, and corrosion resistance.

Currently, the main types of redispersible latex powders used in the market include: vinyl acetate–ethylene copolymer latex powder (VAC/E), ethylene–vinyl chloride–vinyl laurate terpolymer latex powder (E/VC/VL), vinyl acetate–ethylene–higher fatty acid vinyl ester terpolymer latex powder (VAC/E/Veo Va), polyvinyl acetate homopolymer latex powder (PVAC), styrene–butadiene rubber latex powder (SBR), other binary and ternary copolymer latex powders, formulation-based latex powders incorporating functional additives, and formulation-based latex powders that combine functional additives with one or more types of latex powder. The following section will detail the mechanisms by which redispersible latex powders function in cement mortars and their effects on the performance of such mortars.

Composition of Redispersible Latex Powder

Redispersible latex powder is typically a white powder, although a small number of products are available in other colors. The main components of redispersible latex powder include the following:

(1) Polymer resins. Polymer resins constitute the core of the latex powder particles and are the primary active ingredients that enable redispersible latex powder to function, such as polyvinyl acetate–ethylene copolymer, styrene–butadiene copolymer, and ethylene–vinyl chloride–vinyl laurate copolymer, among others.
(2) Additives (internal). Internal additives serve to improve the performance of the resin, such as plasticizers that lower the film-forming temperature of the resin. Not every latex powder contains additive components; for example, vinyl acetate/ethylene copolymer resins do not require the addition of plasticizers.
(3) Protective colloid. The protective colloid is a hydrophilic layer that coats the surface of redispersible latex powder particles. In the vast majority of redispersible latex powders, the protective colloid is polyvinyl alcohol.
(4) Additives (external). To further enhance the performance of redispersible latex powder, other additives besides plasticizers are typically incorporated; these are referred to as “external additives.” In general, superplasticizers are added to redispersible latex powder to improve the flow and dispersibility of the emulsion.
(5) Anti-caking agents. Anti-caking agents are typically fine mineral fillers that prevent latex powder from caking during storage and transportation, and also facilitate the discharge of latex powder from paper bags or tank cars.

The Influence of Rubber Powder on Cement Hydration and Paste Structure

Once cement-based materials containing latex powder come into contact with water, the hydration reaction immediately commences. The calcium hydroxide solution quickly reaches saturation and precipitates as crystals, while ettringite crystals and calcium silicate hydrate gel are simultaneously formed. Polymer particles from the latex emulsion then deposit onto the gel matrix and unhydrated cement particles. As hydration progresses and more hydration products are generated, polymer particles gradually aggregate within the capillary pores and form a densely packed layer on the gel surface and on the surfaces of unhydrated cement particles. These aggregated polymer particles progressively fill the capillaries, though they cannot completely coat the inner surfaces of the pores. With further reduction in pore moisture due to hydration or drying, the tightly packed polymer particles on the gel matrix and within the pores coalesce into a continuous film, creating an interpenetrating composite structure that is intimately intertwined with the hydrated cement paste and enhancing the bond between the hydration products and the aggregates. Because the polymer-modified hydration products form a coating layer at the interface, this may inhibit the growth of ettringite and coarse calcium hydroxide crystals; moreover, the polymer’s condensation into a film within the interfacial transition zone pores results in a denser transition zone in the polymer-modified cement-based material. Certain active functional groups in the polymer molecules can also undergo cross-linking reactions with Ca²⁺, Al³⁺, and other ions present in the cement hydration products, thereby forming specialized bridging bonds that improve the physical microstructure of the hardened cement-based material, relieve internal stresses, and reduce the formation of microcracks. As the cement gel structure develops and moisture is consumed, polymer particles become increasingly confined within the capillary pores. With further cement hydration, the moisture content in the capillaries decreases, causing polymer particles to aggregate on the surfaces of the cement hydration product–gel/unhydrated cement particle mixture and on the aggregate surfaces, thus forming a continuous, densely packed layer; meanwhile, larger pores are filled by viscous or self-adhesive polymer particles.