How polycarboxylate superplasticizers enhance the workability of mortar and concrete
Release Time:
2020-06-05
Source:
Water-reducing agents are typically surfactants, specifically anionic surfactants. They adsorb onto the surface of cement particles, imparting an electrical charge to the particles; because like charges repel one another, the particles are dispersed, releasing excess interparticle water and thereby achieving a water-reducing effect. On the other hand, the formation of an adsorbed film on the surface of cement particles in the presence of a water-reducing agent influences the rate of cement hydration, promoting more complete crystal growth in the hardened cement paste, reducing capillary porosity due to water evaporation, and resulting in a denser network structure. This, in turn, enhances the hardness and structural density of cement mortars.
The primary function of a water-reducing admixture is to disperse cement particles, improve workability, and reduce the water content, thereby enhancing the density and hardness of cement-based materials and increasing their flowability. After cement is mixed with water, interparticle attractive forces cause the cement paste to form a flocculent structure, trapping 10% to 30% of the mixing water within the cement particles. This entrapped water cannot participate in free flow or lubrication, thus impairing the workability of the concrete mix. Upon addition of a water-reducing admixture, the admixture molecules adsorb selectively onto the surface of cement particles, imparting a uniform surface charge—typically negative—on the particles. This generates electrostatic repulsion, which promotes particle dispersion, disrupts the flocculent structure, releases the trapped water, and allows it to contribute to flow, thereby effectively increasing the workability of the concrete mix.
The hydrophilic groups in water-reducing agents are highly polar; consequently, the adsorbed film of the agent on the surface of cement particles can form a stable solvated water layer with water molecules. This water layer provides excellent lubrication, effectively reducing the sliding resistance between cement particles and thereby further enhancing the workability of concrete.
The water-reducing agent features hydrophilic polyether side chains that extend into the aqueous solution, thereby forming a hydrophilic three-dimensional adsorption layer of a certain thickness on the surface of the adsorbed cement particles. As cement particles approach one another, these adsorption layers begin to overlap, generating steric hindrance between the particles. The greater the degree of overlap, the stronger the steric repulsive force, and the more effectively it impedes particle aggregation, thus maintaining good slump retention in concrete.
During the preparation of polycarboxylate superplasticizers, branched side chains are grafted onto the polymer backbone. These side chains not only impart steric hindrance but also gradually cleave under the highly alkaline conditions of cement hydration, thereby releasing polycarboxylic acids with dispersing activity. This enhances the dispersion of cement particles and helps control slump loss.
