A brief discussion on technical knowledge and indicators of water reducing agents

Water reducing agent is a high molecular polymer with amphiphilic properties. It is usually copolymerized with a polyoxyethylene ether macromonomer with terminal double bonds and an unsaturated carboxylic acid small molecule monomer under the action of an initiator. Reaction synthesis, in which the terminal alkenyl groups of large and small monomers form the molecular backbone through copolymerization, and the polyethylene glycol segments of the polyether macromonomers form structural side chains

Features: High water reduction rate, can Reaching 40%, good slump retention performance. High strength growth, increases early strength development and improves later strength of concrete. Stronger plasticizing and dispersing effect on cement particles. Can be used at very low water-cement ratios, such as the production of C100. Reduce concrete shrinkage. Reduce the carbonation rate and chloride ion erosion of concrete. Lower alkali content. Increase the compactness of concrete and improve the appearance quality of concrete. Concrete has good thixotropy and is easy to construct. Good air entrainment performance is beneficial to improving the durability of concrete.

Compared with previous generations, it has the advantages of low dosage, high water reduction rate, high dispersion, high slump retention, low air entrainment, and no bleeding. Water-reducing admixtures have become a new research hotspot because of their highly designable molecular structure, which is conducive to the systematic study of their structure-activity relationships and the development of functional series products. However, compared with the previous two generations of water-reducing agents, the price of water-reducing agents is higher. Since 2016, the environmental protection storm has swept through chemical companies and the supply of crude oil has been tight, causing the raw material costs of water-reducing agents to rise exponentially, further promoting the reduction of water-reducing agents. The unit price of aqueous agents has increased.

Specific functions:

(1) Polymers have a significant retarding effect on cement, mainly because the carboxyl group acts as a retarding component, and R-COO~ interacts with Ca2+ ions It forms a complex, reduces the Ca2+ ion concentration in the solution, delays the formation of Ca(OH)2 crystals, reduces the formation of C-H-S gel, and delays cement hydration.

(2) Carboxyl (-COOH), hydroxyl (-OH), amine (-NH2), polyoxyalkyl (-O-R)n and other polar groups with strong affinity to water are mainly absorbed through adsorption , dispersion, wetting, lubrication and other surface active effects, providing dispersion and flow properties to cement particles, and increasing the workability of fresh concrete by reducing the friction resistance between cement particles and reducing the free energy of the interface between cement particles and water. At the same time, similar substances are adsorbed on the surface of cement particles. Carboxylate ions make the cement particles negatively charged, causing electrostatic repulsion between the cement particles and dispersing the cement particles, which inhibits the agglomeration tendency of the cement slurry and increases the interaction between the cement particles and the cement particles. The water contact area allows the cement to fully hydrate. During the process of diffusing cement particles, free water surrounded by aggregate locks is released, which improves workability and reduces the amount of water mixed.

(3) The steric hindrance of molecular chains (i.e. steric repulsion). The quasi-substance molecules are adsorbed on the surface of cement particles in a “comb-type” form, forming an adsorption layer on the surface of the gel material. When the adsorption layers of polymer molecules are close to crossing each other, physical steric hindrance occurs between the polymer molecular chains, preventing the cement particles from The agglomeration of carboxylic acid superplasticizers is an important reason why they have stronger dispersion ability than other systems.

The dispersion mechanism of type (4) high-efficiency superplasticizers can be understood from the relationship between the elapsed time after mixing of the cement slurry and the zeta potential. Generally speaking, the slump loss of concrete using naphthalene-based and melamine-based high-performance water-reducing admixtures after 60 minutes is significantly higher than that of concrete containing high-performance water-reducing admixtures. This is mainly because the latter is different from the adsorption model of cement particles. The force of the polymer adsorption layer between cement particles is three-dimensional electrostatic repulsion, and the change in Zeta potential is small.

Only using these theories to explain the repulsion between ions is often very different from the experimental results. The steric hindrance effect can successfully explain the dispersion mechanism of the superplasticizer on cement. That is, the polymer is adsorbed on the surface of the cement particles, and its branch chains extending into the solution generate steric hindrance so that the particles cannot get close to each other, thereby making the cement The particles are dispersed and stabilized. This mechanism is now generally accepted. After isothermal adsorption of polymers with similar molecular weights and different branch chain lengths on cement, it was pointed out that polymers with long branch chains have low potential and high spatial repulsion, so they have good dispersion properties in cement after adsorption. The adsorption state of carboxylic acid graft copolymer high-efficiency superplasticizer macromolecules on the surface of cement particles is mostly tooth-shaped. This kind of water reducing agent not only has excellent dispersion of cement particles but also can keep the slump change very little over time. There are three reasons: one is because the graft copolymer has a large number of carboxyl groups, which has a certain chelating ability, and the three-dimensional electrostatic repulsion of the chain constitutes an obstacle to the aggregation of particles; the other is because in a strong alkaline medium such as cement slurry In the body, the graft copolymer chain gradually breaks, releasing carboxylic acid molecules, which makes the above-mentioned first effect continue to be valued; the third is that the absolute value of the Zeta potential of the graft copolymer is lower than that of the naphthalene series water-reducing agent, so it must be achieved In the same dispersion state, the total amount of charge required is not as much as that of naphthalene-based water reducing agent. </p