High-efficiency superplasticizer is widely used to improve the performance of the concrete mixing big fluidity of concrete, improve the strength of concrete or manufacturing high strength concrete, with a variety of high-efficiency superplasticizer in different cementing materials and the application of various kinds of concrete, many cases have also been high-efficiency superplasticizer and cement does not adapt, mainly in low water-reducing effect or increase the liquidity of the result is bad, condenses too fast, retarded, slump loss, fast,Even reduce the strength of concrete, this kind of maladjustment and efficient superplasticizer varieties and action mechanism, the selection of raw materials and manufacturing process, the composition of cementitious materials, fineness, cement grinding stage of the difference, other such as ambient temperature, adding method and admixture dosage will also have an impact.
Mechanism of superplasticizer on rheological properties of concrete
Dispersion effect: after add water mixing cement, due to the hydration of cement particles, cement particles that form the structure of electric double layer, form solvation water film, and the cement particle surface with opposite charges make cement particle between associating with cooperation, to form slurry flocculation structure, make the 10% ~ 30% of the mixing water is wrapped in cement particles, can't participate in free flow and lubrication, thus affecting the liquidity of concrete mixture. When adding superplasticizer, because the superplasticizer molecules adsorbed on the surface of cement particles can redirect, cement particle surface (usually a negative charge) with the same charge, form the electrostatic repulsion, prompted the cement particles disperse mutually, flocculation structure collapse, release the wrapped part of the water, participate in the flow, so as to effectively increase the liquidity of concrete mixture.
Lubrication: superplasticizer hydrophilic polarity is very strong, so the superplasticizer adsorption film on the surface of cement particles can form a stable solvent water film with water molecules, this layer of water film has a good lubrication effect, can effectively reduce the sliding resistance between cement particles, so as to further improve the mobility of concrete.
Steric hindrance: hydrophilic branched chains in the superplasticizer structure extend in the aqueous solution, thus forming a hydrophilic solid adsorption layer with a certain thickness on the surface of the adsorbed cement particles. When cement particles are close to each other, the adsorption layer begins to overlap, that is, the steric hindrance between cement particles is generated. The more the overlap is, the greater the steric hindrance is, and the greater the hindrance is to the cohesion between cement particles so that the slump of concrete remains good.
Graft copolymerization slow-release effect of branched-chain: new type of superplasticizer such as poly (carboxylic acid superplasticizer in the process of preparation, the molecules of superplasticizer on the grafting on some of the branched-chain, the branched-chain not only can provide space steric effect, and, in the high basicity of cement hydration environment, the branched-chain can also slowly is cut off, to release more carboxylic acid with dispersion effect, so that it can improve the effectiveness of the dispersion of cement particles, and control the slump loss.