In a large number of tests on early-strength water-reducing agents, it was found that it has an early-strength effect. In the molecular chain of the water-reducing agent, there are mainly several leading functional groups, which play an early and strong role in the water-reducing agent. The editor of Anhui Yulong New Materials Co., Ltd. will take you to learn more about it:
1. Hydroxyl (-OH): Typical hydroxyl compounds are fatty alcohols. The main function of simple monohydric alcohol is to retard setting. The reason for retarded setting may be that hydroxyl groups are adsorbed on the surface of cement particles and generate hydrogen bonds. Different cement mineral components have different adsorption capacities for hydroxyl groups. C3A has the strongest adsorption capacity, followed by C4AF, C3S and C2S. Among homologues of alcohols, the more hydroxyl groups there are, the stronger the retarding effect. For example, glycerol can stop the cement reaction. Polyols with larger molecular weights, such as monosaccharides and oligomers, have a high retarding effect. At the same time, as the hydrophobic group increases, the surface activity becomes stronger, thereby plasticizing and reducing water on the cement. The retarding effect of hydroxyl compounds can be considered as: (1) The hydroxyl groups are adsorbed by Ca 2+ on the surface of the cement particles, and the adsorption film hinders hydration; (2) The hydroxyl groups and O2- on the surface of cement particles forms hydrogen bonds (-O∙∙∙ H-O-). The latter explanation is preferred for polyols.
2. Carboxyl groups and acid salts (-COOHCOOH COOH, -COOMCOOM COOM): Calcium formate is the simplest compound containing carboxyl groups and can be used as an early agent for concrete. Lower carboxylates all have early strengthening effects. Studies have shown that organic acids with dissociation constants Pk less than 5 have a certain coagulation-promoting effect, such as oxalic acid, acetic acid, pronaphthyl, etc. and their salts; when Pk is greater than 5, the retarding effect increases with the increase of the alkyl group (R). And increase, such as stearic acid and its salts.
3. Hydroxycarboxylic acid (salt) and aminocarboxylic acid (salt): General low-level carboxylic acid (salt) has an early strengthening effect, but if the hydrogen in the α-position or β-position of the group is hydroxyl Or amino substitution will produce obvious retarding effect. The retarding effect of hydroxy acids, amino acids and their salts on cement is mainly due to the interaction between the carboxyl group, hydroxylamine and amino group (-COOHCOOH COOH, -OH, -NH 2) in the molecule with the free hydroxyl group in the alkaline medium of the cement slurry. Ca 2+ and the like form unstable complexes, which inhibit the initial hydration of cement. However, as hydration continues, they will decompose on their own and therefore do not affect the continued hydration of cement.
4. Sulfonate: Sulfonate surfactant plays an important role in cement dispersants. It is a typical anionic surfactant. Since the surface of cement particles is positively charged in the early stage of hydration, it is conducive to the adsorption of anionic surfactants, which in turn delays the cement hydration reaction.
Under the same dosage conditions, high sulfonate concentration on the polymer can give high cement slurry fluidity. However, if sodium chloride or sodium sulfate is added to a cement mixture containing early-strength superplasticizer, the fluidity of the cement slurry will decrease. At the same ionic strength, sodium sulfate reduces the fluidity of cement slurry twice as much as sodium chloride. This is because the increase in ionic strength and the competitive adsorption between carboxyl groups and sulfate groups cause the three-dimensional size of the early-strength superplasticizer to shrink. The sulfate concentration in the liquid phase can be controlled by adding water-soluble polyvalent cations such as CaC12 and soluble sulfates such as Na2SO4. Depending on the charge valence, the added amount of multiply charged cations has an optimal value for mobility.
5. Polyoxyethylene chain: Polyoxyethylene chain is usually copolymerized or grafted on the main chain as a side chain, which is the main source of steric hindrance effect. The length of its chain segments plays a decisive role in the strength of the steric hindrance effect. It has an optimal range of length. The polyoxyethylene branched segment has a thin solvation film associated with water and weak stereorepulsion; the longer the molecular branch, the stronger the force between branches and the stronger the dispersion effect, but Branch chains that are too long will have a "bridging effect" between particles, leading to enhanced flocculation. Copolymers with long graft chains are beneficial to early fluidity, but have poor fluidity retention, so the polymers are matched with chains of different lengths.
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