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Effect and Mechanism of Additives on the Properties of Sulphoaluminate Cement

Generally speaking, in sulphoaluminate cement, the retarding components are mainly ligneous calcium sulphonate, sugar calcium, boric acid, calcium gluconate and sodium gluconate, the early strength components are Li2CO3, Ca (NO2) 2 and so on, and the water reducing components are mainly polycarboxylate superplasticizer (PC), naphthalene superplasticizer (NSF), melamine superplasticizer (MSF), etc.

The retarding components of sulphoaluminate cement

For the retarding component, one Chinese scientist found that lignocellulose and sugar calcium have good retarding effect on the initial setting and final setting time of sulphoaluminate cement. After reaching the optimal content, the initial setting time is gradually shortened with the continuous increase of the content. Other scientists used pure boric acid as retarder and found that with the increase of boric acid content, the initial and final setting time of cement paste will be prolonged, and the appropriate setting time can be obtained with a small amount of boric acid. The suitable dosage of boric acid retarder is 0.1-0.2%. However, excessive boric acid will lead to cement paste non setting for a long time and damage the strength. Some scientists found that sodium silicate has a more obvious retarding effect on sulphoaluminate cement. When the content is less than 0.3%, the setting time of sulphoaluminate cement is obviously delayed. When the content is 0.3-0.5%, the setting time of sulphoaluminate cement will not continue to extend. When the content is more than 0.5%, the setting time will shorten with the increase of the content.

1. Ligneous calcium sulphonate

ligneous calcium sulphonate is an anionic surfactant with hydrophobic and hydrophilic groups. When it is dissolved in water, the hydrophobic group is adsorbed on the surface of cement particles, and the hydrophilic group points to the aqueous solution, so that the cement particles have the same charge, and the cement particles can not agglomerate under the action of electrostatic force. Because the hydrophilic group is anionic, the hydrophilic film with charge is formed on the surface of cement particles, which thickens the water film of diffusion layer, retards the adhesion between cement particles and the condensation of hydration products, increases the distance between cement particles and delays the setting time.

2. Sugar calcium

Sugar calcium is a brown sticky liquid which is neutralized by lime water in the process of sugar production. Its main chemical component is calcium adipate. It is difficult to form ettringite crystal when it is adsorbed on the surface of C3A in cement paste, which hinders the hydration of cement particles.

3. Boric acid

The retarding mechanism of boric acid can be explained as: after adding boric acid, tiny calcium borate grains are formed in cement paste, which cover the clinker surface and form a coating, thus slowing down the speed of H2O, Ca2 + and SO32 - entering anhydrous calcium sulphoaluminate. In order to delay the setting of cement, calcium borate grains should at least cover the surface of clinker. If there is a gap, H2O, Ca2+ and SO32- can easily contact the surface of cement clinker, so that the cement can not be effectively retarded. Therefore, the amount of boric acid should not be too small. However, if the addition amount is too large, the calcium borate coating formed on the clinker surface will be too thick and dense, so that the driving force for the diffusion of H2O, Ca2+ and SO32- is too small, and the formation and growth of ettringite crystal are basically in a stagnant state, making the cement paste always in an inert state, resulting in excessive retarding or even non setting.

4. Calcium gluconate and sodium gluconate

The quick setting of sulphoaluminate cement is determined by the crystallization characteristics of ettringite in cement paste. After adding sodium gluconate into cement, gluconate anion adsorbs on the surface of solid phase and hydration products to form a coating, slowing down the contact rate of H2O, Ca2+ and SO32- with anhydrous calcium sulphoaluminate, slowing down the formation and growth rate of ettringite crystal, thus delaying the setting of cement. When the amount of sodium gluconate is small, the coating is thin enough to prevent the migration of the above ions. Only when the content of sodium silicate reaches a certain amount, the thickness of the coating layer can effectively delay the migration of ions, and then delay the hydration of cement. When the dosage of sodium gluconate continues to increase, it will react with hydration products to form more calcium gluconate crystals in the slurry and body fluid phase, resulting in the slurry thickening and promoting coagulation. Therefore, when sodium gluconate is used as sulphoaluminate cement retarder, its dosage needs to be in the best range.

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