Cellulose ether will delay the hydration of cement to varying degrees, which is manifested in delaying the formation of ettringite, C-S-H gel and calcium hydroxide. At present, the mechanism of cellulose ether delaying cement hydration mainly includes the assumption of hindered ion movement, alkali degradation and adsorption.
1. Hypothesis of hindered ion movement
It is hypothesized that cellulose ethers increase the viscosity of the pore solution, hindering the rate of ion movement, thereby delaying cement hydration. However, in this experiment, the cellulose ether with lower viscosity has a stronger ability to delay cement hydration, so this hypothesis does not hold. In fact, the time for ion movement or migration is very short, which is obviously incomparable with the time of cement hydration delay.
2. Alkaline degradation
Polysaccharides are often easily degraded under alkaline conditions to form hydroxycarboxylic acids that delay cement hydration. Therefore, the reason why cellulose ether delays cement hydration may be that it degrades in alkaline cement slurry to form hydroxycarboxylic acids, but the study found that Cellulose ether is very stable under alkaline conditions, only slightly degraded, and the degraded products have almost no effect on the delay of cement hydration.
3. Adsorption
Adsorption may be the real reason why cellulose ether delays cement hydration. Many organic additives will adsorb to cement particles and hydration products, preventing the dissolution of cement particles and crystallization of hydration products, thereby delaying the hydration and setting of cement. It was found that cellulose ether is easily adsorbed to calcium hydroxide, C. S． The surface of hydration products such as H gel and calcium aluminate hydrate, but it is not easy to be adsorbed by ettringite and unhydrated phase. Moreover, as far as cellulose ether is concerned, the adsorption capacity of HEC is stronger than that of MC, and the lower the content of hydroxyethyl in HEC or hydroxypropyl in HPMC, the stronger the adsorption capacity is: in terms of hydration products, hydrogen The adsorption capacity of calcium oxide C. S． The adsorption capacity of H is stronger. Further analysis also shows that the adsorption capacity of hydration products and cellulose ether has a corresponding relationship with the delay of cement hydration: the stronger the adsorption, the more obvious the delay, but the adsorption of ettringite to cellulose ether is weak, but its formation was significantly delayed. Studies have also shown that cellulose ether has a strong adsorption on tricalcium silicate and its hydration products, thus significantly delaying the hydration of the silicate phase, and has a low adsorption to ettringite, but the formation of ettringite is restricted. Obviously delayed, this is because the delayed formation of ettringite is affected by the Ca2+ balance in the solution, which is the continuation of the delayed silicate hydration of cellulose ether.
In the test results, the retarding ability of HEC is stronger than that of MC, and the ability of cellulose ether to delay the formation of calcium hydroxide is stronger than that of C. S． The ability of H gel and ettringite is strong, which has a corresponding relationship with the adsorption capacity of cellulose ether and cement hydration products. It is further confirmed that adsorption may be the real reason why cellulose ether delays cement hydration, and cellulose ether and cement hydration products have a corresponding relationship. The stronger the adsorption capacity of cement hydration products, the more obvious the formation of delayed hydration products. The previous test results show that different cellulose ethers have different effects on Portland cement hydration delay, and the same cellulose ether has different delay effects on different hydration products, which shows that Portland cement hydration products have different effects on fiber. The adsorption of cellulose ether is selective, and the adsorption of cellulose ether to cement hydration products is also selective.