The modifying effects of cellulose ether on fresh cementitious materials mainly include thickening, water retention, air entrainment and retardation. With the wide application of cellulose ether in cementitious materials, the interaction between cellulose ether and cement slurry is gradually becoming a research hotspot. The focus is on the influence of the molecular structure of cellulose ether on the microstructure and macroscopic properties of cement slurry, and clarifying the mechanism of interaction between cellulose ether and cement slurry.
Hydration heat
According to the curve of the release of hydration heat over time, the hydration process of cement is usually divided into five stages: initial hydration (0–15 min), induction period (15 min–4 h), acceleration and coagulation period (4 h–8 h), deceleration and hardening period (8 h–24 h), and curing period (1 d–28 d).
The test results show that in the pre-induction period (i.e. the initial hydration period), compared to the blank cement slurry, when the HEMC content is 0.1%, the first exothermic peak of the slurry body is advanced and the peak value is significantly increased. When the HEMC content is increased to above 0.3%, the first exothermic peak of the slurry body is delayed, and the peak value gradually decreases with the increase of the HEMC content. HEMC significantly delays the induction period and acceleration period of the cement slurry, and the larger the content, the longer, the acceleration period shifts further back, and the exothermic peak is smaller; changes in the dosage of cellulose ether have no significant effect on the length of the retardation and stabilization periods of the cement slurry, as shown in Figure 3(a); cellulose ether also reduces the hydration heat of the cement slurry within 72h, but when the hydration heat release time is above 36h, changes in the dosage of cellulose ether have little effect on the hydration heat of the cement slurry.
Mechanical properties
A study of two cellulose ethers with viscosities of 60,000 Pa·s and 100,000 Pa·s found that the compressive strength of modified mortars with a single dose of methyl cellulose ether decreased gradually with increasing dosage. The compressive strength of modified mortars with a single dose of hydroxypropyl methyl cellulose ether with a viscosity of 100,000 Pa·s first increased and then decreased with increasing dosage. This shows that the addition of methyl cellulose ether will significantly reduce the compressive strength of cement mortar, and the more it is added, the lower the strength. The lower the viscosity, the greater the impact on the loss of compressive strength of the mortar. Hydroxypropyl methyl cellulose ether can appropriately increase the compressive strength of the mortar when the dosage is less than 0.1%, but when the dosage is greater than 0.1%, the compressive strength of the mortar will decrease with the increase of the dosage. Therefore, the dosage should be controlled at 0.1%.
Setting time
By measuring the setting time of cement slurry with hydroxypropyl methyl cellulose ether of 100,000 Pa·s viscosity at different dosages, it was found that as the HPMC dosage increased, the initial and final setting times of the cement slurry both increased. When the dosage was 1%, the initial setting time reached 510 min and the final setting time reached 850 min. Compared with the blank sample, the initial setting by 210min and the final setting time by 470min. Whether it is HPMC with a viscosity of 50000Pa·s, 100000Pa·s or 200000Pa·s, it can delay cement setting, but compared with the three cellulose ethers, the initial and final setting times are both extended with increasing viscosity, as shown in Figure 6. This is because the cellulose ether adsorbs on the surface of the cement particles, preventing the water from contacting the cement particles, thereby delaying the hydration of the cement. The higher the viscosity of the cellulose ether, the thicker the adsorption layer on the surface of the cement particles, and the more significant the retarding effect.
Methyl cellulose ether and hydroxypropyl methyl cellulose ether can significantly prolong the setting time of cement slurry, ensuring that the slurry has enough time and moisture to carry out the hydration reaction, solving the problems of low strength and cracking after the slurry hardens.
Water retention rate
As the amount of cellulose ether added increases, the water retention rate of the mortar increases. When the amount of cellulose added is greater than 0.6%, the water retention rate tends to stabilise. However, when comparing the three cellulose ethers (HPMC with viscosities of 50,000 Pa·s (MC-5), 100,000 Pa·s (MC-10) and 200,000 Pa·s (MC-20)), there are differences in the effect of viscosity on water retention. At the same dosage, the order of water retention is: MC-5 < MC-10 < MC-20.