1. Overview of cellulose ether (HPMC)
Hydroxypropyl methylcellulose (HPMC) is a commonly used cellulose ether compound, which is chemically modified from natural cellulose. It has excellent water solubility, film-forming, thickening and adhesive properties, so it is widely used in building materials. The application of HPMC in cement-based materials is mainly to improve its fluidity, water retention and adjust the setting time.
2.Basic process of cement setting
The process of cement reacting with water to form hydrates is called hydration reaction. This process is divided into several stages:
Induction period: Cement particles begin to dissolve, forming calcium ions and silicate ions, showing a short-term flow state.
Acceleration period: Hydration products increase rapidly and the setting process begins.
Deceleration period: The hydration rate decreases, cement begins to harden, and a solid cement stone is formed.
Stabilization period: Hydration products gradually mature and strength gradually increases.
Setting time is usually divided into initial setting time and final setting time. Initial setting time refers to the time when cement paste begins to lose plasticity, and final setting time refers to the time when cement paste completely loses plasticity and enters the hardening stage.
3. Mechanism of the influence of HPMC on cement setting time
3.1 Thickening effect
HPMC has a significant thickening effect. It can increase the viscosity of cement paste and form a high-viscosity system. This thickening effect will affect the dispersion and sedimentation of cement particles, and thus affect the progress of hydration reaction. The thickening effect reduces the deposition rate of hydration products on the surface of cement particles, thereby delaying the setting time.
3.2 Water retention
HPMC has good water retention capacity. Adding HPMC to cement paste can significantly improve the water retention of the paste. High water retention can prevent the water on the surface of cement from evaporating too quickly, so as to maintain the water content in the cement paste and prolong the time of hydration reaction. In addition, water retention helps cement paste maintain proper humidity during the curing process and reduce the risk of cracking caused by early water loss.
3.3 Hydration retardation
HPMC can form a protective film covering the surface of cement particles, which will hinder the hydration reaction. This protective film prevents direct contact between cement particles and water, thereby delaying the hydration process of cement and prolonging the setting time. This delay effect is particularly evident in high molecular weight HPMC.
3.4 Enhanced thixotropy
The addition of HPMC can also enhance the thixotropy of cement slurry (i.e., the fluidity increases under the action of external force and returns to the original state after the external force is removed). This thixotropic property helps to improve the workability of cement slurry, but in terms of setting time, this enhanced thixotropy may cause the slurry to redistribute under shear force, further prolonging the setting time.
4. Practical application of HPMC affecting cement setting time
4.1 Self-leveling floor materials
In self-leveling floor materials, cement requires a longer initial setting time for leveling and screeding operations. Adding HPMC can prolong the initial setting time of cement, allowing self-leveling materials to have a longer operating time during construction, avoiding the problem caused by premature setting of cement slurry during construction.
4.2 Premixed mortar
In premixed mortar, HPMC not only improves the water retention of mortar, but also prolongs the setting time. This is especially important for occasions with long transportation and construction time, ensuring that the mortar maintains good operability before use and avoiding construction difficulties caused by too short setting time.
4.3 Dry-mixed mortar
HPMC is often added to dry-mixed mortar to improve its construction performance. The thickening effect of HPMC increases the viscosity of the mortar, making it easy to apply and level during construction, and also prolongs the setting time, giving construction workers sufficient time to make adjustments.
5. Factors affecting the setting time of cement by HPMC
5.1 HPMC addition amount
The amount of HPMC added is a key factor affecting the setting time of cement. Generally, the greater the amount of HPMC added, the more obvious the extension of the setting time of cement. This is because more HPMC molecules can cover more cement particle surfaces and hinder hydration reactions.
5.2 Molecular weight of HPMC
HPMC of different molecular weights has different effects on the setting time of cement. HPMC with high molecular weight usually has stronger thickening effect and water retention capacity, so it can prolong the setting time more significantly. Although HPMC with low molecular weight can also prolong the setting time, the effect is relatively weak.
5.3 Environmental conditions
Ambient temperature and humidity will also affect the effect of HPMC on cement setting time. In high temperature environment, cement hydration reaction is accelerated, but the water retention property of HPMC slows down this effect. In low temperature environment, hydration reaction itself is slow, and the thickening and water retention effect of HPMC may cause cement setting time to be significantly prolonged.
5.4 Water-cement ratio
Changes in water-cement ratio will also affect the effect of HPMC on cement setting time. At a higher water-cement ratio, there is more water in the cement paste, and the water retention effect of HPMC may have less effect on the setting time. At a lower water-cement ratio, the thickening effect of HPMC will be more obvious, and the effect of extending the setting time will be more significant.
As an important cement additive, HPMC significantly affects the setting time of cement through various mechanisms such as thickening, water retention, and retardation of hydration reaction. The application of HPMC can prolong the initial and final setting time of cement, provide longer construction operation time, and improve the performance of cement-based materials. In practical applications, factors such as the amount of HPMC added, molecular weight, and environmental conditions jointly determine its specific effect on cement setting time. By rationally adjusting these factors, precise control of cement setting time can be achieved to meet the needs of different construction projects.
Post time: Jun-21-2024