HEMC (Hydroxyethyl Methyl Cellulose) is a water-soluble polymer material commonly used in building materials. It mainly plays a role in improving the fluidity of cement paste and delaying cement hydration reaction. In the hydration process of cement, HEMC has a certain influence on the hydration reaction and performance of cement.

1. Basic properties of HEMC
HEMC is a polymer formed by chemical modification of natural cellulose. Its molecular structure contains two substituents, hydroxyethyl and methyl, which makes it have good solubility, viscosity adjustment ability and film-forming properties. As an admixture of cement, HEMC can improve its fluidity, construction performance, and operability in cement paste, and can improve the strength and durability after hardening to a certain extent.

2. Effect of HEMC on Cement Hydration Process
Cement hydration is the process of cement and water reaction. Through this reaction, cement paste gradually hardens to form a solid cement matrix. As an admixture, HEMC can play a variety of roles in the cement hydration process. The specific effects are as follows:

2.1 Improving the fluidity of cement slurry
In the early stage of cement hydration, the fluidity of cement slurry is poor, which may be affected during construction. HEMC can effectively improve the fluidity of cement slurry due to its high viscosity and good water solubility. It disperses cement particles and reduces the aggregation between cement particles, thereby improving the consistency of cement slurry, making it easier to operate and pour during construction.

2.2 Delaying cement hydration reaction
The hydroxyethyl group in HEMC has strong hydrophilicity. It can form a hydration film on the surface of cement particles, slowing down the contact speed between cement particles and water, thereby delaying the hydration process of cement. This delaying effect is particularly important in high temperature or fast construction. It can prevent uneven strength development caused by excessive hydration of cement, and can extend the construction time to avoid early drying problems.

2.3 Improving the stability of cement slurry
During the hydration process of cement slurry, HEMC can effectively improve the stability of the slurry. The hydroxyethyl and methyl groups in the HEMC molecule can interact with cement particles through hydrogen bonds and van der Waals forces to form a stable paste structure. This stability helps prevent stratification in the cement paste, thereby improving the uniformity of the cement paste and ensuring the stability of the cement hydration process.

2.4 Improving the microstructure of cement hydration products
HEMC can improve the microstructure of cement hydration products by adjusting the fluidity and viscosity of cement paste. For example, in the later stage of cement hydration, HEMC can affect the formation and distribution of hydration products in cement paste, such as hydrated calcium silicate (C-S-H) gel. During the cement hydration process, the formation of C-S-H gel is a key factor in determining the strength and durability of cement. HEMC can promote the uniform distribution of C-S-H gel and improve the density and strength of cement by adjusting the ion concentration in the cement hydration reaction.

2.5 Effect on cement strength
The effect of HEMC on cement strength is closely related to the cement hydration process. In the early stage of cement hydration, the early strength of cement may decrease slightly due to the retarding effect of HEMC. However, as the cement hydration reaction continues, HEMC can help form a denser cement structure, thereby improving the ultimate strength of cement during long-term curing. In addition, HEMC can improve the crack resistance of cement, enhance the impermeability of cement structure, and improve the durability of cement.

3. Other effects of HEMC on cement
In addition to the above-mentioned effects on the cement hydration process, HEMC also has certain effects on other properties of cement, mainly including:

3.1 Improving the frost resistance and impermeability of cement
HEMC can improve the microstructure of cement, so that it can generate a denser cement matrix during the hydration process. This dense structure can effectively reduce the porosity inside the cement, thereby improving the frost resistance and impermeability of cement. Under extreme climatic conditions, the frost resistance and impermeability of cement structure are crucial to the long-term stability of buildings.

3.2 Enhance the corrosion resistance of cement
Since HEMC improves the density of cement paste, it can also effectively reduce the presence of pores inside cement, reduce the penetration of water, gas or chemicals, and thus improve the corrosion resistance of cement. Especially in some humid or acid-base environments, HEMC can help extend the service life of cement structures.

4. The amount and effect of HEMC
The effect of the amount of HEMC on the cement hydration process is a key factor. Generally speaking, the amount of HEMC added should be adjusted according to actual needs. Too much HEMC may cause the cement slurry to have a high consistency and affect the construction performance; while insufficient addition may not fully play its role in improving the performance of cement. Usually, the amount of HEMC added to cement is 0.2% to 1.0% (by cement mass), and the specific amount used should be determined according to different cement types and use environments.

As a cement admixture, HEMC plays an important role in improving the fluidity of cement slurry, delaying the cement hydration process, and improving the microstructure and strength of cement. Reasonable use of HEMC can improve the performance of cement to a certain extent, especially in improving the workability of cement, extending the operability time, and enhancing the strength and durability of hardened cement. However, the amount of HEMC used needs to be adjusted according to actual conditions to ensure its optimal performance. In construction projects, the application of HEMC will help improve the quality and durability of cement products.