Hydroxypropyl methylcellulose (HPMC), as one of the most commonly used organic polymer additives in cement-based materials, mainly functions to improve the workability, water retention, and adhesion of mortar or concrete. However, as a hydrophilic polymer, HPMC also affects the hydration process and microstructure of cement to varying degrees, thus significantly influencing the properties of the hardened material. In-depth research into its mechanism of action helps to more scientifically control its dosage in engineering applications, improving system stability and final strength.
1. The most direct impact of HPMC on the cement hydration process stems from its water retention effect. Cement hydration requires sufficient free water, but during mortar preparation, due to factors such as water absorption and evaporation from the substrate, water may be rapidly lost, leading to incomplete hydration. HPMC, by forming a dense three-dimensional network structure, increases the viscosity and water-holding capacity of the slurry, retaining water within the system and delaying the outward migration of free water. This process is equivalent to “delayed water supply.” Therefore, HPMC can slow down the hydration rate in the early stages, but it continuously provides moisture to cement particles, allowing for more complete hydration in the middle and later stages, thus improving structural density.
2. HPMC’s influence on the surface behavior of cement particles also regulates the hydration process. After swelling, HPMC forms a thin film, partially adsorbed on the surface of cement particles, inhibiting the hydration reaction interface. The presence of the adsorbed film delays the initial contact between minerals such as C3S and C2S and water, leading to a prolonged induction period and a delayed hydration peak. This explains why mortars containing HPMC generally exhibit prolonged setting times. Furthermore, HPMC with high substitution degrees or high viscosity has a stronger adsorption capacity and a more significant impact on hydration.
3. HPMC increases the viscosity of the system, reducing the fluidity of the cement paste and affecting the migration and crystallization morphology of hydration products. Higher viscosity restricts crystal growth space, resulting in finer C-S-H gel and Ca(OH)₂ crystals, promoting structural homogenization. Although early strength may decrease, later-stage strength is improved due to more complete hydration and reduced capillary porosity.
4. In terms of microstructure, the addition of HPMC results in a denser network structure of hydration products. It may form weak physical adsorption with C-S-H gel, improving interfacial adhesion and reducing porosity. Some studies indicate that, at appropriate amounts, HPMC can improve structural defects in the interfacial transition zone (ITZ), enhancing the overall toughness and crack resistance of the mortar.
5. The effect of HPMC on cement hydration is two-sided. Excessive addition can significantly delay hydration, leading to excessive reduction in early strength; simultaneously, increased organic content may decrease the high-temperature resistance of cementitious materials. Therefore, the dosage should be rationally controlled according to the mortar type, cementitious material type, and construction conditions. For example, in thin-layer construction materials such as tile adhesive and putty powder, HPMC’s water retention and time-delay effect are advantageous; however, in repair mortars requiring high early strength, the viscosity and addition ratio should be carefully selected.
6. HPMC impact on the cement hydration process is mainly reflected in three aspects: delaying hydration and promoting later-stage reactions through water retention; extending the induction period through adsorption; and altering the morphology and structure of hydration products by increasing viscosity. Reasonable control of the type and dosage of HPMC can improve mortar performance while avoiding excessive impact on early strength, achieving the optimal balance between workability and mechanical properties.
Post time: Nov-17-2025