1 Introduction
Cement-based tile adhesive is currently the largest application of special dry-mixed mortar, which is composed of cement as the main cementitious material and supplemented by graded aggregates, water-retaining agents, early strength agents, latex powder and other organic or inorganic additives mixture. Generally, it only needs to be mixed with water when used. Compared with ordinary cement mortar, it can greatly improve the bonding strength between the facing material and the substrate, and has good slip resistance and excellent water and water resistance. It is mainly used to paste decorative materials such as building interior and exterior wall tiles, floor tiles, etc. It is widely used in interior and exterior walls, floors, bathrooms, kitchens and other building decoration places. It is currently the most widely used Tile bonding material.
Usually when we judge the performance of a tile adhesive, we not only pay attention to its operational performance and anti-sliding ability, but also pay attention to its mechanical strength and opening time. Cellulose ether in tile adhesive not only affects the rheological properties of porcelain adhesive, such as smooth operation, sticking knife, etc., but also has a strong influence on the mechanical properties of tile adhesive
2. The impact on the opening time of tile adhesive
When rubber powder and cellulose ether co-exist in wet mortar, some data models show that rubber powder has stronger kinetic energy to attach to cement hydration products, and cellulose ether exists more in the interstitial fluid, which affects more Mortar viscosity and setting time. The surface tension of cellulose ether is higher than that of rubber powder, and more cellulose ether enrichment on the mortar interface will be beneficial to the formation of hydrogen bonds between the base surface and cellulose ether.
In the wet mortar, the water in the mortar evaporates, and the cellulose ether is enriched on the surface, and a film will be formed on the surface of the mortar within 5 minutes, which will reduce the subsequent evaporation rate, as more water is removed from the thicker mortar Part of it migrates to the thinner mortar layer, and the film formed at the beginning is partially dissolved, and the migration of water will bring more cellulose ether enrichment on the mortar surface.
Therefore, the film formation of cellulose ether on the surface of the mortar has a great influence on the performance of the mortar. 1) The formed film is too thin and will be dissolved twice, which cannot limit the evaporation of water and reduce the strength. 2) The formed film is too thick, the concentration of cellulose ether in the mortar interstitial liquid is high, and the viscosity is high, so it is not easy to break the surface film when the tiles are pasted. It can be seen that the film-forming properties of cellulose ether have a greater impact on the open time. The type of cellulose ether (HPMC, HEMC, MC, etc.) and the degree of etherification (substitution degree) directly affect the film-forming properties of cellulose ether, and the hardness and toughness of the film.
3. The influence on the drawing strength
In addition to imparting the above-mentioned beneficial properties to mortar, cellulose ether also delays the hydration kinetics of cement. This retarding effect is mainly due to the adsorption of cellulose ether molecules on various mineral phases in the cement system being hydrated, but generally speaking, the consensus is that cellulose ether molecules are mainly adsorbed on water such as C-S-H and calcium hydroxide. On the chemical products, it is rarely adsorbed on the original mineral phase of clinker. In addition, cellulose ether reduces the mobility of ions (Ca2+, SO42-, …) in the pore solution due to the increase in the viscosity of the pore solution, thereby further delaying the hydration process.
Viscosity is another important parameter, which represents the chemical characteristics of cellulose ether. As mentioned above, the viscosity mainly affects the water retention capacity and also has a significant effect on the workability of the fresh mortar. However, experimental studies have found that the viscosity of cellulose ether has almost no effect on cement hydration kinetics. Molecular weight has little effect on hydration, and the maximum difference between different molecular weights is only 10min. Therefore, molecular weight is not a key parameter to control cement hydration.
“Application of Cellulose Ether in Cement-Based Dry-Mixed Mortar Products” clearly pointed out that the retardation of cellulose ether depends on its chemical structure. And the general trend concluded is that for MHEC, the higher the degree of methylation, the smaller the retarding effect of cellulose ether. In addition, the retarding effect of hydrophilic substitution (such as substitution to HEC) is stronger than that of hydrophobic substitution (such as substitution to MH, MHEC, MHPC).
In the test, we consider HPMC, which is a compound ether. For HPMC, it needs a certain degree of absorption to ensure its water solubility and light transmittance. We know that the content of substituents also determines the gel temperature of HPMC, that is, It determines the use environment of HPMC, so the group content of HPMC that is usually applicable is also framed within a range. In this range, how to combine methoxy and hydroxypropoxy to achieve the best effect is the content of our research. Within a certain range, an increase in the content of methoxyl groups will lead to a downward trend in the pull-out strength, while an increase in the content of hydroxypropoxyl groups will lead to an increase in the pull-out strength. There is a similar effect for opening hours.
The change trend of the mechanical strength under the open time condition is consistent with the normal temperature condition, which is also consistent with the toughness of the cellulose ether film we talked about in Section 2. The methoxyl (DS) content is high and the hydroxypropoxyl HPMC with low (MS) content has good toughness of the film, but it will affect the wettability of the wet mortar to the surface material.