Tile adhesive is a mixture primarily composed of cement as the main bonding material, supplemented with graded aggregates, water-retaining agents, early strength agents, latex powder, and other organic or inorganic additives. Typically, it only needs to be mixed with water. Compared to regular cement mortar, it significantly enhances the bonding strength between the finishing material and the substrate. It possesses excellent anti-slip properties and advantages such as water resistance, heat resistance, and resistance to freeze-thaw cycles. Tile adhesive is mainly used for bonding decorative materials such as wall tiles and floor tiles in both interior and exterior construction. It is widely applicable in areas such as interior and exterior walls, floors, bathrooms, kitchens, and other architectural decorative spaces.
In addition to considering its workability and anti-slip capability, the performance of tile adhesive also focuses on its mechanical strength and open time. Cellulose ethers, such as hydroxypropyl methyl cellulose (HPMC), not only affect the rheological properties of tile adhesive, such as workability and slip resistance but also exert a strong influence on the mechanical properties and open time of the adhesive.
When both cement powder and cellulose ethers are present in wet mortar, some data suggests that cement powder has a stronger affinity for the hydration products of cement, while cellulose ethers are more present in the interstitial liquid, affecting the viscosity and setting time of the mortar. The surface tension of cellulose ethers is greater than that of cement powder, and a higher concentration of cellulose ethers at the mortar interface is beneficial for the formation of hydrogen bonds between the substrate and cellulose ethers. Cement powder changes the system’s consistency and workability in the wet state, and the addition of latex powder improves cohesion. After drying, it provides bonding strength for a smooth and dense surface layer, improving the interface action between sand and stone, and pores. With a guaranteed amount of addition, a film is formed at the interface, giving the tile adhesive flexibility, reducing the elastic modulus, absorbing thermal deformation stress to a large extent, and providing waterproofing when exposed to water later, buffering temperature, and inconsistent material deformation. The flexibility and rigidity of cement powder are generally determined by the glass transition temperature, with temperatures below 0 degrees indicating flexibility. The choice of cement powder in mortar generally depends on the performance characteristics of the product, and for tile adhesives, it is advisable to use cement powder with good bonding strength.
In wet mortar, as moisture evaporates from the mortar, cellulose ethers accumulate on the surface, forming a film within 5 minutes, reducing subsequent evaporation rates. As more water migrates from thicker to thinner parts of the mortar layer, the initially formed film partially dissolves. The migration of water brings more cellulose ethers to the surface. Cellulose provides good water retention and workability for freshly mixed mortar, especially in wetting areas, playing a critical role. To ensure the smooth progress of hydration reactions, excessive water absorption by the substrate and surface layer moisture evaporation are prevented. Due to its air-entraining properties, tile adhesive density is reduced, saving materials, and lowering the elastic modulus of hardened mortar.
The film formation of cellulose ethers on the surface of the mortar has a significant impact on the mortar’s performance. If the formed film is too thin, it may dissolve again, unable to restrict water evaporation, leading to reduced strength. If the film is too thick, cellulose ethers in the interstitial liquid of the mortar have a high concentration and viscosity, making it difficult to break the film when tiles are applied. Therefore, the film-forming performance of cellulose ethers has a significant impact on the open time. The type (HPMC, HEMC, MC, etc.) and degree of etherification of cellulose ethers directly affect their film-forming performance, including hardness and toughness.
Apart from imparting various beneficial properties to the mortar, cellulose ethers also delay the kinetic process of cement hydration. This retarding effect is mainly due to cellulose ether molecules adsorbing onto various mineral phases in the hydrating cement system, with a consensus that cellulose ether molecules primarily adsorb on C-S-H and hydrated lime, rarely on the original clinker mineral phases. Additionally, due to the increase in pore solution viscosity, cellulose ethers reduce the activity of ions (Ca2+, SO42-, etc.) in the pore solution, further slowing down the hydration process.
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