The advanges of HPMC & MHEC in dry mix mortar products
November 22nd 2024
Cellulose ethers are part of the formulation in cement based tile adhesives. They provide water retention and a special rheological profile to the wet mortar. They also determine to a large extent the complete performance properties of the set adhesive.
Cellulose ethers are part of the formulation in cement based tile adhesives. They provide water retention and a special rheological profile to the wet mortar. They also determine to a large extent the complete performance properties of the set adhesive. New hydroxyethyl methyl cellulose (HEMC) were developed showing special dissolution and thermogelation properties compared to conventional celllulose ethers. We know that methylcellulose forms strong gels upon heating of an aqueous solution. They also provide long open times in tile adhesive formulations.
The film formation of cellulose ether on the surface of the mortar has a great influence on the performance of the mortar. 1) The film formed 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 interstitial liquid of the mortar is high, and the viscosity is large, and 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 (degree of substitution) directly affect the film-forming properties of the cellulose ether, and the hardness and toughness of the film.
Cellulose ether not only imparts the above-mentioned beneficial properties to the mortar, but also delays the hydration kinetics process of cement. This retardation 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 view is that cellulose ether molecules are mainly adsorbed on water such as CSH and calcium hydroxide. The chemical products are rarely adsorbed on the original mineral phase of the clinker. In addition, due to the increase in the viscosity of the pore solution, the cellulose ether reduces the mobility of ions (Ca2+, SO42-, …) in the pore solution, thereby further delaying the hydration process.
Cellulose ethers are essential additives in many applications of dry-mix mortar products. The most important property of cellulose ether is its water retention in mineral building materials. If cellulose ether is not added, the thin layer of fresh mortar will dry out quickly, making the cement unable to be hydrated in the normal way, resulting in the mortar being unable to harden and obtaining good adhesion to the base layer.
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