Both carboxymethyl cellulose and methyl cellulose can be used as water-retaining agents for plaster, but the water-retaining effect of carboxymethyl cellulose is much lower than that of methyl cellulose, and carboxymethyl cellulose contains sodium salt, so it is not suitable for plaster of paris. Has retarding effect and reduces the strength of plaster of paris. Methyl cellulose is an ideal admixture for gypsum cementitious materials integrating water retention, thickening, strengthening, and viscosifying, except that some varieties have a retarding effect when the dosage is large. higher than carboxymethyl cellulose. For this reason, most gypsum composite gelling materials adopt the method of compounding carboxymethyl cellulose and methyl cellulose, which not only exert their respective characteristics (such as the retarding effect of carboxymethyl cellulose, the reinforcing effect of methyl cellulose ), and exert their common advantages (such as their water retention and thickening effect). In this way, both the water retention performance of the gypsum cementitious material and the comprehensive performance of the gypsum cementitious material can be improved, while the cost increase is kept at the lowest point.
How important is the viscosity of methyl cellulose ether for gypsum mortar?
Viscosity is an important parameter of methyl cellulose ether performance.
Generally speaking, the higher the viscosity, the better the water retention effect of gypsum mortar. However, the higher the viscosity, the higher the molecular weight of methyl cellulose ether, and the corresponding decrease in its solubility will have a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the more viscous the wet mortar will be. During construction, it is manifested as sticking to the scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of the wet mortar itself. In addition, during construction, the anti-sag performance of wet mortar is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
How important is the fineness of cellulose ether to mortar?
Fineness is also an important performance index of methyl cellulose ether. The MC used for dry powder mortar is required to be powder with low water content, and the fineness also requires 20% to 60% of the particle size to be less than 63m. The fineness affects the solubility of methyl cellulose ether. Coarse MC is usually granular, which is easy to disperse and dissolve in water without agglomeration, but the dissolution rate is very slow, so it is not suitable for use in dry powder mortar. Some domestic products are flocculent, not easy to disperse and dissolve in water, and easy to agglomerate . In dry powder mortar, MC is dispersed between aggregates, fine fillers and cement and other cementing materials. Only fine enough powder can avoid methyl cellulose ether agglomeration when mixing with water. When MC is added with water to dissolve the agglomerates, it is very difficult to disperse and dissolve. Coarse MC is not only wasteful, but also reduces the local strength of the mortar. When such a dry powder mortar is applied in a large area, the curing speed of the local mortar will be significantly reduced, and cracks will appear due to different curing times. For the sprayed mortar with mechanical construction, the requirement for fineness is higher due to the shorter mixing time.
The fineness of MC also has a certain impact on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, under the same addition amount, the finer the finer the better the water retention effect.