The effects of several common cellulose single ethers and mixed ethers in dry-mixed mortar on the water retention and thickening, fluidity, workability, air-entraining effect, and strength of dry-mixed mortar are reviewed. It is better than a single ether; the development direction of the application of cellulose ether in dry-mixed mortar is prospected.
Key words: cellulose ether; dry-mixed mortar; single ether; mixed ether
Traditional mortar has problems such as easy cracking, bleeding, poor performance, environmental pollution, etc., and will gradually be replaced by dry-mixed mortar. Dry-mixed mortar, also known as pre-mixed (dry) mortar, dry powder material, dry mix, dry powder mortar, dry-mixed mortar, is a semi-finished mixed mortar without mixing water. Cellulose ether has excellent properties such as thickening, emulsification, suspension, film formation, protective colloid, moisture retention, and adhesion, and is an important admixture in dry-mixed mortar.
This paper introduces the advantages, disadvantages and development trend of cellulose ether in the application of dry-mixed mortar.
1. Characteristics of dry-mixed mortar
According to the construction requirements, the dry-mixed mortar can be used after being accurately measured and fully mixed in the production workshop, and then mixed with water at the construction site according to the determined water-cement ratio. Compared with traditional mortar, dry-mixed mortar has the following advantages: ①Excellent quality, dry-mixed mortar is produced according to scientific formula, large-scale automation, coupled with appropriate admixtures to ensure that the product can meet special quality requirements; ②Variety Abundant, various performance mortars can be produced according to different requirements; ③Good construction performance, easy to apply and scrape, eliminating the need for substrate pre-wetting and subsequent watering maintenance; ④Easy to use, just add water and stir, easy to transport and store , convenient for construction management; ⑤ green and environmental protection, no dust on the construction site, no various piles of raw materials, reducing the impact on the surrounding environment; ⑥ economical, dry-mixed mortar avoids unreasonable use of raw materials due to reasonable ingredients, and is suitable for mechanization Construction shortens the construction cycle and reduces construction costs.
Cellulose ether is an important admixture of dry-mixed mortar. Cellulose ether can form a stable calcium-silicate-hydroxide (C-S-H) compound with sand and cement to meet the requirements of high-performance new mortar materials.
2. Cellulose ether as admixture
Cellulose ether is a modified natural polymer in which the hydrogen atoms on the hydroxyl group in the cellulose structural unit are replaced by other groups. The type, quantity and distribution of the substituent groups on the cellulose main chain determine the type and nature.
The hydroxyl group on the cellulose ether molecular chain produces intermolecular oxygen bonds, which can improve the uniformity and completeness of cement hydration; increase the consistency of mortar, change the rheology and compressibility of mortar; improve the crack resistance of mortar; Entraining air, improving the workability of mortar.
2.1 Application of carboxymethyl cellulose
Carboxymethylcellulose (CMC) is an ionic water-soluble single cellulose ether, and its sodium salt is usually used. Pure CMC is white or milky white fibrous powder or granules, odorless and tasteless. The main indicators to measure the quality of CMC are degree of substitution (DS) and viscosity, transparency and stability of solution.
After adding CMC to the mortar, it has obvious thickening and water retention effects, and the thickening effect largely depends on its molecular weight and degree of substitution. After adding CMC for 48 hours, it was measured that the water absorption rate of the mortar sample decreased. The lower the water absorption rate, the higher the water retention rate; the water retention effect increases with the increase of CMC addition. Due to the good water retention effect, it can ensure that the dry-mixed mortar mixture does not bleed or segregate. At present, CMC is mainly used as an anti-scouring agent in dams, docks, bridges and other buildings, which can reduce the impact of water on cement and fine aggregates and reduce environmental pollution.
CMC is an ionic compound and has high requirements on cement, otherwise it can react with Ca(OH)2 dissolved in cement after being mixed into cement slurry to form water-insoluble calcium carboxymethylcellulose and lose its viscosity, greatly reducing the The water retention performance of CMC is impaired; the enzyme resistance of CMC is poor.
2.2 Application of hydroxyethyl cellulose and hydroxypropyl cellulose
Hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC) are non-ionic water-soluble single cellulose ethers with high salt resistance. HEC is stable to heat; easily soluble in cold and hot water; when the pH value is 2-12, the viscosity changes little. HPC is soluble in water below 40°C and a large number of polar solvents. It has thermoplasticity and surface activity. The higher the degree of substitution, the lower the water temperature in which HPC can be dissolved.
As the amount of HEC added to the mortar increases, the compressive strength, tensile strength and corrosion resistance of the mortar decrease in a short period of time, and the performance changes little over time. HEC also affects the distribution of pores in the mortar. After adding HPC to the mortar, the porosity of the mortar is very low, and the required water is reduced, thus reducing the working performance of the mortar. In actual use, HPC should be used together with plasticizer to improve the performance of mortar.
2.3 Application of methyl cellulose
Methylcellulose (MC) is a non-ionic single cellulose ether, which can quickly disperse and swell in hot water at 80-90°C, and dissolve quickly after cooling down. The aqueous solution of MC can form a gel. When heated, MC does not dissolve in water to form a gel, and when cooled, the gel melts. This phenomenon is completely reversible. After adding MC to the mortar, the water retention effect is obviously improved. The water retention of MC depends on its viscosity, degree of substitution, fineness, and addition amount. Adding MC can improve the anti-sagging property of mortar; improve the lubricity and uniformity of dispersed particles, make the mortar smoother and more uniform, the effect of troweling and smoothing is more ideal, and the working performance is improved.
The amount of MC added has a great influence on the mortar. When the MC content is greater than 2%, the strength of the mortar is reduced to half of the original. The water retention effect increases with the increase of the viscosity of MC, but when the viscosity of MC reaches a certain value, the solubility of MC decreases, the water retention does not change much, and the construction performance decreases.
2.4 Application of hydroxyethylmethylcellulose and hydroxypropylmethylcellulose
A single ether has the disadvantages of poor dispersibility, agglomeration and rapid hardening when the amount added is small, and too many voids in the mortar when the amount added is large, and the hardness of the concrete deteriorates; therefore, workability, compressive strength, and flexural strength The performance is not ideal. Mixed ethers can overcome the shortcomings of single ethers to a certain extent; the amount added is less than that of single ethers.
Hydroxyethylmethylcellulose (HEMC) and hydroxypropylmethylcellulose (HPMC) are nonionic mixed cellulose ethers with the properties of each single substituent cellulose ether.
The appearance of HEMC is white, off-white powder or granule, odorless and tasteless, hygroscopic, insoluble in hot water. The dissolution is not affected by the pH value (similar to MC), but due to the addition of hydroxyethyl groups on the molecular chain, HEMC has higher salt tolerance than MC, is easier to dissolve in water, and has a higher condensation temperature. HEMC has stronger water retention than MC; viscosity stability, mildew resistance, and dispersibility are stronger than HEC.
HPMC is white or off-white powder, non-toxic, tasteless and odorless. The performance of HPMC with different specifications is quite different. HPMC dissolves in cold water into a clear or slightly turbid colloidal solution, soluble in some organic solvents, and also soluble in water. Mixed solvents of organic solvents, such as ethanol in an appropriate proportion, in water. The aqueous solution has the characteristics of high surface activity, high transparency and stable performance. The dissolution of HPMC in water is also not affected by pH. Solubility varies with viscosity, the lower the viscosity, the greater the solubility. With the decrease of methoxyl content in HPMC molecules, the gel point of HPMC increases, the water solubility decreases, and the surface activity also decreases. In addition to the common characteristics of some cellulose ethers, HPMC also has good salt resistance, dimensional stability, enzyme resistance, and high dispersibility.
The main functions of HEMC and HPMC in dry-mixed mortar are as follows. ① Good water retention. HEMC and HPMC can ensure that the mortar will not cause problems such as sanding, powdering and strength reduction of the product due to lack of water and incomplete cement hydration. Improve uniformity, workability and product hardening. When the amount of HPMC added is greater than 0.08%, the yield stress and plastic viscosity of the mortar also increase with the increase of the amount of HPMC. ②As an air-entraining agent. When the content of HEMC and HPMC is 0.5%, the gas content is the largest, about 55%. The flexural strength and compressive strength of mortar. ③ Improve workability. The addition of HEMC and HPMC facilitates carding of thin-layer mortar and paving of plastering mortar.
HEMC and HPMC can delay the hydration of mortar particles, DS is the most important factor affecting hydration, and the effect of methoxyl content on delayed hydration is greater than that of hydroxyethyl and hydroxypropyl content.
It should be noted that cellulose ether has a double effect on the performance of mortar, and it can play a good role if used properly, but it will have a negative effect if used improperly. The performance of dry-mixed mortar is firstly related to the adaptability of cellulose ether, and the applicable cellulose ether is also related to factors such as the amount and order of addition. In practical applications, a single type of cellulose ether can be selected, or different types of cellulose ether can be used in combination.
3. Outlook
The rapid development of dry-mixed mortar provides opportunities and challenges for the development and application of cellulose ether. Researchers and producers should seize the opportunity to improve their technical level, and work hard to increase varieties and improve product stability. While meeting the requirements for the use of dry-mixed mortar, it has achieved a leap in the cellulose ether industry.