Application progress of cellulose ethers in dry mixed mortar

Traditional mortar easy cracking and bleeding, the performance is not up to the requirements, environmental pollution and other issues, will be gradually replaced by dry mixed mortar. Dry mixed mortar, also known as pre temperature (dry), dry powder mortar, dry mixing, dry mortar, dry mortar, semi-finished products, does not contain cellulose ether mixed mortar mixing water with excellent thickening, emulsifying, suspension, performance film, protective colloid, keep moisture, adhesive, is dry mixed mortar admixture in the important.

This paper introduces the advantages and disadvantages of cellulose ethers in dry mixed mortar and their development trend.
1. Characteristics of dry mixed mortar
Dry mixed mortar is in accordance with construction requirements, in the production workshop in accordance with accurate measurement, fully mixing evenly, to the site, according to a certain water cement ratio, mixing water can be used. Compared with the traditional mortar, dry mixed mortar has the following advantages: (1) excellent quality, dry mixed mortar is in accordance with the scientific formula, large-scale automatic production, coupled with appropriate additives to ensure product quality can meet the special requirements; (2) variety, according to the different requirements of production service performance of mortar; (3) the construction of good performance, easy to apply, save material pre moderate late water curing process; (4) convenient, mixing water can be used for transportation and storage, convenient construction and management; (5) construction of green environmental protection, no dust, no raw materials to reduce the influence of pile up like a mountain hand. The surrounding environment; (6) the economy, because of the dry mixed mortar ingredients reasonable and avoid the unreasonable use of raw materials, suitable for mechanized construction, shorten the construction period, reduce construction cost.
Cellulose ether is an important admixture of dry mixed mortar. Cellulose ether can form a stable calcium silicate hydroxide (C-S-H) complex with sand and cement to meet the requirements of high performance new mortar materials.

2. Cellulose ether as additive
Cellulose ether is a kind of modified natural polymer chlorine atom on hydroxyl cellulose structure unit is replaced by another group, the type and number of substituents and their distribution in the cellulose backbone determines the types and properties of cellulose ether.
Cellulose ether on the molecular chains of hydroxyl of intermolecular hydrogen bonds can improve the uniformity and completeness of cement hydration, increase of mortar consistency, change the mortar rheological properties and compressibility, improve crack resistance of mortar, air entraining, improve workability of mortar.
(1) application of carboxymethyl cellulose
Carboxymethyl cellulose (CMC) is an ionic water-soluble cellulose ether, which is usually used as a sodium salt. Pure CMC is white or creamy white fibrous powder or granule, odorless and tasteless. The main indexes of CMC quality are degree of substitution (DS) and viscosity, transparency and stability of the solution. After adding CMC into mortar, it has obvious thickening, water retention and thickening action, and depends largely on its molecular weight and degree of substitution. After adding CMC 48 h, the water absorption of mortar samples decreased. The lower the water absorption rate, the higher the water retention rate, and the effect of water retention increased with the addition of CMC. Because the water retaining effect is good, can guarantee the dry mixed mortar mix not secrete water, not the segregation. At present, CMC is mainly used as anti erosion agent for the dam, pier, bridge construction in with reduced water on the water
Erosion of mud and fine aggregate and the effect of environmental pollution reduction. CMC is an ionic compound with high requirements for cement, otherwise it can be reacted with Ca (OH) in cement after mixing with cement paste
Producing water-insoluble carboxymethyl cellulose calcium and losing viscosity, greatly reducing the water retention property of CMC, and the poor enzyme resistance of CMC.
(2) application of hydroxyethyl cellulose and hydroxypropyl cellulose
Hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose (HPC) are nonionic water-soluble cellulose ethers with high salt resistance. HEC is stable to heat and soluble in cold and hot water. When pH is 2-12, the viscosity changes little. HPC is soluble in water below 40 DEG C and in a large number of polar solvents. It has thermoplastic and surface activity. The higher the degree of substitution, the lower the water temperature of HPC can be dissolved.
With the addition of HEC in mortar, the compressive strength, tensile strength and corrosion resistance of mortar decrease in the short term, and the properties change little with the increase of time. HEC also affects the distribution of pores in mortar. In the mortar after joining the HPC mortar porosity is very low, the required water drop thus reducing the working performance of mortar. In actual use, HPC should be used in conjunction with plasticizers to improve mortar performance.
(3) application of methyl cellulose
Methyl cellulose (MC) is a non-ionic, single cellulose ether. It is dispersed and swelled rapidly in hot water at 80-90 degrees centigrade. It dissolves rapidly after cooling. MC aqueous solutions form gels. When heated, MC does not dissolve in water and forms gel. When cooled, the gel dissolves. This phenomenon is completely reversible.
After adding MC into the mortar, the water retention is obviously improved. The water retention of MC depends on its viscosity, degree of substitution, fineness and amount of addition. Adding MC can improve the anti dropping mortar, improve the lubricity of dispersed particles and uniformity, make more smooth and uniform mortar, smeared and smoothing effect is more ideal, to improve the performance of. The amount of MC has great influence on the mortar. When the amount of MC is greater than 2%, the strength of the mortar falls to half of the original strength. Water retention effect increased with the increase of viscosity of MC, but when the viscosity of MC reached a certain value, the solubility of MC decreased, the water holding capacity changed little, and the performance of construction decreased.

(4) application of hydroxyethyl methyl cellulose and hydroxypropyl methylcellulose
There is less when adding a single ether dispersion is not very good, easy to agglomerate and quickly add a large amount of mortar hardening, void too much, concrete hardness is poor, therefore, the workability, compressive strength, flexural strength and other performance is not ideal. The mixed ether can overcome the deficiency of a single ether to a certain extent, and the amount of added ether is less than that of a single ether. Hydroxyethyl methyl cellulose (HEMC) and hydroxypropyl methyl cellulose (HPMC) are non-ionic mixed cellulose ethers with the properties of various single substituent cellulose ethers. HEMC appearance is white, white powder or particle, odorless, tasteless, hygroscopic, insoluble in hot water. Dissolution is not affected by pH value (similar to MC), but because of the addition of hydroxyethyl groups in the molecular chain, HEMC is more salt tolerant, soluble in water, and has higher condensation temperature than MC. The water retention of HEMC is stronger than that of MC, and its viscosity stability, mildew resistance and dispersion are stronger than HEC. HPMC is a white powder, non-toxic, tasteless, odorless. The performance of HPMC with different specifications is quite different.
HPMC dissolves into clear or slightly turbid colloidal solutions in cold water, soluble in certain organic solvents and dissolved in a mixed solvent of water organic solvents, such as an appropriate proportion of ethanol / water. Aqueous solution has the characteristics of high surface activity, high transparency and stable performance. The dissolution of HPMC in water is not affected by pH. Solubility changes with viscosity. The lower the viscosity, the greater the solubility. With the decrease of methoxyl group in HPMC molecule, the gel point of HPMC increases, the solubility of water decreases, and the surface activity decreases. In addition to the common properties of some cellulose ethers, HPMC also has good salt resistance, size 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 sand, powder and strength reduction due to water shortage and incomplete hydration of cement. Improve uniformity, usability and product hardening. When the amount of HPMC is added
In 0.08% with the increase in the amount of HPMC mortar, the yield stress and plastic viscosity also increased. As an air entraining agent. When HEMC and
When the content of HPMC is 0.5%, the maximum gas content is about 55%. When the content is more than 0.5%, the gas content of mortar decreases gradually with the increase of the amount of the mortar, and the flexural strength and compressive strength of the mortar are reduced. Improvement and workability. The addition of HEMC and HPMC facilitates the thin layer mortar to be combed and the plastering mortar spread out. HEMC and HPMC delay the hydration of mortar particles, DS is the most important factor affecting hydration, and the influence of methoxyl content on the hydration is greater than the influence of hydroxyethyl and hydroxypropyl content. It should be noted that cellulose ether has double influence on the performance of mortar and can play a good role when used properly. The properties of dry mixed mortar are firstly related to the adaptability of cellulose ether, and to the applicable cellulose ether, it is also related to the amount of admixture and the order of addition. In practical applications, a variety of cellulose ethers can be ordered, or different varieties of cellulose ethers 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 ethers. Researchers and producers should seize the opportunity to improve their own technical level, and strive to increase varieties and improve the stability of products. In order to meet the requirement of dry mixed mortar, a leap of cellulose ether industry can be realized.