1 Introduction
China has been promoting ready-mixed mortar for more than 20 years. Especially in recent years, relevant national government departments have attached importance to the development of ready-mixed mortar and issued encouraging policies. At present, there are more than 10 provinces and municipalities in the country that have used ready-mixed mortar. More than 60%, there are more than 800 ready-mixed mortar enterprises above the ordinary scale, with an annual design capacity of 274 million tons. In 2021, the annual production of ordinary ready-mixed mortar was 62.02 million tons.
During the construction process, the mortar often loses too much water and does not have enough time and water to hydrate, resulting in insufficient strength and cracking of the cement paste after hardening. Cellulose ether is a common polymer admixture in dry-mixed mortar. It has the functions of water retention, thickening, retardation and air entrainment, and can significantly improve the performance of the mortar.
In order to make the mortar meet the transportation requirements and solve the problems of cracking and low bonding strength, it is of great significance to add cellulose ether to the mortar. This article briefly introduces the characteristics of cellulose ether and its influence on the performance of cement-based materials, hoping to help solve the related technical problems of ready-mixed mortar.
2 Introduction to cellulose ether
Cellulose Ether (Cellulose Ether) is made from cellulose through the etherification reaction of one or more etherification agents and dry grinding.
2.1 Classification of cellulose ethers
According to the chemical structure of ether substituents, cellulose ethers can be divided into anionic, cationic and nonionic ethers. Ionic cellulose ethers mainly include carboxymethyl cellulose ether (CMC); non-ionic cellulose ethers mainly include methyl cellulose ether (MC), hydroxypropyl methyl cellulose ether (HPMC) and hydroxyethyl fiber Ether (HC) and so on. Non-ionic ethers are divided into water-soluble ethers and oil-soluble ethers. Non-ionic water-soluble ethers are mainly used in mortar products. In the presence of calcium ions, ionic cellulose ethers are unstable, so they are rarely used in dry-mix mortar products that use cement, slaked lime, etc. as cementing materials. Non-ionic water-soluble cellulose ethers are widely used in the building materials industry because of their suspension stability and water retention effect.
According to the different etherification agents selected in the etherification process, cellulose ether products include methyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, cyanoethyl cellulose, carboxymethyl cellulose, Ethyl cellulose, benzyl cellulose, carboxymethyl hydroxyethyl cellulose, hydroxypropyl methyl cellulose, benzyl cyanoethyl cellulose and phenyl cellulose.
Cellulose ethers used in mortar usually include methyl cellulose ether (MC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl methyl cellulose ether (HEMC) and hydroxyethyl cellulose ether (HEMC) Among them, HPMC and HEMC are the most widely used.
2.2 The chemical properties of cellulose ether
Each cellulose ether has the basic structure of cellulose-anhydroglucose structure. In the process of producing cellulose ether, the cellulose fiber is first heated in an alkaline solution and then treated with an etherifying agent. The fibrous reaction product is purified and ground to form a uniform powder with a certain fineness.
In the production of MC, only methyl chloride is used as an etherifying agent; in addition to methyl chloride, propylene oxide is also used to obtain hydroxypropyl substituents in the production of HPMC. Various cellulose ethers have different methyl and hydroxypropyl substitution rates, which affect the organic compatibility and thermal gel temperature of the cellulose ether solution.
2.3 The dissolution characteristics of cellulose ether
The dissolution characteristics of cellulose ether have a great influence on the workability of cement mortar. Cellulose ether can be used to improve the cohesiveness and water retention of cement mortar, but this depends on the cellulose ether being completely and fully dissolved in water. The main factors affecting the dissolution of cellulose ether are dissolution time, stirring speed and powder fineness.
2.4 The role of sinking in cement mortar
As an important additive of cement slurry, Destroy has its effect in the following aspects.
(1) Improve the workability of the mortar and increase the viscosity of the mortar.
Incorporating flame jet can prevent the mortar from separating and obtain a uniform and uniform plastic body. For example, booths that incorporate HEMC, HPMC, etc., are convenient for thin-layer mortar and plastering. , Shear rate, temperature, collapse concentration and dissolved salt concentration.
(2) It has an air-entraining effect.
Due to impurities, the introduction of groups into the particles reduces the surface energy of the particles, and it is easy to introduce stable, uniform and fine particles into the mortar mixed with the stirring surface in the process. “Ball efficiency” improves the construction performance of the mortar, reduces the moisture of the mortar and reduces the thermal conductivity of the mortar. Tests have shown that when the blending amount of HEMC and HPMC is 0.5%, the gas content of the mortar is the largest, about 55%; when the blending amount is greater than 0.5%, the content of the mortar gradually develops into a gas content trend as the amount increases.
(3) Keep it unchanged.
The wax can dissolve, lubricate and stir in the mortar, and facilitate the smoothing of the thin layer of mortar and plastering powder. It does not need to be wetted in advance. After construction, the cementitious material can also have a long period of continuous hydration along the coast to improve the adhesion between the mortar and the substrate.
The modification effects of cellulose ether on fresh cement-based materials mainly include thickening, water retention, air entrainment and retardation. With the widespread use of cellulose ethers in cement-based materials, the interaction between cellulose ethers and cement slurry is gradually becoming a research hotspot.
Post time: Dec-16-2021