1. The necessity of water retention
All kinds of bases that require mortar for construction have a certain degree of water absorption. After the base layer absorbs the water in the mortar, the constructability of the mortar will be deteriorated, and in severe cases, the cementitious material in the mortar will not be fully hydrated, resulting in low strength, especially the interface strength between the hardened mortar and the base layer , causing the mortar to crack and fall off. If the plastering mortar has suitable water retention performance, it can not only effectively improve the construction performance of the mortar, but also make the water in the mortar difficult to be absorbed by the base layer and ensure the sufficient hydration of the cement.
2. Problems with traditional water retention methods
The traditional solution is to water the base, but it is impossible to ensure that the base is evenly moistened. The ideal hydration target of cement mortar on the base is that the cement hydration product absorbs water along with the base, penetrates into the base, and forms an effective “key connection” with the base, so as to achieve the required bond strength. Watering directly on the surface of the base will cause serious dispersion in the water absorption of the base due to differences in temperature, watering time, and watering uniformity. The base has less water absorption and will continue to absorb the water in the mortar. Before the cement hydration proceeds, the water is absorbed, which affects the cement hydration and the penetration of hydration products into the matrix; the base has a large water absorption, and the water in the mortar flows to the base. The medium migration speed is slow, and even a water-rich layer is formed between the mortar and the matrix, which also affects the bond strength. Therefore, using the common base watering method will not only fail to effectively solve the problem of high water absorption of the wall base, but will affect the bonding strength between the mortar and the base, resulting in hollowing and cracking.
3. Requirements of different mortars for water retention
The water retention rate targets for plastering mortar products used in a certain area and in areas with similar temperature and humidity conditions are proposed below.
①High water absorption substrate plastering mortar
High water absorption substrates represented by air-entrained concrete, including various lightweight partition boards, blocks, etc., have the characteristics of large water absorption and long duration. The plastering mortar used for this kind of base layer should have a water retention rate of not less than 88%.
②Low water absorption substrate plastering mortar
Low water absorption substrates represented by cast-in-place concrete, including polystyrene boards for external wall insulation, etc., have relatively small water absorption. The plastering mortar used for such substrates should have a water retention rate of not less than 88%.
③Thin layer plastering mortar
Thin-layer plastering refers to the plastering construction with a plastering layer thickness between 3 and 8 mm. This kind of plastering construction is easy to lose moisture due to the thin plastering layer, which affects the workability and strength. For the mortar used for this type of plastering, its water retention rate is not less than 99%.
④Thick layer plastering mortar
Thick layer plastering refers to the plastering construction where the thickness of one plastering layer is between 8mm and 20mm. This kind of plastering construction is not easy to lose water due to the thick plastering layer, so the water retention rate of the plastering mortar should not be less than 88%.
⑤Water-resistant putty
Water-resistant putty is used as an ultra-thin plastering material, and the general construction thickness is between 1 and 2mm. Such materials require extremely high water retention properties to ensure their workability and bond strength. For putty materials, its water retention rate should not be less than 99%, and the water retention rate of putty for exterior walls should be greater than that of putty for interior walls.
4. Types of water-retaining materials
Cellulose ether
1) Methyl cellulose ether (MC)
2) Hydroxypropyl Methyl Cellulose Ether (HPMC)
3) Hydroxyethyl cellulose ether (HEC)
4) Carboxymethyl cellulose ether (CMC)
5) Hydroxyethyl Methyl Cellulose Ether (HEMC)
Starch ether
1) Modified starch ether
2) Guar ether
Modified mineral water-retaining thickener (montmorillonite, bentonite, etc.)
Five, the following focuses on the performance of various materials
1. Cellulose ether
1.1 Overview of Cellulose Ether
Cellulose ether is a general term for a series of products formed by the reaction of alkali cellulose and etherification agent under certain conditions. Different cellulose ethers are obtained because alkali fiber is replaced by different etherification agents. According to the ionization properties of its substituents, cellulose ethers can be divided into two categories: ionic, such as carboxymethyl cellulose (CMC), and nonionic, such as methyl cellulose (MC).
According to the types of substituents, cellulose ethers can be divided into monoethers, such as methyl cellulose ether (MC), and mixed ethers, such as hydroxyethyl carboxymethyl cellulose ether (HECMC). According to the different solvents it dissolves, it can be divided into two types: water-soluble and organic solvent-soluble.
1.2 Main cellulose varieties
Carboxymethylcellulose (CMC), practical degree of substitution: 0.4-1.4; etherification agent, monooxyacetic acid; dissolving solvent, water;
Carboxymethyl hydroxyethyl cellulose (CMHEC), practical degree of substitution: 0.7-1.0; etherification agent, monooxyacetic acid, ethylene oxide; dissolving solvent, water;
Methylcellulose (MC), practical degree of substitution: 1.5-2.4; etherification agent, methyl chloride; dissolving solvent, water;
Hydroxyethyl cellulose (HEC), practical degree of substitution: 1.3-3.0; etherification agent, ethylene oxide; dissolving solvent, water;
Hydroxyethyl methylcellulose (HEMC), practical degree of substitution: 1.5-2.0; etherification agent, ethylene oxide, methyl chloride; dissolving solvent, water;
Hydroxypropyl cellulose (HPC), practical degree of substitution: 2.5-3.5; etherification agent, propylene oxide; dissolving solvent, water;
Hydroxypropyl methylcellulose (HPMC), practical degree of substitution: 1.5-2.0; etherification agent, propylene oxide, methyl chloride; dissolving solvent, water;
Ethyl cellulose (EC), practical degree of substitution: 2.3-2.6; etherification agent, monochloroethane; dissolving solvent, organic solvent;
Ethyl hydroxyethyl cellulose (EHEC), practical degree of substitution: 2.4-2.8; etherification agent, monochloroethane, ethylene oxide; dissolving solvent, organic solvent;
1.3 Properties of cellulose
1.3.1 Methyl cellulose ether (MC)
①Methylcellulose is soluble in cold water, and it will be difficult to dissolve in hot water. Its aqueous solution is very stable in the range of PH=3-12. It has good compatibility with starch, guar gum, etc. and many surfactants. When the temperature reaches the gelation temperature, gelation occurs.
②The water retention of methylcellulose depends on its addition amount, viscosity, particle fineness and dissolution rate. Generally, if the addition amount is large, the fineness is small, and the viscosity is large, the water retention is high. Among them, the amount of addition has the greatest impact on water retention, and the lowest viscosity is not directly proportional to the level of water retention. The dissolution rate mainly depends on the degree of surface modification of cellulose particles and particle fineness. Among cellulose ethers, methyl cellulose has a higher water retention rate.
③The change of temperature will seriously affect the water retention rate of methyl cellulose. Generally, the higher the temperature, the worse the water retention. If the mortar temperature exceeds 40°C, the water retention of methyl cellulose will be very poor, which will seriously affect the construction of the mortar.
④ Methyl cellulose has a significant impact on the construction and adhesion of mortar. The “adhesion” here refers to the adhesive force felt between the worker’s applicator tool and the wall substrate, that is, the shear resistance of the mortar. The adhesiveness is high, the shearing resistance of the mortar is large, and workers need more strength during use, and the construction performance of the mortar becomes poor. Methyl cellulose adhesion is at a moderate level in cellulose ether products.
1.3.2 Hydroxypropyl Methyl Cellulose Ether (HPMC)
Hydroxypropyl methylcellulose is a fiber product whose output and consumption are increasing rapidly in recent years.
It is a non-ionic cellulose mixed ether made from refined cotton after alkalization, using propylene oxide and methyl chloride as etherification agents, and through a series of reactions. The degree of substitution is generally 1.5-2.0. Its properties are different due to the different ratios of methoxyl content and hydroxypropyl content. High methoxyl content and low hydroxypropyl content, the performance is close to methyl cellulose; low methoxyl content and high hydroxypropyl content, the performance is close to hydroxypropyl cellulose.
①Hydroxypropyl methylcellulose is easily soluble in cold water, and it will be difficult to dissolve in hot water. But its gelation temperature in hot water is significantly higher than that of methyl cellulose. The solubility in cold water is also greatly improved compared with methyl cellulose.
② The viscosity of hydroxypropyl methylcellulose is related to its molecular weight, and the higher the molecular weight, the higher the viscosity. Temperature also affects its viscosity, as temperature increases, viscosity decreases. But its viscosity is less affected by temperature than methyl cellulose. Its solution is stable when stored at room temperature.
③The water retention of hydroxypropyl methylcellulose depends on its addition amount, viscosity, etc., and its water retention rate under the same addition amount is higher than that of methyl cellulose.
④Hydroxypropyl methylcellulose is stable to acid and alkali, and its aqueous solution is very stable in the range of PH=2-12. Caustic soda and lime water have little effect on its performance, but alkali can speed up its dissolution and slightly increase its viscosity. Hydroxypropyl methylcellulose is stable to common salts, but when the concentration of salt solution is high, the viscosity of hydroxypropyl methylcellulose solution tends to increase.
⑤Hydroxypropyl methylcellulose can be mixed with water-soluble polymers to form a uniform and transparent solution with higher viscosity. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
⑥ Hydroxypropyl methylcellulose has better enzyme resistance than methylcellulose, and its solution is less likely to be degraded by enzymes than methylcellulose.
⑦The adhesion of hydroxypropyl methylcellulose to mortar construction is higher than that of methylcellulose.
1.3.3 Hydroxyethyl cellulose ether (HEC)
It is made from refined cotton treated with alkali, and reacted with ethylene oxide as etherification agent in the presence of acetone. The degree of substitution is generally 1.5-2.0. It has strong hydrophilicity and is easy to absorb moisture.
①Hydroxyethyl cellulose is soluble in cold water, but it is difficult to dissolve in hot water. Its solution is stable at high temperature without gelling. It can be used for a long time under high temperature in mortar, but its water retention is lower than that of methyl cellulose.
②Hydroxyethyl cellulose is stable to general acid and alkali. Alkali can accelerate its dissolution and slightly increase its viscosity. Its dispersibility in water is slightly worse than that of methyl cellulose and hydroxypropyl methyl cellulose.
③Hydroxyethyl cellulose has good anti-sag performance for mortar, but it has a longer retarding time for cement.
④The performance of hydroxyethyl cellulose produced by some domestic enterprises is obviously lower than that of methyl cellulose due to its high water content and high ash content.
1.3.4 Carboxymethyl cellulose ether (CMC) is made of natural fibers (cotton, hemp, etc.) after alkali treatment, using sodium monochloroacetate as etherification agent, and undergoing a series of reaction treatments to make ionic cellulose ether. The degree of substitution is generally 0.4-1.4, and its performance is greatly affected by the degree of substitution.
①Carboxymethyl cellulose is highly hygroscopic, and it will contain a large amount of water when stored under general conditions.
②Hydroxymethyl cellulose aqueous solution will not produce gel, and the viscosity will decrease with the increase of temperature. When the temperature exceeds 50 ℃, the viscosity is irreversible.
③ Its stability is greatly affected by pH. Generally, it can be used in gypsum-based mortar, but not in cement-based mortar. When highly alkaline, it loses viscosity.
④ Its water retention is far lower than that of methyl cellulose. It has a retarding effect on gypsum-based mortar and reduces its strength. However, the price of carboxymethyl cellulose is significantly lower than that of methyl cellulose.
2. Modified starch ether
Starch ethers generally used in mortars are modified from natural polymers of some polysaccharides. Such as potato, corn, cassava, guar beans, etc. are modified into various modified starch ethers. The starch ethers commonly used in mortar are hydroxypropyl starch ether, hydroxymethyl starch ether, etc.
Generally, starch ethers modified from potatoes, corn, and cassava have significantly lower water retention than cellulose ethers. Because of its different degree of modification, it shows different stability to acid and alkali. Some products are suitable for use in gypsum-based mortars, while others cannot be used in cement-based mortars. The application of starch ether in mortar is mainly used as a thickener to improve the anti-sagging property of mortar, reduce the adhesion of wet mortar, and prolong the opening time.
Starch ethers are often used together with cellulose, resulting in complementary properties and advantages of the two products. Since starch ether products are much cheaper than cellulose ether, the application of starch ether in mortar will bring about a significant reduction in the cost of mortar formulations.
3. Guar gum ether
Guar gum ether is a kind of etherified polysaccharide with special properties, which is modified from natural guar beans. Mainly through the etherification reaction between guar gum and acrylic functional groups, a structure containing 2-hydroxypropyl functional groups is formed, which is a polygalactomannose structure.
①Compared with cellulose ether, guar gum ether is easier to dissolve in water. PH basically has no effect on the performance of guar gum ether.
②Under the conditions of low viscosity and low dosage, guar gum can replace cellulose ether in an equal amount, and has similar water retention. But the consistency, anti-sag, thixotropy and so on are obviously improved.
③Under the conditions of high viscosity and large dosage, guar gum cannot replace cellulose ether, and the mixed use of the two will produce better performance.
④The application of guar gum in gypsum-based mortar can significantly reduce the adhesion during construction and make the construction smoother. It has no adverse effect on the setting time and strength of gypsum mortar.
⑤ When guar gum is applied to cement-based masonry and plastering mortar, it can replace cellulose ether in an equal amount, and endow the mortar with better sagging resistance, thixotropy and smoothness of construction.
⑥In the mortar with high viscosity and high content of water retaining agent, guar gum and cellulose ether will work together to achieve excellent results.
⑦ Guar gum can also be used in products such as tile adhesives, ground self-leveling agents, water-resistant putty, and polymer mortar for wall insulation.
4. Modified mineral water-retaining thickener
The water-retaining thickener made of natural minerals through modification and compounding has been applied in China. The main minerals used to prepare water-retaining thickeners are: sepiolite, bentonite, montmorillonite, kaolin, etc. These minerals have certain water-retaining and thickening properties through modification such as coupling agents. This kind of water-retaining thickener applied to mortar has the following characteristics.
① It can significantly improve the performance of ordinary mortar, and solve the problems of poor operability of cement mortar, low strength of mixed mortar, and poor water resistance.
② Mortar products with different strength levels for general industrial and civil buildings can be formulated.
③The material cost is low.
④ The water retention is lower than that of organic water retention agents, and the dry shrinkage value of the prepared mortar is relatively large, and the cohesiveness is reduced.
Post time: Mar-03-2023