Admixtures play a key role in improving the performance of construction dry mortar, but the addition of dry mortar makes the material cost of dry mortar products significantly higher than traditional mortar, accounting for more than 40% of the material cost of dry mortar.
Table of Contents
Water retention agent
Water retaining agent is the key additive to improve the water retaining performance of dry mortar, and it is also one of the key additives that determine the cost of dry mortar materials.
Cellulose ether is the general name for a series of products produced by the reaction of alkali cellulose and etherifying agent under certain conditions. Alkali cellulose is replaced by different etherifying agents to obtain different cellulose ethers.
According to the ionization properties of the substituents, cellulose ethers can be divided into two categories: ionic (such as carboxymethyl cellulose) and non-ionic (such as methyl cellulose).
According to the type of substituents, cellulose ethers can be divided into monoethers (such as methylcellulose) and mixed ethers (such as hydroxypropylmethylcellulose).
According to different solubility, it can be divided into water-soluble (such as hydroxyethyl cellulose) and organic solvent solubility (such as ethyl cellulose). Dryed mortar is mainly water-soluble cellulose, and water-soluble cellulose is divided into instant type and surface-treated delayed dissolution type.
Methyl cellulose (MC)
After the refined cotton is treated with alkali, cellulose ether is produced through a series of reactions using methyl chloride as the etherifying agent. Generally, the degree of substitution is 1.6~2.0, and the solubility will be different with different degrees of substitution. It is a non-ionic cellulose ether.
Hydroxypropyl methylcellulose (HPMC)
It is made by reacting refined cotton with alkali treatment and using ethylene oxide as etherifying agent in the presence of acetone. Its degree of substitution is generally 1.5~2.0. It has strong hydrophilicity and is easy to absorb moisture:
- Hydroxyethyl cellulose is soluble in cold water, but is more difficult to dissolve in hot water.
- Hydroxyethyl cellulose is stable to general acids and alkalis. Alkali can speed up its dissolution and slightly increase its viscosity. Its dispersibility in water is slightly worse than that of methyl cellulose and hydroxypropyl methyl cellulose.
- Hydroxyethyl cellulose has good anti-sag properties for mortar, but has a long retardation time for cement.
- The performance of hydroxyethyl cellulose produced by some companies is significantly lower than that of methyl cellulose due to its high water content and high ash content.
Carboxymethyl cellulose (CMC)
Ionic cellulose ether is made from natural fibers (cotton, etc.) after alkali treatment, using sodium monochloroacetate as the etherifying agent, and a series of reaction treatments. Its degree of substitution is generally 0.4~1.4, and its performance is greatly affected by the degree of substitution.
Starch ethers used in mortar are modified from some natural polymers of polysaccharides. Such as potatoes, corn, cassava, guar beans, etc.
Starch ethers modified from potatoes, corn, cassava, etc. have significantly lower water retention than cellulose ethers. Starch ethers are often used together with cellulose, making the properties and advantages of the two products complementary.
Guar gum ether
Guar gum ether is a starch ether with special properties modified from natural guar beans.
Modified mineral water-retaining thickener
Water-retaining thickeners made from modified and compounded natural minerals have been used 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 by coupling agents and other modifications.
Redispersible polymer powder
Redispersible rubber powder is produced by spray drying a special polymer emulsion. During the processing, protective colloids, anti-caking agents, etc. become indispensable additives. The dried rubber powder is a collection of 80~100mm spherical particles.
These particles are soluble in water and form a stable dispersion slightly larger than the original emulsion particles. This dispersion will form a film after losing water and drying. This film is as irreversible as the general emulsion film formation and will not disperse again when exposed to water dispersions.
Redispersible rubber powder can be divided into: styrene-butadiene copolymer, tertiary ethylene carbonate copolymer, ethylene-acetic acid copolymer, etc., and based on this, silicone, vinyl laurate, etc. are grafted to improve performance.
Wood fiber is made from plants as the main raw material and processed using a series of technologies. Its properties are different from cellulose ether. The main features are:
- Insoluble in water and solvents, and insoluble in weak acid and weak alkali solutions
- When used in mortar, it will overlap to form a three-dimensional structure in a static state, increasing the thixotropy and sag resistance of the mortar and improving the constructability.
- Due to the three-dimensional structure of wood fiber, it has the ability to “lock water” in the mixed mortar, and the water in the mortar will not be easily absorbed or removed. But it does not have the high water retention of cellulose ethers.
- The good capillary effect of wood fiber has the function of “conducting water” in the mortar, making the surface and internal moisture content of the mortar consistent, thereby reducing cracks caused by uneven shrinkage.
Polypropylene fiber is made of polypropylene as raw material and an appropriate amount of modifier. The fiber diameter is generally about 40 microns, the tensile strength is 300~400mpa, the elastic modulus is ≥3500mpa, and the ultimate elongation is 15~18%. Its performance characteristics:
- Polypropylene fibers are uniformly distributed in three-dimensional directions in the mortar, forming a network reinforcement system. If 1 kg of polypropylene fiber is added to each ton of mortar, more than 30 million monofilament fibers can be obtained.
- Adding polypropylene fiber to the mortar can effectively reduce shrinkage cracks in the plastic state of the mortar. Whether these cracks are visible or invisible. And it can significantly reduce the surface bleeding and aggregate settlement of fresh mortar.
- For mortar hardened bodies, polypropylene fibers can significantly reduce the number of deformation cracks.
- The efficient dispersion of polypropylene fibers in mortar production can become a problem.
Plastic water reducing agent
Plastic water-reducing admixture is the most commonly used admixture in cement concrete. Almost all water-reducing agents are composed of surface-active substances, and the performance of the water-reducing agent is determined by the interface interaction between the molecular structure of the surface-active substances and the cement particles.
Since the cement particles have different polarities during the hydration process, they attract each other and wrap a lot of mixing water to produce a flocculation structure.
Lignin water reducing agent
Lignin superplasticizers are usually produced as by-products of paper pulp produced by the sulfurous acid process. It generally includes three types: calcium lignin, sodium lignin and magnesium lignin. Calcium lignin and sodium lignin are commonly used, namely calcium lignosulfonate and sodium lignosulfonate, usually in powder form.
The dosage of lignin water-reducing agent is small and the price is low. It is suitable for mortars with low water-reducing rate requirements. Better results will be achieved when used in conjunction with high-efficiency water-reducing admixtures.
Cai series water reducing agent
Naphthalene-based water reducing agent is prepared by using industrial naphthalene, formaldehyde, concentrated sulfuric acid and liquid alkali as the main raw materials under certain reaction conditions. The main component is naphthalene sulfonic acid formaldehyde condensate. Usually in liquid or powder form as the final product, it is one of the most widely used water reducing agents at present. The dosage of powdery products is generally 0.5~1.0% of cement weight, and the water reduction rate can reach about 20%.
Superplasticizer is a high-efficiency water-reducing agent, and the water-reducing rate can generally reach more than 30%. Powdered superplasticizer is generally used in special dryed mortars, such as floor self-leveling agents, grouting materials, refractory castables and other products.
Air-entraining agent is a surfactant that uses physical methods to form stable bubbles in fresh concrete or mortar. Mainly include: rosin and its thermopolymers, non-ionic surfactants, alkyl benzene sulfonates, lignosulfonates, carboxylic acids and their salts, etc.
Air-entraining agents are often used to prepare plastering mortars and masonry mortars. The addition of air-entraining agents will cause some changes in the properties of the mortar.
Early Strengthening Agent
Used to improve the early strength of concrete and mortar, sulfate early strength agents are commonly used, mainly including sodium sulfate, sodium thiosulfate, aluminum sulfate and potassium aluminum sulfate.
Generally, anhydrous sodium sulfate is widely used. Its dosage is lower and the early strength effect is better. However, when the dosage is too large, it will cause expansion and cracking in the later period. At the same time, it will produce alkali return, which will affect the appearance and surface decoration layer effect.
Calcium formate is also a good antifreeze, with good early-strengthening effect and good compatibility with other admixtures. Many properties are better than sulfate early-strengthening agents, but the price is higher.
These are common dry mortar additives that improve performance. I hope they can help everyone. At the same time, there are other excellent articles on our website, you can continue to browse.