How Many Additives in dry mix mortar?

1. Water retention and thickening material

The main type of water-retaining thickening material is cellulose ether. Cellulose ether is a high-efficiency admixture that can greatly improve the specific performance of mortar with only a small amount of addition. It is converted from water-insoluble cellulose into water-soluble fiber through etherification reaction. It is made of plain ether and has the basic structural unit of anhydroglucose. It has different properties according to the type and number of substituting groups on its substitution position. It can be used as a thickener to adjust the consistency of mortar; its water retention It can well adjust the water demand of the mortar, and can gradually release water within a certain period of time, which can well ensure that the slurry and the water-absorbing substrate are better bonded. At the same time, cellulose ether can adjust the rheological properties of mortar, increase workability and workability. The following cellulose ether compounds can be used as chemical additives in dry-mixed mortar: ①Na-carboxymethyl cellulose; ②Ethyl cellulose; ③Methyl cellulose; ④Hydroxy cellulose ether; ⑤Hydroxypropyl methyl Cellulose; ⑥starch ester, etc. The addition of the above-mentioned various cellulose ethers improves the performance of the dry-mixed mortar: ①Increase the workability; ②Increase the adhesion; ③The mortar is not easy to bleed and separate; Excellent crack resistance; ⑥ Mortar is easy to construct in thin layers. In addition to the above properties, different cellulose ethers also have their own special properties. Cai Wei from Chongqing University summarized the improvement mechanism of methyl cellulose ether on the performance of mortar. He believed that after adding MC (methyl cellulose ether) water retaining agent to the mortar, many tiny air bubbles would be formed. It acts like a ball bearing, which improves the workability of the freshly mixed mortar, and the air bubbles are still retained in the hardened mortar body, forming independent pores and blocking the capillary pores. MC water retaining agent can also Improve the water retention of the freshly mixed mortar to a large extent, which can not only prevent the mortar from bleeding and segregating, but also prevent the water from evaporating too quickly or being absorbed by the substrate too quickly in the early stage of curing, so that the cement can be better hydrated, so that the bond Strength is improved. The incorporation of MC water-retaining agent will improve the shrinkage of the mortar. This is a fine-powder water-retaining agent that can be filled in the pores, so that the interconnected pores in the mortar will be reduced, and the evaporation loss of water will be reduced, thereby reducing the dry shrinkage of the mortar. value. Cellulose ether is generally mixed in dry-mix adhesive mortar, especially when used as tile adhesive. If cellulose ether is mixed into the tile adhesive, the water retention capacity of tile mastic can be greatly improved. Cellulose ether inhibits the rapid loss of water from the cement to the substrate or bricks, so that the cement has enough water to fully solidify, prolongs the correction time, and improves the bonding strength. In addition, cellulose ether also improves the plasticity of the mastic, makes construction easier, increases the contact area between the mastic and the brick body, and reduces the slipping and sagging of the mastic, even if the mass per unit area is large and the surface density is high. The tiles are glued to vertical surfaces without slippage of the mastic. Cellulose ether can also delay the formation of the cement skin, prolong the open time, and increase the utilization rate of the cement.

2. Organic fiber

The fibers used in mortar can be divided into metal fibers, inorganic fibers and organic fibers according to their material properties. Adding fibers into the mortar can greatly improve its anti-crack and anti-seepage performance. Organic fibers are usually added to dry-mixed mortar to improve the impermeability and crack resistance of the mortar. Commonly used organic fibers are: polypropylene fiber (PP), polyamide (nylon) (PA) fiber, polyvinyl alcohol (vinylon) (PVA) fiber, polyacrylonitrile (PAN), polyethylene fiber, polyester fiber, etc. Among them, polypropylene fiber is currently the most practically used. It is a crystalline polymer with regular structure polymerized by propylene monomer under certain conditions. It has chemical corrosion resistance, good processability, light weight, small creep shrinkage, and low price. And other characteristics, and because polypropylene fiber is resistant to acid and alkali, and does not chemically react with cement-based materials, it has received widespread attention at home and abroad. The anti-cracking effect of fibers mixed with mortar is mainly divided into two stages: one is the plastic mortar stage; the other is the hardened mortar body stage. In the plastic stage of the mortar, the evenly distributed fibers present a three-dimensional network structure, which plays a role in supporting the fine aggregate, prevents the settlement of the fine aggregate, and reduces segregation. The segregation is the main reason for the cracking of the mortar surface, and the addition of fibers reduces the segregation of the mortar and reduces the possibility of cracking of the mortar surface. Due to the evaporation of water in the plastic stage, the shrinkage of the mortar will produce tensile stress, and the addition of fibers can bear this tensile stress. In the hardening stage of mortar, due to the existence of drying shrinkage, carbonization shrinkage, and temperature shrinkage, stress will also be generated inside the mortar. microcrack extension. Yuan Zhenyu and others also concluded through the analysis of the crack resistance test of the mortar plate that adding polypropylene fiber to the mortar can significantly reduce the occurrence of plastic shrinkage cracks and improve the crack resistance of the mortar. When the volume content of polypropylene fiber in the mortar is 0.05% and 0.10%, the cracks can be reduced by 65% and 75%, respectively. Huang Chengya and others from the School of Materials, South China University of Technology, also confirmed through the mechanical performance test of modified polypropylene fiber cement-based composite materials that adding a small amount of polypropylene fiber to cement mortar can improve the flexural and compressive strength of cement mortar. The optimal amount of fiber in cement mortar is about 0.9kg/m3, if the amount exceeds this amount, the strengthening and toughening effect of fiber on cement mortar will not be significantly improved, and it is not economical. Adding fibers to the mortar can improve the impermeability of the mortar. When the cement matrix shrinks, due to the role of the fine steel bars played by the fibers, energy is effectively consumed. Even if there are micro-cracks after coagulation, under the action of internal and external stress, the expansion of cracks will be hindered by the fiber network system. , It is difficult to develop into larger cracks, so it is difficult to form a through seepage path, thereby improving the impermeability of the mortar.

3. Expansion agent

Expansion agent is another important anti-crack and anti-seepage component in dry-mix mortar. The most widely used expansion agents are AEA, UEA, CEA and so on. AEA expansion agent has the advantages of large energy, small dosage, high post-strength, dry shrinkage, and low alkali content. The calcium aluminate minerals CA in the high-alumina clinker in the AEA component first react with CaSO4 and Ca(OH)2 to hydrate to form calcium sulfoaluminate hydrate (ettringite) and expand. UEA also generates ettringite to generate expansion, while CEA mainly generates calcium hydroxide. AEA expansion agent is a calcium aluminate expansion agent, which is an expansion admixture made by co-grinding a certain proportion of high-alumina clinker, natural alunite and gypsum. The expansion formed after the addition of AEA is mainly due to two aspects: in the early stage of cement hydration, the calcium aluminate mineral CA in the high alumina clinker in the AEA component first reacts with CaSO4 and Ca(OH)2, and hydrates to form calcium sulfoaluminate hydrate (ettringite) and expand, the amount of expansion is large. The generated ettringite and hydrated aluminum hydroxide gel make the expansion phase and the gel phase reasonably match, which not only ensures the expansion performance but also ensures the strength. In the middle and late stages, ettringite also generates ettringite under the excitation of lime gypsum to produce micro-expansion, which improves the microstructure of cement aggregate interface. After AEA is added to the mortar, a large amount of ettringite generated in the early and middle stages will expand the volume of the mortar, make the internal structure more compact, improve the pore structure of the mortar, reduce the macropores, reduce the total porosity, and greatly improve the impermeability. When the mortar is in a dry state in the later stage, the expansion in the early and middle stages can offset all or part of the shrinkage in the later stage, so that the crack resistance and seepage resistance are improved. UEA expanders are made from inorganic compounds such as sulfates, alumina, potassium sulfoaluminate and calcium sulfate. When UEA is mixed into cement in an appropriate amount, it can achieve the functions of compensating shrinkage, crack resistance and anti-leakage. After UEA is added to ordinary cement and mixed, it will react with calcium silicate and hydrate to form Ca(OH)2, which will generate sulfoaluminic acid. Calcium (C2A·3CaSO4·32H2O) is ettringite, which makes the cement mortar moderately expand, and the expansion rate of cement mortar is proportional to the content of UEA, making the mortar dense, with high crack resistance and impermeability. Lin Wentian applied cement mortar mixed with UEA to the exterior wall, and achieved good anti-leakage effect. CEA expansion agent clinker is made of limestone, clay (or high alumina clay), and iron powder, which is calcined at 1350-1400°C, and then ground to make CEA expansion agent. CEA expansion agents have two expansion sources: CaO hydration to form Ca(OH)2; C3A and activated Al2O3 to form ettringite in a medium of gypsum and Ca(OH)2.

4. Plasticizer

Mortar plasticizer is a powdery air-entraining mortar admixture compounded by organic polymers and inorganic chemical admixtures, and is an anionic surface-active material. It can significantly reduce the surface tension of the solution, and produce a large number of closed and tiny bubbles (generally 0.25-2.5mm in diameter) during the mixing process of mortar with water. The distance between microbubbles is small and the stability is good, which can significantly improve the workability of mortar. ; It can disperse cement particles, promote cement hydration reaction, improve mortar strength, impermeability and freeze-thaw resistance, and reduce part of cement consumption; it has good viscosity, strong adhesion of mortar mixed with it, and can be well Prevent common building problems such as shelling (hollowing), cracking, and water seepage on the wall; it can improve the construction environment, reduce labor intensity, and promote civilized construction; it is a very significant economic and social benefit that can improve project quality and reduce Environmentally friendly and energy-saving products with low construction costs. Lignosulfonate is a plasticizer commonly used in dry powder mortar, which is waste from paper mills, and its general dosage is 0.2% to 0.3%. Plasticizers are often used in mortars that require good self-leveling properties, such as self-leveling cushions, surface mortars or leveling mortars. Adding plasticizers into the masonry mortar can improve the workability of the mortar, improve the water retention, fluidity and cohesion of the mortar, and overcome the shortcomings of cement-mixed mortar such as explosive ash, large shrinkage and low strength, so as to ensure The quality of the masonry. It can save 50% lime paste in plastering mortar, and the mortar is not easy to bleed or separate; the mortar has good adhesion to the substrate; the surface layer has no salting-out phenomenon, and has good crack resistance, frost resistance and weather resistance.

5. Hydrophobic additive

Hydrophobic additives or water repellents prevent water from entering the mortar while also keeping the mortar open to allow the diffusion of water vapor. Hydrophobic additives for dry-mixed mortar products should have the following characteristics: ①It should be a powder product; ②Have good mixing properties; ③Make the mortar as a whole hydrophobic and maintain long-term effect; ④Bond to the surface Strength has no obvious negative impact; ⑤ friendly to the environment. Hydrophobic agents currently used are fatty acid metal salts, such as calcium stearate; silane. However, calcium stearate is not a suitable hydrophobic additive for dry-mixed mortar, especially for plastering materials for mechanical construction, because it is difficult to mix quickly and uniformly with cement mortar. Hydrophobic additives are commonly used in plastering mortars for thin plastering external thermal insulation systems, tile grouts, decorative colored mortars, and waterproof plastering mortars for exterior walls.

6. Other additives

The coagulant is used to adjust the setting and hardening properties of the mortar. Calcium formate and lithium carbonate are widely used. Typical loadings are 1% calcium formate and 0.2% lithium carbonate. Like accelerators, retarders are also used to adjust the setting and hardening properties of mortar. Tartaric acid, citric acid and their salts, and gluconate have been used successfully. The typical dosage is 0.05%~0.2%. Powdered defoamer reduces the air content of fresh mortar. Powdered defoamers are based on different chemical groups such as hydrocarbons, polyethylene glycols or polysiloxanes adsorbed on inorganic supports. Starch ether can significantly increase the consistency of the mortar, and thus slightly increase the water demand and yield value, and reduce the sagging degree of the freshly mixed mortar. This allows the mortar to be made thicker and the tile adhesive to adhere to heavier tiles with less sagging.