Dry powder mortar additives
Dry powder mortar additives
Dry powder mortar generally uses portland cement as the cementing material. The amount of cementing material generally accounts for 20% to 40% of the dry powder mortar; most of the fine aggregate is quartz sand, and the amount is large, which generally needs to be dried and sieved. Grading pretreatment to ensure that the particle size and quality meet the requirements of the formula; sometimes fly ash, slag powder, etc. are used as admixtures; the general dosage of admixtures is very small, about 1% to 3%, but the effect is huge It is often selected according to the requirements of the product formula to improve the mortar's workability, layering degree, strength, shrinkage and antifreeze performance and other indicators.
Compared with the additives for mixing mortar on site, dry powder mortar additives have the following characteristics: ① No water, generally solid; ② When mixed with cement, sand and other additives, the dry state does not react, but after water is added, it can be fully dispersed and efficient Play its role.
Water retention thickener
Water-retaining thickeners mainly include: cellulose ethers, starch ethers, etc. The cellulose ethers used in the dry powder mortar are mainly methyl hydroxyethyl cellulose ether (MHEC) and hydroxypropyl methyl cellulose ether (HPMC).
The basic function of the water reducing agent is to reduce the water demand of the mortar, thereby improving the compressive strength of the mortar. The main water reducing agents used in dry powder mortar are: casein, naphthalene based water reducing agent, melamine formaldehyde condensate, polycarboxylic acid. Casein is a superplasticizer with excellent performance, especially for thin-layer mortar, but because it is a natural product, quality and price often fluctuate. Naphthalene-based water-reducing agents commonly used β-naphthalenesulfonic acid formaldehyde condensate.
There are two types of accelerators: quick-setting accelerator and retarder. The quick setting agent is used to accelerate the setting and hardening of the mortar. Calcium formate and lithium carbonate are widely used, and aluminate and sodium silicate can also be used as the quick setting agent. Retarder is used to slow down the coagulation and hardening of mortar. Tartaric acid, citric acid and its salts and gluconate have been successfully used.
Waterproofing agents mainly include: ferric chloride, organic silane compounds, fatty acid salts, polypropylene fibers, styrene-butadiene rubber and other polymer compounds. Ferric chloride waterproofing agent has good waterproof effect, but it is easy to produce corrosion of steel bars and metal embedded parts. The insoluble calcium salt generated by the reaction of fatty acid salts and calcium ions in the cement phase is deposited on the wall of the capillary to block the pores, and at the same time make the capillary tube wall become a hydrophobic surface, thus playing a waterproof role. The unit cost of these products is relatively low, but it takes a long time to mix with water when mixing mortar.
The fibers used for dry powder mortar are: alkali-resistant glass fibers, polyolefin fibers (polypropylene fibers), high-strength high-modulus polyvinyl alcohol fibers (vinylon fibers), wood fibers, etc. The most widely used are high-strength high-modulus polyvinyl alcohol fibers and polypropylene fibers. High-strength high-modulus PVA fiber has better performance and lower price than imported polypropylene fiber. The fibers are irregularly and evenly distributed in the cement matrix, and are tightly combined with the cement, thereby preventing the formation and development of micro-cracks, and making the mortar matrix dense, so that it has waterproof performance and excellent impact resistance and crack resistance. The length is 3 ~ 19 mm.