Dry powder mortar and its additives

Dry powder mortar is polymer dry mixed mortar or dry powder prefabricated mortar. It is a kind of cement and gypsum as the main base material. According to different building function requirements, dry powder building aggregates and additives are added in a certain proportion. It is a mortar building material that can be mixed evenly, transported to the construction site in bags or in bulk, and can be used directly after adding water.

Common dry powder mortar products include dry powder tile adhesive, dry powder wall coating, dry powder wall mortar, dry powder concrete, etc.

Dry powder mortar generally has at least three components: binder, aggregate, and mortar additives.

Raw material composition of dry powder mortar:

1. Mortar bonding material

(1) Inorganic adhesive:
Inorganic adhesives include ordinary Portland cement, high alumina cement, special cement, gypsum, anhydrite, etc.
(2) Organic adhesives:
Organic adhesive mainly refers to redispersible latex powder, which is a powdery polymer formed by correct spray drying (and selection of appropriate additives) of polymer emulsion. The dry polymer powder and water become emulsion. It can be dehydrated again, so that the polymer particles form a polymer body structure in the cement mortar, which is similar to the polymer emulsion process, and plays a role in modifying the cement mortar.
According to different proportions, the modification of dry powder mortar with redispersible polymer powder can improve the bonding strength with various substrates, and improve the flexibility, deformability, bending strength and wear resistance of the mortar , toughness, cohesion and density as well as water retention capacity and construction.
The redispersible latex powder for dry mix mortar mainly includes the following types: ① styrene-butadiene copolymer; ② styrene-acrylic acid copolymer; ③ vinyl acetate copolymer; ④ polyacrylate homopolymer; ⑤ Styrene Acetate Copolymer; ⑥ Vinyl Acetate-Ethylene Copolymer.

2. Aggregate:

Aggregate is divided into coarse aggregate and fine aggregate. One of the main constituent materials of concrete. It mainly acts as a skeleton and reduces the volume change caused by the shrinkage and swelling of the cementitious material during the setting and hardening process, and it is also used as a cheap filler for the cementitious material. There are natural aggregates and artificial aggregates, the former such as gravel, pebbles, pumice, natural sand, etc.; the latter such as cinder, slag, ceramsite, expanded perlite, etc.

3. Mortar additives

(1) Cellulose ether:
In dry mortar, the addition amount of cellulose ether is very low (generally 0.02%-0.7%), but it can significantly improve the performance of wet mortar, and it is a main additive that affects the construction performance of mortar.
In dry powder mortar, because ionic cellulose is unstable in the presence of calcium ions, it is rarely used in dry powder products that use cement, slaked lime, etc. as cementing materials. Hydroxyethyl cellulose is also used in some dry powder products, but the share is very small.
The cellulose ethers used in dry powder mortar are mainly hydroxyethyl methylcellulose (HEMC) and hydroxypropyl methylcellulose ether (HPMC), referred to as MC.
MC characteristics: Adhesiveness and construction are two factors that influence each other; water retention, to avoid rapid evaporation of water, so that the thickness of the mortar layer can be significantly reduced.

(2) anti-crack fiber
It is not an invention of modern people to mix fibers into mortar as anti-crack reinforcement materials. In ancient times, our ancestors have used natural fibers as reinforcement materials for some inorganic binders, such as mixing plant fibers and lime mortar to build Temples and halls, use hemp silk and mud to shape Buddha statues, use wheat straw short joints and yellow mud to build houses, use human and animal hair to repair hearths, use pulp fibers, lime, and gypsum to paint walls and make various gypsum products, etc. wait. Adding fibers into cement base materials to make fiber reinforced cement-based composites is only a matter of recent decades.
Cement products, components or buildings will inevitably produce many microcracks due to the change of microstructure and volume during the hardening process of cement, and will expand with changes in drying shrinkage, temperature changes, and external loads. When subjected to external force, the fibers play a role in limiting and hindering the expansion of micro-cracks. The fibers are criss-crossed and isotropic, consume and relieve stress, prevent the further development of cracks, and play a role in blocking cracks.
The addition of fibers can make the dry-mixed mortar have high quality, high performance, high strength, crack resistance, impermeability, burst resistance, impact resistance, freeze-thaw resistance, wear resistance, aging resistance and other functions.

(3) Water reducing agent
Water reducer is a concrete admixture that can reduce the amount of mixing water while maintaining the slump of concrete basically unchanged. Most of them are anionic surfactants, such as lignosulfonate, naphthalenesulfonate formaldehyde polymer, etc. After being added to the concrete mixture, it can disperse the cement particles, improve its workability, reduce the unit water consumption, improve the fluidity of the concrete mixture; or reduce the unit cement consumption and save cement.
According to the water reducing and strengthening ability of the water reducing agent, it is divided into ordinary water reducing agent (also known as plasticizer, the water reducing rate is not less than 8%, represented by lignosulfonate), high-efficiency water reducing agent (also known as superplasticizer) Plasticizer, the water reducing rate is not less than 14%, including naphthalene, melamine, sulfamate, aliphatic, etc.) and high-performance water reducing agent (water reducing rate is not less than 25%, polycarboxylic acid It is represented by superplasticizer), and it is divided into early strength type, standard type and retarded type.
According to the chemical composition, it is usually divided into: lignosulfonate-based superplasticizers, naphthalene-based superplasticizers, melamine-based superplasticizers, sulfamate-based superplasticizers, and fatty acid-based superplasticizers. Water agents, polycarboxylate-based superplasticizers.
The application of water reducing agent in dry powder mortar has the following aspects: cement self-leveling, gypsum self-leveling, mortar for plastering, waterproof mortar, putty, etc.
The choice of water reducing agent should be selected according to different raw materials and different mortar properties.

(4) Starch ether
Starch ether is mainly used in construction mortar, which can affect the consistency of mortar based on gypsum, cement and lime, and change the construction and sag resistance of mortar. Starch ethers are usually used in conjunction with non-modified and modified cellulose ethers. It is suitable for both neutral and alkaline systems, and is compatible with most additives in gypsum and cement products (such as surfactants, MC, starch and polyvinyl acetate and other water-soluble polymers).
The characteristics of starch ether mainly lie in: improving sag resistance; improving construction; improving mortar yield, mainly used for: hand-made or machine-sprayed mortar based on cement and gypsum, caulk and adhesive; tile adhesive; masonry Build mortar.

Note: The usual dosage of starch ether in mortar is 0.01-0.1%.

(5) Other additives:
The air-entraining agent introduces a large number of uniformly distributed micro-bubbles during the mixing process of the mortar, which reduces the surface tension of the mortar mixing water, thereby leading to better dispersion and reducing the bleeding and segregation of the mortar-concrete mixture. Additives, mainly fat Sodium sulfonate and sodium sulfate, the dosage is 0.005-0.02%.
Retarders are mainly used in gypsum mortars and gypsum-based joint fillers. It is mainly fruit acid salts, usually added in an amount of 0.05%-0.25%.
Hydrophobic agents (water repellents) prevent water from penetrating into the mortar, while the mortar remains open for water vapor to diffuse. Hydrophobic polymer redispersible powders are mainly used.
Defoamer, to help release the air bubbles entrained and generated during mortar mixing and construction, improve compressive strength, improve surface condition, dosage 0.02-0.5%.


Post time: Feb-09-2023