With additives, the modern dry-mixed mortar would be developed, and many of the technical properties of dry-mixed mortar would be possible. However, in the powder ratio, the content of the additives generally only accounts for 0.1% ~ 10% of the mass percentage. Additives are divided into two types: organic and inorganic, and usually exist in the form of polymers. Additives can improve the mixing of dry powder mortar and water, such as improving the rheological properties or construction performance of the mortar as well as the hardening performance of it.
1) Cellulose ether
1)In a dry-mixed mortar, cellulose ether is used as a thickener and water retention agent. Even though the addition ratio of cellulose ether is meager (0.02%~0.7% under standard conditions), it is an essential additive. Using cellulose ether and re-dispersible powder can affect the performance of dry-mixed mortar among all additives. The cellulose cement used in dry-mixed mortar is mainly Methyl hydroxyethyl cellulose(MHEC), and Methyl hydroxypropyl cellulose (MHPC) accounts for a market share of at least 90%. Other relevant cellulose cores that have a small proportion in the dry-mixed mortar market are Ethyl hydroxyethyl cellulose ether(EHEC) and Hydroxyethylcellulose ether(HEC). As methyl cellulose (sodium) is unstable in the presence of calcium ions, it is used as a thickener in only a few applications.
2) Starch Ether
The starch ether added to the dry-mixed mortar is mainly hydroxypropyl, although its viscosity is low (2% solution is usually 100~500MPas), but after being added to the dry-mixed mortar containing MC, it can increase significantly—the consistency of the mortar. The ratio traditionally added in cement-based plastering materials and mortar is 0.01%-0.04%; in gypsum-based dry-mixed mortar, the balance is 0.02%-0.06%. The addition of starch pebbles caused a slight increase in the water demand of the mortar, and at the same time, the coating amount also increased slightly, but the water retention rate of the mortar did not increase.
3) Air-entraining agent
Introduce micro-bubbles into dry-mixed mortar through physical activity so that the density is reduced, the workability is improved, and the output is increased. The air trapped in the mortar makes the concrete more insulating and reduces its strength. The air-entraining agent is generally powder-like, mainly sodium sulfate. The addition ratio in mortar and masonry mortar is usually between 0.01% -0.06% and can determine the optimal dosage by observing the air content of the mortar and its workability.
4) Coagulant
Coagulants are often used in cement-based systems to achieve the desired setting time. Calcium formate is the most commonly used coagulant and has achieved good results in practical applications. If using calcium formate as a coagulant, the addition ratio is about 0.5% ~ 2.5%.
5) Retarder
Retarders are mainly used in gypsum mortars and gypsum-based joint fillers. If not applied retarders, the plaster will set too quickly. The retarder has different types, primarily fruit acid salts, such as tartaric or citrate, and synthetic acid salts, usually between 0.05% and 0.25%.
6) Hydrophobic agent (water-repelling agent)
A water-repelling agent prevents water from penetrating the mortar but, at the same time, leaves the mortar open for the diffusion of water vapor. The performance of a water-repelling agent can be measured by capillary water absorption. The main application occasions are outdoor plastering, inorganic waterproof slurry, and mortar for ceramic tiles. There are two types of water-repelling agents on the market: Polysilicate-Metals (such as zinc stearate or sodium oleate); polymeric re-dispersible powders with hydrophobic properties. The advantage of the first type is that the addition ratio is low (0.1%~1%). The second type has a significant service life because the hydrophobic re-dispersible powder will not be washed out of the mortar by rain after many years. In addition, using hydrophobic re-dispersible powders does not cause wetting difficulties when the dry-mixed mortar is mixed with water; therefore, it improves the adhesion between the hardened mortar and the substrate.
7) Superplasticizer
Superplasticizer, also known as a high-efficiency plasticizer, significantly impacts the water demand of mortar when mixed into dry-mixed mortar. Mortars containing superplasticizers require less water to obtain the same consistency than mortars without superplasticizers. The superplasticizer’s mechanism of action is that the cement particles’ surface has a different electric charge causing them to contain water when they agglomerate. Through the adsorption of the superplasticizer, the surface charge is neutralized, and the water is released. Depending on the technical advantages, either casein (the main product) or synthetic superplasticizers. Superplasticizers are mainly used in self-leveling dry powder mortars with high requirements, such as self-leveling materials, adhesives for floors, and pourable floor tiles, and the dosage is usually between 0.2% and 1%.
8)Fiber
Fiber can divide into two groups: K fibers and short fibers. Long fibers are mainly used for reinforcement of dry powder mortar; short fibers are used to improve mortar’s performance and water demand.
9)Antifoaming agent
The primary function of the antifoaming agent is to reduce the air content in the mortar. It is currently used mainly in powdered antifoaming agents based on different chemical agents (particularly hydrocarbons on inorganic carriers, polyethylene glycol, or polysiloxane).
