Main Functions of Additives in Dry-Mix Mortar

Main Functions of Common Additives for Dry-Mix Mortar

　　Due to their limited variety and poor performance, traditional mortars are increasingly unable to meet the demands of practical construction projects. Dry-mix mortars are being progressively promoted and applied, and the development of dry-mix mortar admixtures is the key technical factor in this promotion. As societal expectations for environmental protection and construction quality continue to rise, a wide range of high-performance admixtures have emerged—characterized by excellent technical properties, superior product quality, broad application scope, strong adaptability, and significant economic benefits. Dry-mix mortar admixtures not only enhance key performance attributes such as workability, water retention, and strength, but also play a crucial role in reducing the consumption of cementitious materials, lowering project costs, decreasing energy use, and ensuring construction quality.

Redispersible Latex Powder
　　Redispersible polymer powder delivers excellent performance in applications such as thin-layer plastering mortars, tile adhesives, external thermal insulation systems for exterior walls, and self-leveling floor coatings. In mortar formulations, redispersible polymer powder acts as a secondary binder that synergizes with the inorganic cement binder, thereby enhancing the mortar’s bonding properties. When incorporated into dry-mix mortars, it significantly improves workability; moreover, it markedly increases both the flexural-to-compressive strength ratio and the tensile-to-compressive strength ratio. Additionally, the addition of redispersible latex powder leads to a progressive increase in the dry apparent density of the mortar with rising dosage, while the modified mortar exhibits improved wear resistance and impermeability.

Cellulose
　　The hardening of cement and gypsum is driven by their hydration reactions; however, when mortar is applied in thin layers, water rapidly penetrates into the substrate or evaporates. Cellulose ethers, owing to intermolecular forces—specifically van der Waals forces—exhibit excellent water-retention properties and can also ensure the uniformity and completeness of the hydration of inorganic binders, as they possess vaporization enthalpies and capillary diffusion characteristics that are broadly comparable to those of water. Mortar is a heterogeneous system in which water and aqueous solutions constitute the continuous phase; upon initial addition of water, frictional and mobilization resistances increase substantially, thereby making mortar application difficult, particularly for thin coats. The incorporation of cellulose ether-based plasticizers lubricates the slippage of the mortar while simultaneously modifying its workability, thus mitigating the risk of cracking during thick-layer application. Furthermore, cellulose ethers dissolve in the liquid phase of the mortar paste, leading to volumetric expansion and, through bridging effects, an increase in paste viscosity. This helps to impede the settling of fine aggregates within the cement paste and the sedimentation of cement particles in the water dispersion—i.e., stratification—thereby improving the mortar’s workability and homogeneity, enhancing bond strength between the mortar and the substrate, and reducing shrinkage, all of which are of considerable significance.

Water-reducing agent
　　Water-reducing admixtures exhibit excellent plasticizing effects. Their primary function is to reduce the water demand of mortar while maintaining the same workability, with the reduction becoming more pronounced as the dosage of the admixture increases. Moreover, due to their dispersing and slight air-entraining properties, mortars containing water-reducing admixtures also show improved water retention. These admixtures are commonly used in applications requiring good self-leveling performance, such as self-leveling underlays, finishing screeds, and leveling mortars. Dry-mix mortars typically incorporate one of three types of superplasticizers: casein, melamine-formaldehyde condensate, or polycarboxylate. Casein is an outstanding superplasticizer, particularly for thin-layer mortars; however, as a natural product, its quality and price can be subject to considerable variation. Melamine-formaldehyde condensate is a high-performance superplasticizer, but its effectiveness is limited in thin-layer applications and it may release formaldehyde. Polycarboxylates represent the latest technological advancement, offering exceptional efficiency without formaldehyde emission.

Hydrophobic agent
　　Organosilane compounds and their modified derivatives possess reactive functional groups. Under the highly alkaline conditions that prevail after cement hydration, the hydrophilic organic functional groups in the silane undergo hydrolysis, generating highly reactive silanol groups. These silanol groups then undergo irreversible reactions with the hydroxyl groups present in the cement hydration products, forming chemical bonds. As a result, the silane molecules, cross-linked with one another, are firmly anchored to the pore wall surfaces within the cement mortar. Because the hydrophobic organic functional groups orient outward toward the pore walls, the pore surfaces acquire hydrophobic properties, thereby imparting overall water-repellency to the mortar.

Defoamer
　　Since the addition of finely dispersed organic polymers increases the porosity of mortar, it is essential to concurrently incorporate an antifoam agent to eliminate air bubbles. Powdered antifoams, depending on their chemical functional groups, include hydrocarbons adsorbed onto inorganic carriers, stearic acid and its esters, tributyl phosphate, polyethylene glycol, and polysiloxanes. Currently, the powdered antifoams most commonly used in dry-mix mortars are primarily polyols and polysiloxanes. In addition to controlling bubble content, the use of antifoams can also reduce shrinkage; an effective antifoam must exhibit good chemical stability, a surface tension lower than that of the medium being defoamed, and insolubility in the medium. Moreover, while antifoaming may produce air-entraining effects in other systems, it is therefore crucial to select the most efficient single antifoam or a composite antifoam tailored to the specific system.