Dry powder mortar, also known as mortar dry powder (mixed) material, refers to a granular or powdery mixture of fine aggregates, cemented materials and additives which are produced by professional manufacturers and dried and sieved in a certain proportion. It is transported to the construction site in the form of bags or bulk, and the mortar-like building materials can be directly used after mixing with water. The basic components of dry mortar include cementitious materials, aggregates and various additives. Additives for dry mortar can be divided into mineral additives, chemical additives and fibers.

1 Mineral additives

  1. 1 fly ash

The main components of fly ash are oxides of silicon, aluminum, iron, calcium and magnesium, which have potential chemical activity. Fly ash alone does not have water coagulating activity when mixed with water, but mixed with cement and then mixed with water, pulverized coal. The chemically active SiO2 and Al2O3 in the ash can react with Ca(OH) 2 precipitated by cement to form a gelling substance similar to gels such as hydrated calcium silicate and hydrated calcium aluminate produced by cement hydration. And has a certain strength. The ratio of active SiO2 to active Al2O3 in fly ash is called the “silicon to aluminum ratio”. Fly ash is often used in ordinary dry mortar such as masonry mortar and plastering mortar. The addition of fly ash in the dry mortar not only reduces the cost of the mortar, but also ensures that the mortar increases the binder and reduces the loss of the fluidity of the mortar without increasing the amount of cement.

  1. 2 silica fume

Silica fume is a fine dust collected from the flue gas emitted from the production of ferrosilicon or silicon steel. The main component of silica fume is silica, which generally accounts for about 90%. Most of it is amorphous silica, and it also contains other components such as iron oxide, calcium oxide, sulfur oxide, etc. [1]. When silica fume is mixed into the dry mortar, the calcium hydroxide formed by hydration of ordinary Portland cement accounts for about 20% of the volume. The silica fume can react with the calcium hydroxide to form calcium silicate, which is evenly distributed in water and Between the particles, a dense structure is formed to increase the strength of the mortar. Due to the fineness and high activity of silica fume, the addition of silica fume has no adverse effect on the early strength of the mortar. The mortar with silica fume will have smaller capillary pores, which will significantly improve the impermeability, frost resistance and corrosion resistance of the mortar. It is worth noting that due to the high specific surface area of ​​silica fume, it requires a large amount of water. When it is incorporated into the mortar, it needs to be matched with a water reducing agent to ensure the workability of the mortar.

  1. 3 slag powder

The main chemical components of slag are SiO2, CaO, Al2O3. In general, the content of these three oxides is about 90%, and there are also a small amount of magnesium oxide, ferric oxide, sodium oxide, potassium oxide and the like. Active SiO2 and active Al2O3 in the slag interact with Ca(OH) 2 in the liquid phase to form hydrated calcium silicate, calcium hydrated calcium aluminate, etc., and thus have high activity [2]. When the mortar is mixed with slag powder, the cement clinker is first hydrated to form hydrated calcium silicate, hydrated calcium aluminate, hydrated iron aluminate, and hydrated calcium sulphoaluminate. The hydration of cement clinker will induce the hydration of clinker minerals in the slag and greatly promote the hydration reaction process. As the hydration reaction of the slag and cement clinker accelerates, the concentration of Ca(OH) 2 in the liquid phase is ensured, and the alkali-excited reaction of the slag is promoted. Therefore, the proper amount of slag is added to the mortar to increase the strength of the mortar to some extent.

  1. 4 metakaolin

Metakaolin is a highly active artificial pozzolanic material that undergoes a pozzolanic reaction under the action of cement hydration product Ca(OH)2 to act as an auxiliary cementitious material. The appropriate amount of calcined kaolin is mixed with the cement powder. The active SiO2 and Al2O3 in the calcined kaolin can react with Ca(OH) 2 and gypsum to form fine acicular ettringite, hydrated calcium silicate and calcium hydrated calcium aluminate. . Incorporation of calcined kaolin can also increase the ratio of SiO2/CaO and Al2O3/CaO content near the interface between cement slurry and aggregate (sand or gravel), and reduce the possibility of aggregation of calcium hydroxide crystals near the interface. The interface structure is favorable. The fine particles in the calcined kaolin can also provide a large number of crystal cores for the crystallization of calcium hydroxide, and have an inhibitory effect on the growth of calcium hydroxide crystals [3]. The addition of calcined kaolin increases the number of components and the degree of freedom of the reaction system, and is advantageous for increasing the hydration reaction and the coagulation rate and improving the microstructure. At the same time, kaolin has a high adsorption performance, and can increase the suspension force, so as to have a certain water retention thickening performance. Adding the right amount of metakaolin in the mortar is not only beneficial to the working performance of the mortar, but also can improve the strength of the mortar to some extent.

2 chemical additives

  1. 1 redispersible powder

The dispersible polymer powder is also used as an organic cementing material in a dry powder mortar. It is a polymer of vinyl acetate-ethyl ester which is spray-dried to form spherical particles of 80 μm to 1 002 μm from the initial 2 μm. The surface of these particles is coated with an inorganic anti-hard structure powder to give a dry polymer powder. When the powder is mixed with water, cement or gypsum as the substrate mortar, it can be redispersed. The basic particles (2μm) will be reformed to the same state as the original latex, and its chemical properties are exactly the same as the initial solution. When the dry mortar is added to water and stirred, the rubber powder particles are dispersed into the water under the action of a hydrophilic protective colloid and mechanical shearing force. During the construction process, as the moisture decreases, the resin particles gradually approach, the interface gradually blurs, and the resin gradually merges with each other, eventually forming a continuous polymer film on the pores of the mortar and the surface of the solid. Thereby improving the crack resistance of the mortar. At the same time, the addition of the dispersible polymer powder causes the increase of the gas content of the mortar to lubricate the construction of the mortar, and the affinity of the rubber powder, especially the protective colloid, and the subsequent viscosity to the cohesion of the construction mortar. Improve, thus improving the workability. Due to the bleed effect of the dispersible polymer powder, it is disadvantageous to the compressive strength of the cement mortar.

  1. 2 cellulose ether

The addition of cellulose ether in dry mortar is very low, but can significantly improve the performance of the mortar. Water retention is an important property of methyl cellulose ethers. The water retention effect of cellulose ethers is closely related to its amount, viscosity, fineness and temperature used. As the amount of addition increases, the water retention effect becomes more pronounced. However, when the dosage is too large (> 0.2%), the viscosity of the mortar increases sharply, and the sticking knife phenomenon appears during construction, which is not conducive to construction. Moreover, as the amount of the addition increases, a large amount of bubbles are introduced into the mortar, and the viscosity is too large to be removed, thereby greatly affecting the strength of the mortar. In addition, cellulose ether will also reduce the bulk density, which will significantly delay the hydration heat release rate of cement and delay the occurrence of hydration exothermic peaks, which will significantly reduce the hydration heat release within 48 h, prolong the clotting time of mortar and increase it. The dry shrinkage value of the mortar.

3 fiber

The addition of fiber to dry mortar mainly plays the following roles:

1) Preventing and expanding the cracks of the original original defects of the mortar, and effectively preventing and delaying the appearance of new cracks; 2) improving the compactness of the mortar body, preventing external water intrusion, improving water resistance and impermeability; 3) improving mortar The anti-freeze and anti-fatigue properties of the matrix improve the durability; 4) Improve the rigidity of the mortar matrix, increase the toughness, reduce the brittleness, and improve the deformation force and impact resistance of the mortar body; 5) Improve the surface morphology of the cement mortar to make it more Dense, delicate, smooth and beautiful.

When the cement slurry is mixed, the fiber continues to develop strength and the strain capacity of the mortar is increased, which shows great advantages especially in places where high tensile strength and cracking are required. When chopped fibers are added to the cement mixture, when the mortar is cured, the large amount of microfibers that are strongly dispersed and cemented with the cement are weakened and inhibited, and the early deformation and shrinkage stress of the cement are weakened and suppressed. It is less prone to stress concentration during the curing process, thereby reducing significant cracks.

The choice and amount of additives have an important impact on improving the performance of dry mortar. The selection of mineral additives not only improves the performance of all aspects of dry mortar, but also contributes a lot to the environment. Redispersible powders and cellulose ethers have become essential additives for dry mortars, and have a great impact on improving mortar toughness and construction performance. Adding fiber to dry mortar is an important measure to control the cracking of mortar matrix. At the same time, the use of additives such as water reducer, retarder, air entraining agent, quick-setting agent and early strength agent has an important influence on the performance of dry mortar. Understanding the effects of various additives on mortar performance and the mechanism of action are helpful for both the dry mortar required for formulation and the various problems associated with the treatment of hardened mortar.

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