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  • Loose and binding materials and mixtures /

    Secrets of the bag with the dry mix.

    In the last decade of the XX century, the use of dry mix during construction, finishing and repair work has finally become the norm in our country. Materials for floors, waterproofing and repair pastes, adhesives for ceramic tiles and natural stone, grouts (jointers), plasters of various functions — this is not a complete list of products available in the form of dry mixtures. Unfortunately, the majority of consumers do not have a clue about what lies behind the beautiful (or not so beautiful) package, and the main criterion for the selection of a material often is the cost, which in principle is not true. Why? Try to understand.

    Modern dry mixes — it is not just cement, sand, and the product of high technologies, the use of which makes it possible not only to significantly increase productivity, but also get a very different qualitative results unattainable in the case of traditional cement-sand mixtures. Naturally, manufacturers kept their recipe mixes in deepest secrecy, but the general principles of the specific characteristics of the materials and components that make them are well known. The composition of the modified dry mixtures can be divided into four main groups of components: mineral binders, excipients, additives to obtain special properties, including water retention and polymeric binders, which work in the same direction as the mineral, but have completely different mechanism action.

    Mineral binders.
    As a hydraulic mineral binder are most commonly used Portland cement (white and gray) quick-vysokoalyuminatnye cements, a- and .-hemihydrate gypsum, and combinations thereof. When combined hydration of cement and plaster, a large number of mineral called ettringite, the presence of which can lead to the destruction of the material obtained from the dry mixture, however, such a combination has limited use, for example non-shrink device for floors with a rigid surface. Hydrated lime (pushonka) is cured by the reaction of c carbon dioxide (CO2), but because the air is astringent.
    Secrets of the bag with the dry mix. Currently, lime is used as an additive (5 — 30%) to cement materials to significantly improve the processability of mixing the solution, and as the primary binder — Only in special formulations intended for the restoration of historic buildings. If the filler is properly selected, i.e. has a low porosity and a dense packing of sufficient intrinsic strength, the strength and density of the mineral composition obtained depend only on the properties of the binder.
    Unfortunately, the technical characteristics of cement, even a single manufacturer may vary considerably from batch to batch, which leads to a change in the properties of the dry mixture in the same recipe. Despite the presence in our country, a huge number of fields CaCO3, there are big problems with the quality of the lime. In this connection, not to mention the international quality management system DIN ISO 9001. There is a misconception that having ISO 9001 certification is a guarantee of absolute quality. This is not quite true. The ISO 9001 does not define the quality characteristics of materials, and ensures only the stability properties of the product. The Special Commission is conducting a survey of the enterprise, evaluates the condition of its equipment, technical documentation, the organization of the production process and quality of raw materials.
    The decision to grant (or refusal to grant) the certificate is made based on a comprehensive analysis of the results. In the future, if introduced into the production technology of any changes that may affect product conformity with requirements of normative documents, the manufacturer shall notify the Commission, issued the certificate. In turn, the Commission, having considered the application, may decide to hold a re-certification.
    So, mark ISO 9001, is present on the packaging of the dry mixture or in the technical documentation indicates that the product is manufactured at the company, certified, and the information contained in the technical description corresponds to reality.

    In the production of dry mixtures as fillers are most often used limestone powder and quartz sand. The use of dolomite instead of limestone powder is not recommended because it contained more soluble salts of magnesium, which may lead to the formation of efflorescence.
    The composition of thermal insulation and plaster mixtures include porous fillers such as expanded clay, perlite and vermiculite, and in some cases — granulated polystyrene. If you need to use high-quality cement does not cause any shadow of doubt, the feasibility of using fillers and, in particular, certain sand of conditions, from the standpoint of the layman quite obvious.
    Secrets of the bag with the dry mix. In the meantime, the quality of the dry mixture depends not only on the particle size distribution of filler, but also on the content of extraneous additives, the presence of which significantly affects the final result. In particular, even a small content of impurities clay can significantly reduce the adhesion solution, and the presence of certain soluble compounds initiates destruction of costly organic additives.
    Furthermore, in the composition inorganic filler may contain chemical compounds (e.g., active alumina), can affect the cement hydration process. To avoid such trouble makers tend to use a filling of a field that allows for consistent quality of the final product.
    For example, leveling mass brand « Vetonit », is quite popular among our builders are manufactured at the company Optiroc, which is in the center of the quarry, from which sand is used as filler.
    Unfortunately, the volume of a journal article does not allow to consider in detail such important parameters of the filler, the hardness and porosity. Let us briefly examine the impact of screen performance. Most solutions for thin-film deposition, such as tile adhesives and thin plasters generally comprise grains larger than 0.6 mm is not. Formulations suitable for thick application, including fine, include larger grains. The size of 0.1 mm is typical for such smooth solutions, like jointing and filler weight. Maximum mechanical parameters, for example the peel force with a minimum consumption of chemical additives can be achieved when the filler has sufficiently dense packing, that is, contains approximately the same proportion of the various fractions.

    Modifying additives.
    The use of special additives (modifiers) when creating recipes modified dry mixes due to the need to obtain specific technical and technological characteristics of these materials and primarily, — need to retain water in the mixing solution after application. Water soaks into the ground and evaporates from the solution, thereby reducing the residence time of the cement in the gel phase, decrease in the degree of hydration and, consequently, to a decrease in strength. The smaller the thickness of the solution, the greater the impact on the quality of the resulting cement paste these shortcomings.

    Secrets of the bag with the dry mix.

    In the beginning of the last century in Germany was developed a method of producing water-soluble cellulose ethers. Studies have shown that as a result of the weak intermolecular interaction with water molecules, these polymers have excellent water-holding capacity. Each polymer molecule can hold up to 20 thousand. Of water molecules.
    Secrets of the bag with the dry mix. The energy of this interaction is comparable with the energy of evaporation and capillary diffusion in the base, which is an obstacle to leaving water. In turn, this energy is somewhat smaller than the energy of water diffusion in the cement hydration, which allows him to select the water.
    fact water solution is replaced with a homogeneous solution of methylcellulose jelly, wherein the suspended particles of cement and aggregate. High water-holding capacity of the system contributes to the full hydration of cement and allows the solution to gain the necessary strength even with thin layers. After leaving the water in the form of very thin polymer film remains on the surface between the cement matrix and filler, without affecting the mechanical properties of the hardened solution. Thus, adding a small amount (0.02 — 7%) of water-soluble cellulose ethers in cement-sand mixture substantially increases the open time and allows the solution to hydrate uniformly throughout the volume, and provides a significant increase in adhesion to the base and improve the surface quality .
    The thicker layer of slurry, the less of methylcellulose is required to ensure the necessary degree of initial hydration, so label on the dry mixture shall be clearly specified minimum permissible thickness of coating composition . In turn, and unacceptably high build (10 mm or more) application of the solution with a high content of cellulose ether, intended for thin layer technology. In this case, it may occur « the effect of caramel », when the surface is cured properly, and not retained within the hardened cement. For this reason, for the preparation of uneven grounds (with differences of more than 10 — 15 mm) we recommend the use of materials, consisting of a dry mixture for rough leveling and thin-layer leveling mass, provides a smooth finishing layer , the surface of which laid flooring. System approach not only avoids these troubles, but also reduce costs for materials .
    By following group of additives are dispersed powders, which, in contrast to water-soluble cellulose derivatives, when mixed with water do not form solutions, biphasic systems consisting of polymer particles (based on copolymers of vinyl acetate and ethylene, vinyl chloride, styrene-acrylate, etc. n.), dispersed in water. The addition of these compounds in the products of building chemistry allows you to actively influence the properties of the final material and produces results that are not attainable by using only traditional mineral binders.
    The first attempts to cement mixes modified polymers is to add to the mixing water dispersion of vinyl acetate, the known as the PVA glue. In the process of formation of vinyl acetate film there is a significant (10%) shrinkage, which leads to cracking of polymer cement material, so the use of PVA quickly abandoned.
    The next step was the use of two-component, consisting of cement-sand mixture prepared at the factory, and the polymer dispersion, supplied in liquid form, which are mixed on site. Two-component solutions used until now, but the aqueous dispersion loses its properties during freezing, so the coldest time of the year its transportation and preparation of the working solution cause some difficulties.
    Start of production of one-component dry mixes refers to 1953, when the experts of the company Wacker (Germany) managed to get dry redispersible powder formed after mixing with water two-phase system with the original properties of the polymer dispersion.
    The dispersions are different from methyl cellulose and mechanism of action. As the consumption of water is concentrated in the pores of the cement paste and the same is concentrated dispersion to form a « elastic bridges », working on stretching and bending much better than cement. The combination of mineral and polymer binders enables the production of construction chemicals, has not only increased strength properties and improved adhesion (including such « problem » bases, such as metal, wood, plastic, glazed tiles, etc.) but controlled rheology (thixotropy, plasticity) and special (hydrophobicity, fluidity) characteristics.
    For example, leveling mortar for floors contain a combination of special additives to the dispersion of organic and synthetic plasticizers, which determine the presence of such specific properties of these materials, such as spreadability and smoothness of the resulting surface. Dispersion modifiers included in the adhesives for tile work, improve working conditions, prolong life and improve the mixing of the solution thixotropy (ability to thicken at rest and thinning stirring) of the material.
    In addition to improving the adhesion to the fundamentals of complex, dispersion powders serve as a polymeric binder in cases where the greatness in the shear stress exceeds the capacity of cement-sand mortar, only the modified cellulose ether. Plasticity of adhesives allows, in particular, to compensate for thermal stresses arising between the facing material and osnovaniem.V primarily refers to the facade systems where daily fluctuations in temperature can reach 70 — 80 & deg; C and « warm » floors, where temperature fluctuations are also very large.
    Very high (20 — 30%) polymer content leads to the fact that the cement will not form a continuous crystal lattice, and some of the cement stone fragments linked only elastic polymer chains. Such materials are used for waterproofing. Assortment of many component manufacturers of construction chemistry (CLARIANT, BAYER, WACKER et al.) Includes a dispersion powders with a temperature of film formation order 0 & deg; C, which allows the use of dry mixes supplemented with these materials at low (but not negative) temperatures, and some brands dispersion powders have a film

    Secrets of the bag with the dry mix.

    -forming temperature at – 15 & deg; C. The solutions which are used in the formulation of these powders intended for stroitelnootdelochnyh winter operation (at temperatures below – 10 & deg; C).
    formulating dry mixes — it is a complex and long process, is not possible without the participation of qualified personnel and necessarily presuppose the existence of a modern laboratory for the

     analysis and testing of samples, equipped with high quality equipment. In this regard, I want to warn consumers against attempts handicraft « modification » simple cement mortars.
    In particular, a common practice in our country « improved » ordinary cement-sand mixtures of vinyl acetate dispersion (PVA glue) leads to very poor results. Low resistance to alkaline solutions inherent in vinyl acetate, saponification causes it in places with high humidity (bathrooms, etc.). This circumstance is the reason that falls off tiles, laid on a solution with the addition of PVA glue.

    The price and quality.
    A comparative analysis of the cost of dry mixes, on the market, shows that most of the products produced in the vast former Soviet space is much cheaper than Western counterparts. Domestic consumer perceives this as something for granted. Indeed, cheap labor, local raw materials, minimal transport costs and the absence of customs duties are, at first glance, real prerequisites for the reduction of the price characteristics of building materials. However, upon closer inspection, the situation looks quite different.




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