RU2351556C2 - Modified component of magnesia cement - Google Patents

Abstract

FIELD: construction.

SUBSTANCE: invention is related to composition of modified component of magnesia cement. Modified component of magnesia cement contains active oxide of magnesium and metakaolin - thermally activated kaolin with preferable size of particles of less than 10 mcm, in amount from 2% to 50% of active magnesia oxide mass. Invention is developed in dependable claims of formula.

EFFECT: increased compression strength on magnesia cement in water saturated and dry form, increased sedimentation resistance of concrete mix and mortar, increased water-retaining capacity of magnesia mortars and concretes, increased adhesion strength, resistance to blooming.

4 cl, 4 ex

Description

The invention relates to the field of building materials containing an inorganic binder, in particular to magnesia cements, and is intended primarily for the manufacture of screeds and floors, and can also be used for masonry, fastening of tiles, the manufacture of protective and decorative coatings and products.

Magnesia binder (cement) is a combination of fine powder, the active ingredient of which is magnesium oxide MgO (oxide component) with a salt of a strong acid with a divalent or trivalent metal (salt component) mainly magnesium chloride MgCl ₂ , ferric chloride FeCl ₃ and / or sulfate magnesium MgSO ₄ .

Magnesium oxide, in turn, is a product of moderate calcination of natural carbonate rocks of magnesite or dolomite. When mixing it with a salt component, a strong cement stone is formed. Many properties of magnesia cements are better than Portland cement: they harden quickly, are dustless, have elasticity, resistance to oils, greases, organic solvents, alkalis and salts, provide high fire resistance and low thermal conductivity, good wear resistance and compressive and bending strength. Very significant is the fact that magnesian binders are characterized by increased adhesion to various types of aggregates, both inorganic and organic.

All these qualities determine their use in abrasive production (grindstone), for the manufacture of heat-insulating products (foam and gas magnesite) and partitions, window sills, stair steps, less often for facing tiles of the interior and small architectural forms. However, their main use was and remains the device of seamless monolithic floors, since magnesian cements allow to obtain high-strength and wear-resistant coatings with high resistance to vibration and shock, as well as oil and petrol resistance.

These floors are hygienic, non-combustible and durable. However, this is their significant drawback compared with Portland cement floors, magnesia concrete floors themselves are characterized by low water resistance and require protection from moisture, including from the bottom of the capillary suction of water through the base and sideways through the walls. Using various polymers, floor manufacturers can thus primer the surface of the base on which magnesian concrete is laid, so that the primer serves as a waterproofing. Polymer impregnation of the top layer allows you to protect from moisture penetration into the concrete from above. However, such measures significantly complicate the technology and increase the cost of the product, and also do not give a full guarantee of the water resistance of the structure, since the waterproofing layer can be damaged when the structure is deformed or as a result of poor-quality manufacturing.

In addition, a component with active magnesium oxide, such as caustic magnesite, has a significantly higher water demand compared to Portland cement (45-55% compared to 23-27%). This means that equal-moving mortars and concretes with a magnesian binder contain a significantly larger amount of liquid compared to compositions based on Portland cement. This leads to a more acute problem of the separation of magnesite-concrete mixtures and to the separation of liquid on their surface than in compositions based on Portland cement. As a result of this, the strength first of all of the surface layer of hardened concrete is reduced, as well as the strength in volume due to a change in its structure. This stratification can lead to significant warping (deformation) of thin-walled products from magnesian cements, since they turn out to be inhomogeneous in thickness.

Improving the properties of magnesia cements is possible by modifying its components.

Magnesia cement is known in which dehydrated aluminosilicates of the metakaolinite type are the main component of burned rock or fly ash used to modify the binder. However, the reaction of formation of calcium and magnesium hydrogranates, due to which the binder’s water resistance increases, proceeds during hydrothermal treatment of the composition, that is, at elevated temperature and pressure in the borehole, and liquid glass (sodium silicate) is used to activate the modifying additives used. The necessity of activating the aluminosilicate ingredient used is explained by its low activity due to the uncontrolled and uneven temperature of its self-firing, a large variation in particle sizes and composition of each of the particles, and instability in the structure and chemical composition. When sodium silicate is mixed with magnesium chloride, an instant reaction occurs to form magnesium silicate and sodium chloride. In this case, magnesium silicate is released in the form of a thick gel, and sodium chloride no longer enters into chemical reactions and remains in the hardened solution in the form of a water-soluble salt (SU No. 1101542, prototype).

The disadvantages of this cement are the need for hydrothermal treatment of the composition to form water-resistant compounds, which does not allow the use of this cement for concrete and mortars hardening under normal conditions (temperature 20 ± 5 ° C, atmospheric pressure), as well as deterioration of the consistency of the solution due to the rapid formation of thick magnesium silicate gel and the appearance of efflorescence on the resulting surfaces or products due to the presence of water-soluble sodium chloride in the hardened solution or concrete.

Cement is known in which caustic magnesite is used only to activate the aluminosilicate component. That is, the hardening mechanism of this cement is the hydration of the aluminosilicate component, and not the magnesian binder (SU No. 1346608).

The disadvantages of this cement, which is not magnesian by the mechanism of product formation, are the narrow range of use due to the inability to obtain mechanical and other characteristics inherent in magnesian cements, as well as the need for autoclaving of the resulting products.

Known magnesia cement, in which one of the components of the composition is kaolin. In this case, kaolin is a passive filler of the composition and can play the role of only a mineral plasticizer (makes the mixture more plastic, "greasy"). To increase water resistance, hydrophobization of the mixture with polyorganosiloxane is performed. Since polyorganosiloxanes are not miscible with water, they require a mineral carrier for incorporation into aqueous systems. This carrier in this invention is kaolin, which alone cannot provide an improvement in the water resistance of cement (RU No. 2062763).

The disadvantages of this cement are the low efficiency of the modification in terms of water resistance, associated with a limited period of hydrophobization with polyorganosiloxane, the complexity of the composition and high cost of the components used, in particular polyorganosiloxane.

Known magnesia cement, which uses a mixed binder, consisting of magnesia cement (caustic magnesite and a solution of magnesium chloride) and ground blast furnace slag containing 6-9% alumina. In this invention, the component ratio is such that blast furnace slag is used 1.5 times more than caustic magnesite, so the main mechanism of its hardening is the hydration of slag minerals (and not the reaction of magnesite with magnesium chloride to form magnesium oxychloride, which is the mechanism for the formation of magnesian products cement). As a result, hardening significantly slows down, which is indicated by the setting time - 7-8 hours. Steel reinforcing aggregate significantly increases the strength of products, especially bending, but this filler in wet conditions will intensely rust in concrete (since the pH of the concrete is low, based on its composition), expanding and destroying the resulting product (RU No. 2096380).

The disadvantages of this cement are the long setting time, the need for heat treatment of products, low modification efficiency in terms of water resistance, low sedimentation resistance (resistance to delamination) and the complexity of the composition.

Magnesia cement is known in which ground basic granular blast furnace slag and magnesium hydrosilicate are added to modify the binder (RU No. 2238251).

The disadvantages of this cement are the low strength of the products in the dry state, which could be significantly higher without the use of additives, low sedimentation resistance (resistance to delamination) and the complexity of the composition (two components are used simultaneously to modify the binder). Some increase in water resistance of concrete, i.e. an increase in strength in a water-saturated state is insufficient and is accompanied by a decrease in dry strength, which means the loss of the advantages inherent in magnesian cements.

Known technical solutions, as a rule, indicate the positive effect of active silica (silicon dioxide) on the water resistance of a magnesian binder. The results of repeated experimental checks of this statement by the applicant showed that although the water resistance actually increased slightly, all the advantages of magnesia cement were lost at the same time. Also, Portland cement, which is based on calcium silicates, even in small amounts (3-5%), enters the magnesia cement, halving the strength of magnesia concrete or mortar.

Magnesium hydrosilicates exhibit very weak astringent properties. That is, during the formation of magnesium hydrosilicates, a durable artificial stone does not form, but a loose gel forms. Thus, silica is a harmful component for a magnesian binder, along with calcium silicates, which are also found in large quantities in materials such as basic blast furnace slag and fly ash. In known technical solutions, when known modifying additives containing various forms of silica and aluminosilicates are added to magnesia cements, hydrosilicates and hydroaluminosilicates of magnesium and calcium are formed in parallel with the formation of the main reaction product of the hardening of magnesia cements (magnesium oxychlorides or oxysulfates). According to the experimental results, magnesium hydrosilicates, which are the products of the reaction of active magnesium oxide with silica and calcium silicates, weaken the astringent properties of magnesia cement. As a result, water resistance increases while reducing the strength characteristics of the product in a dry state. In other words, in the case of such a modification, all the advantages of the magnesian binder are lost.

An object of the invention is to obtain a durable, water-resistant and sedimentation-resistant magnesia cement while maintaining its strength characteristics and the remaining above-mentioned advantages of products from magnesia cement.

The technical result consists in increasing the water resistance, i.e. increasing the strength of products in water-saturated form from 45-55% to 90-110%, without compromising dry performance. At the same time, a significant increase in sedimentation resistance (resistance to delamination) of modified magnesia concrete and mortar is provided. Higher sedimentation resistance of modified concrete and mortar provides a higher strength in volume and especially on the surface of concrete or mortar, higher impermeability, as well as the stability of the shape of the products. In addition, an increase in the water-holding capacity of magnesia mortars and concretes is provided, which is expressed in a decrease in the suction of the mixing fluid into the base and, therefore, an increase in the adhesion of magnesia concrete and mortars to absorbent substrates by reducing the weakening of the contact zone. At the same time, decorativeness and resistance to salt formation increase.

The essence of the invention lies in the fact that the modified component of magnesia cement, which includes an ingredient containing active magnesium oxide, as well as an aluminosilicate modifying mineral additive, contains metakaolin, which is a thermally activated kaolin with a predominant particle size of less than 10 microns, in the amount of from 2% to 50% by weight of active magnesium oxide.

Preferably, metakaolin is kaolin thermally activated for at least 5 minutes at a temperature of 500-1000 ° C, with at least 60% of particles smaller than 10 microns. As an ingredient containing active magnesium oxide, the component contains caustic magnesite with a mass fraction of active magnesium oxide from 0.7 to 0.95 or caustic dolomite with a mass fraction of active magnesium oxide from 0.15 to 0.23.

The basis of metakaolin are natural aluminosilicates, which are characterized by a complex chemical composition and internal structure. Their structure is based on a silicon-oxygen tetrahedron, in the center of which there is a silicon ion Si ⁴⁺ or aluminum ion Al ³⁺ , and at the vertices - oxygen ions O ^2- . Silicon ions create tetravalent, and aluminum ions create a trivalent radical with an additional negative charge. Natural kaolin is a mineral from the group of aqueous aluminum silicates. The chemical composition of Al ₄ [Si ₄ , O ₁₀ ] (OH) ₈ ; contains 39.5% Al ₂ O ₃ , 46.5% SiO ₂ and 14% H ₂ O. The crystal structure of kaolin is based on infinite sheets of Si-O ₄ tetrahedra having three common oxygen and connected in pairs through aluminum by free vertices and hydroxyl.

When heated to 500 ° C, kaolin begins to lose water with the formation of active metakaolin Al ₂ Si ₂ O ₇ , and at 925-1050 ° C it can decompose with heat, giving first spinel Si ₃ Al ₄ O ₁₂ , and then mullite Si ₂ Al ₆ O ₁₃ and cristobalite SiO ₂ are chemically inactive substances.

The process of chemical transformations during firing is not instantaneous, it takes at least 5 minutes, as does the process of heating the fired material, and depends on factors such as the size of the fired material, firing time, heat transfer rate (type of furnace) and, of course, temperature (i.e. E. To what temperature does the calcined material have time to warm up on average, and how long it is in this state). Therefore, even at a temperature in the range of 925 - 1000 ° С, it is possible to select such mode parameters at which the desired result is obtained and kaolin does not go into a chemically inactive form.

The mixture into which the modified component is introduced contains the salt component mainly in the form of solid crystalline hydrate or in the form of an aqueous solution with a density of 1.1-1.35 kg / l of salts of strong acids and divalent metals, at least one of the following: chloride magnesium MgCl, ferric chloride FeCl ₂ , magnesium sulfate MgSO ₄ . Strong acids include acids that are completely dissociated in aqueous solutions. Preferably, salts of hydrochloric and / or sulfuric acid, divalent magnesium metal and / or ferric metal are used as salts of strong acids.

The modified component of magnesia cement is used as follows.

Based on the set properties of the next batch of the mixture and the range of available starting materials, the necessary quantities of the modified active component and the remaining components of magnesia cement are determined.

The calculated quantities of ingredients for the next batch of solution are loaded into a mixer or in a special container with the necessary amount of water added and mixed thoroughly until a homogeneous mass is obtained. The mass fraction of the solution of the salt component in the mixture is, as a rule, at least 9%, the modified component is at least 7%, aggregates and fillers are the rest (see examples).

Metakaolin is contained in the modified component of magnesia cement in an amount of from 2% to 50% by weight of the amount of magnesium oxide. To obtain metakaolin - to impart the desired properties (activation) - natural kaolin aluminosilicate is subjected to heat treatment for at least 5 minutes at a temperature of 500-1000 ° C, which ensures dehydration, but retains its structure and does not allow the conversion of metakaolin to inactive minerals. Metakaolin for modifying a component of magnesia cement is taken with a predominant, i.e. at least 60%, content of particles less than 10 microns in size, in most cases no more than 3 microns.

The result is a ready-to-use mortar or concrete. The manufacture of the floor or other structures or products is carried out by pouring the mortar onto the surface prepared accordingly or into the mold and, if necessary, leveling and sealing it. During the first day, 30-50% of the strength of the structure or product is achieved, which allows you to remove the product from the mold or move freely on the surface of the structure. Full normalized strength and quality indicators of the hardened mortar (concrete) are achieved after 28 days. In this case, the adhesion strength to the concrete of the base may exceed its tensile strength.

Example 1

To prepare a batch of the mixture for each ton of the mixture, based on the required compressive strength of 35 MPa, 200 kg of the salt component are taken, which is an aqueous solution of two metal salts with a density of 1.205 kg / l, with a mass fraction (content) of magnesium chloride of 0.16 , and a mass fraction of ferric chloride of 0.05.

As a modified component containing active magnesium oxide, caustic magnesite with a mass fraction of active magnesium oxide of 0.83 and a metakaolin content of 2.5% by weight of active magnesium oxide is used.

Metakaolin is a kaolin thermally activated for 5 minutes at a temperature of 1000 ° C with a content of 60% of particles less than 10 microns in size.

The modified component take 244 kg. Silica sand with a grain size of up to 0.5 mm is used as a filler, and powdered silica is used as filler. The amount of quartz sand is 475 kg. As additives, 2.5 kg of methyl cellulose, 6 kg of plasticizer, 1.8 kg of antifoam are taken. Ground quartz, the rest is up to 1000 kg. The mobility of the freshly prepared mixture according to the Suttard viscometer is 25 cm. The strength of the hardened solution from this mixture under compression of 36.8 MPa after 28 days from the beginning of hardening, the strength in the water-saturated form of the solution from this mixture is 35.4 MPa (water resistance 96%), the adhesion strength to base concrete 4.5 MPa (destruction of concrete base). Delamination does not exceed 5%. No efflorescence. The surface is smooth, homogeneous, without signs of water separation.

Example 2

For the manufacture of magnesia concrete, 90 kg of a 15% percent solution of magnesium chloride with a density of 1.149 kg / l, 104 kg of a modified component containing caustic magnesite with a mass fraction of 0.75 magnesium oxide and metakaolin content in the amount of 27% of the mass of the latter, 252 kg of sand are taken fractions of 0.1-5 mm, 554 kg of crushed stone of a granite fraction of 5-20 mm and 0.5 kg of sulfonated naphthalene-formaldehyde resin. Metakaolin is a kaolin thermally activated for 20 minutes at a temperature of 800 ° C with a content of 65% of particles less than 10 microns in size.

The mobility of the freshly prepared concrete mix is 7.5 cm cone sediment. The strength of the hardened concrete from this mixture under compression of 44.4 MPa after 28 days from the beginning of hardening, the strength in the water-saturated form of the product from this mixture is 45.3 MPa (water resistance 102%), the adhesion strength to the base concrete is 2.1 MPa (mixed fracture border with concrete and concrete base). Delamination does not exceed 3%. No efflorescence.

Example 3

To make a batch of repair mortar, they take 133 kg of a 15% percent solution of magnesium chloride, 294 kg of caustic dolomite with a mass fraction of magnesium oxide of 0.21 and a metakaolin content of 18% by weight of the latter and 573 kg of quartz sand. Metakaolin is kaolin thermally activated for 40 minutes at a temperature of 600 ° C with a content of 70% of particles less than 10 microns in size.

This mixture has increased ductility. Its freshly prepared mobility is 4 cm dipping cone. The strength of the hardened mortar from this mixture under compression of 31.3 MPa after 28 days from the beginning of hardening, the strength in the water-saturated form of the hardened solution from this mixture is 30.0 MPa (water resistance is about 96%), the adhesion strength to base concrete is 3.5 MPa (failure on concrete base).

Delamination does not exceed 3%. No efflorescence.

Example 4

To produce a batch of decorative mixture, 223 kg of a 13% percent solution of magnesium sulfate, 756 kg of a modified component containing caustic dolomite with a mass fraction of magnesium oxide of 0.18 and metakaolin content in the amount of 48% by weight of the latter are taken, 1 kg of sulfonated naphthalene-formaldehyde resin is added and 20 kg of pigment (ocher). This mixture does not require the introduction of aggregate or filler in connection with the use of caustic dolomite containing a sufficient amount of ground limestone with a particle size of up to 0.1 mm. Metakaolin is a kaolin thermally activated for 60 minutes at a temperature of 500 ° C with a content of 75% of particles less than 10 microns in size. This mixture is both decorative and economical, since cheap caustic dolomite is used as a component containing active magnesium oxide.

The mobility of the freshly prepared mortar is 9.5 cm immersion cone. The compressive strength of the hardened putty is 13.6 MPa after 28 days from the beginning of hardening, the strength in the water-saturated form of the putty is 14.8 MPa (water resistance 109%), the adhesion strength to the base concrete is 1.5 MPa (destruction along the contact boundary). Separation is absent. Salting is not observed either before or after water saturation and drying.

The results of experimental checks, including the above examples, confirm the fact that the presence of metakaolin as a modifying active mineral additive in the composition of magnesia cement ingredients leads to the formation of water-resistant and astringent magnesium aluminosilicates in parallel with the formation of non-water-resistant magnesium oxychlorides or oxysulfates. As a result, an increase in water resistance - an increase in characteristics in a water-saturated form, is not accompanied by a decrease in the characteristics of products in dry form, in particular, the coefficient of water resistance increases from 45-55% to 90-110% with the addition of metakaolin, without a noticeable decrease in the strength of concrete in dry form compared to formulations without metakaolin. This is due to the fact that it is magnesium hydroaluminosilicates that have astringent properties (and magnesium hydrosilicates, which are the products of the reaction of active magnesium oxide with silica and calcium silicates in known technical solutions, only weaken the astringent properties of magnesia cement). Thus, metakaolin is an ideal additive for modifying a component of magnesia cement, since it is almost 100% aluminum silicate, has a uniform composition and structure, and is very active due to purposefully established modes of its production (firing).

The introduction of metakaolin as a modifying additive leads to a significant increase in the sedimentation resistance (resistance to delamination) of modified magnesia concrete and mortar. This is facilitated by the particle size of metakaolin (predominantly less than 10 microns). At the same time, the water holding capacity of modified magnesia concrete and mortar, i.e. decreases the suction of moisture (mixing solution) into the base on which the magnesia composition is applied. In turn, this leads to increased adhesion of magnesia concrete and mortars to absorbent substrates by reducing the weakening of the contact zone as a result of suction of the mixing fluid into the substrate.

Also, metakaolin helps to increase the decorativeness and resistance of mortars and concrete to salt formation, due to the binding of alkali (Na ₂ O, K ₂ O) with metakaolin to water-insoluble zeolite-like compounds, as well as a general increase in the uniformity, water resistance and impermeability of the compositions.

Claims (4)

1. A modified component of magnesia cement, which includes an ingredient containing active magnesium oxide, as well as an aluminosilicate modifying mineral additive, characterized in that as a modifying additive it contains metakaolin, which is a thermally activated kaolin with a predominant particle size of less than 10 microns, in an amount of from 2 to 50 wt.% active magnesium oxide. 2. The component according to claim 1, characterized in that metakaolin is kaolin thermally activated for at least 5 minutes at a temperature of 500-1000 ° C with a content of at least 60% of particles smaller than 10 microns. 3. A component according to any one of claims 1 and 2, characterized in that, as an ingredient containing active magnesium oxide, the component contains caustic magnesite with a mass fraction of active magnesium oxide from 0.7 to 0.95. 4. A component according to any one of claims 1 and 2, characterized in that, as an ingredient containing active magnesium oxide, the component contains caustic dolomite with a mass fraction of active magnesium oxide from 0.15 to 0.23.