Glass-cement materials and the glass-fiber plastics

Glass-cement materials. This group of materials are obtained by the application of inorganic binding agents and glass (including basaltic) fibers. The application of both hydraulic binders (Portland cement and its varieties, alumina cement) and also air binders is possible (gypsum, caustic magnesite). The cables and clusters, twisted from thin glass fibers, and grids made of the alkali-free fiberglass serve as the reinforcing materials. The fibers, obtained by drawing from basaltic fusion are also used.

The strength-weight ratio of glass-cement is about 1.5 times higher than the strength-weight ratio of steel, and the density is lower than the density of aluminum alloys which is 1.5-2 times. The relative deformations of glass-cement at the moment of the microscopic cracks forming in the matrix are in 30-60 times higher than in reinforced concrete. The use of glass-cement compositions instead of the reinforced concrete makes it possible to decrease the mass of structures by 8-10 times and to reduce the cement content in 2-4 times.

The properties of fiberglass depend on the chemistry of glass and method of obtaining it. According to the chemistry the glass fibers are divided into three groups: alkali-free (content of alkaline oxides not more than 1-2% throughout the mass), low-alkali (from 2 to 10%) and alkaline (more than 10% of alkaline oxides throughout the mass. The tensile strength of fiberglass is considerably higher than the strength of massive glass, which is explained by its lower heterogeneity, presence of the strengthened surface layer. The observed increase in the strength as the diameter of fibers decreased(Fig. 14.12) is explained by the smaller thermal gradients, which appear during the glass cooling and with the respectively smaller intensity of crack formation.

The average strength of the elementary fiberglass is 3500 MPa. Atmospheric moisture decreases the strength of glass (Fig. 14.13, 14.14). After several months of storage the strength of fiberglass decreases by 10 to 15% in comparison with the initial. To prevent the reduction in the strength of fiber under the action of atmospheric moisture, they are protected by hydrophobic substances during the drawing.

Fiber from alkali-free aluminum-borosilicate glass has the largest strength. Alkaline oxides decrease the strength of fiberglass. Fibers from phosphate and borate glass has the lowest strength.

Glass in the system of Na₂O-MgO-Al₂O₃-SiO₂ in the alkaline environment of the hardening cement stone has relatively high durability if it composition (mol %) is such as Na₂O 0.14-0.3; MgO 10-30; Al₂O₃ 0-15; SiO₂ 50-70. Industrial fibers are produced in the form of the short (up to 60 cm) models of elementary fibers (staple fiber) and continuous fibers with length of hundred and thousands of meters.

An increase of fiberglass strength is achieved by action on its surface for the purpose of the reducing the quantity of micro-flaws by heat treatment.

Staple and continuous fibers are obtained from basaltic fusions just as in the case of glass. Continuous fiber is processed into the roving (plait), woven and non-woven materials. Glass (basaltic) wool, roll materials, mats, linens and others are made of the staple fibers.

Two basic methods of reinforcement of inorganic materials by fibers are used: directed and chaotic. The tentative influence of the method of reinforcement on the strength of fiberglass materials along the fibers is given below:

The tensile strength of glass-cement grows linearly with an increase of the glass content (Fig. 14.15).

On the stress-strain diagram of glass-cement, two sections are observed. The linear section characterizes the joint operation of fibers and matrix. At the point of inflection (a) the matrix undergoes splitting and load is redistributed to the fibers.

There is an experience of the successful application of glass-cement as the decoration layer for different forms of concrete and reinforced concrete products, roof claddings, facing slabs, light-weight facing panels, materials for the construction of oil storages, silos, tubes, pipes, chutes, waterproofing, etc.

A new type of glass-cement is "transparent concrete" , that appeared on the construction market in 2004 (LiTraCon - Kight Transparent Concrete). The matrix of this concrete is optical fiberglass, whose volumetric content is 4-5%. Fibers are arranged parallel to each other and transfer light from one front surface of the block to another. Walls from such blocks become luminescent, different shadows and figures can appear on them. With the use of " transparent concrete " a sensation of easiness”, is created “the airiness " of structures, the work of optical fibers ensures light transmission almost without losses within a distance of 20 m.

Glass-fiber plastics. The plastic masses, reinforced by fiberglass materials, belong to the group of glass-fiber materials. A number of the valuable properties is characteristic of this type of materials: high strength, easiness, low thermal conductance and others. Use of light structures on the basis of glass-fiber plastics makes it possible to decrease the mass of buildings by about 16 times in comparison with brick. Glass-fiber plastics are lighter by 1.5 times than products made of aluminum alloys, substantially exceeding the last in terms of mechanical strength. The light transmission of glass-fiber materials can reach 90% with a thickness of 1.5 mm, and up to 30% in the ultraviolet spectrum. Its thermal conductance of 6-10 times is lower than in ceramics and concrete. Industry makes glass-cloth laminates, sheet glass-fiber materials, glass-fiber materials with oriented arrangement of fibers, roll materials.

Glass-cloth laminates – These are laminate sheet materials, produced by the method of the hot pressing of the widths of fiberglass fabric, impregnated with synthetic binding agent and packed by parallel layers. The fiberglass laminate in the form of sheets with a thickness of between 0.5…7 mm is used for the envelope of sandwiches-panels, elements of three-dimensional structures, the device of electrical switchboards and others.

The basic representatives of glass-fiber materials with the oriented fibers are the glass-fiber anisotropic materials, obtained by placing of elementary glass fibers parallel to each other with the simultaneous putting of binders on them. Obtained by such means glass-fiber veneer is dried and stacked into packages, which are pressed on the hydraulic presses at raised temperatures. The strength of the sheets in the tension achieves 100 MPa. The strength of the anisotropic glass-fiber material in the longitudinal and transverse directions of the sheet depends on the arrangement of glass fibers in the glass-fiber veneer and the method of laying the sheets into the packets and the glassfiber content (Fig. 14.16).

Phenol-formaldehyde, polyester, epoxy and silicon polymers are used usually for preparing the glass-fiber materials as the binding agents. The regulation of the properties of binders is provided by the introduction of fillers.

Date: 2015-12-18; view: 738