Asbestos - cement products

PRODUCTS BASED ON MINERAL BINDERS

Products based on mineral binders form the largest group of construction materials. Concrete and reinforced concrete products take a lead place among them. Reinforced concrete products are presented in chapter 14. Un-reinforced concrete is used for a limited range of products which are subjected to relatively low tensile loads.

Cement stone, reinforced with the asbestos fibers is used for asbestos-cement products, which have high compressive and tensile strength, frost-resistance and low water permeability. Asbestos-cement products are heat-resistant, have reduced thermal conductivity. Under the influence of moisture, their strength increases over time and they do not corrode.

Using clinker-free binders like lime-silica binders, which include a variety of raw materials from waste products is effective for a group of building products. Autoclave technology, based on formation of hydrosilicates and other binding compounds in conditions of water vapor impact at a temperature of 175 - 300°C and pressure of 0.8 -3 MPa, opens ample opportunities for using lime-silica binders. The most popular products that are produced using autoclave technology are lime-sand bricks and stones, and other products from dense and porous silicate concretes.

Gypsum products have acquired more importance in modern construction. They are fire-resistant and biostable, chemically neutral, have low thermal conductivity, and low density. They possess an inherent architectural and decorative expressiveness. Ability to change in a wide range of properties of gypsum and concrete mortars allows the use of highly-mechanized and automated methods of product formation such as: casting, pressing, rolling and extrusion.

Asbestos - cement products

Asbestos is a natural mineral which is mined mainly in Russia, Canada and South Africa. Once mined the asbestos rock is crushed producing fibres which can then be woven into fabrics and used to reinforce cement and plastics.

Some asbestos fibers, when inhaled, constitute a health hazard leading to asbestosis, a form of lung cancer. These health risks prompted the establishment of strict environmental regulations with regards to working with asbestos.. Health risks were shown to be greatest during the mining and production processes, but minimal during installation and use of asbestos-cement products. According to the Environmental Protection Agency (EPA), a material containing asbestos is deemed potentially hazardous only in a friable state, which means when it can be crumbled, pulverized, or reduced to a powder by hand pressure. Asbestos-cement is not considered friable, and therefore not hazardous, because the cement binds the asbestos fibers and prevents their release into the air under normal use conditions. However, asbestos-cement products are classified as friable when severe deterioration disturbs the asbestos or when mechanical means are used for chipping, grinding, sawing, or sanding, therefore allowing particles to become airborne.

The most propagated asbestos-cement products are corrugated and flat sheets, pipes and connective sockets. Asbestos-cement products have relatively good processabilty. They are much lighter than the metal and concrete. Average density of asbestos-cement products changes from 1400 to 2100 kg/m³, ultimate compressive strength 40-60 MPa, tensile strength - 8 - 15 MPa, and absorption of water is in the range of 10…30%. The main imperfections of asbestos-cement products are the low shock strength and the ability to buckling.

The source components for the production of asbestos-cement are asbestos, Portland cement and water.

There are three common types of asbestos: Crocidolite (blue asbestos), Amosite (brown asbestos) and Chrysotile (white asbestos) and each have different characteristics. All three forms of asbestos can be used to make asbestos-cement but the most effective is Chrysolite.

Due to the strong linear and very weak transversal ties crystals asbestos has a high tensile strength (300 MPa) and the ability to split into thin fibers.

For asbestos-cement products a special type of Portland cement is used, the properties of which contribute to the filtering process of the solid phase in asbestos-cement suspensions and accelerating of products hardening. Fineness of grinding of the cement is characterized by specific surface area in the range of 2200 - 3200 cm²/g. The content of free CaO in the clinker should not exceed 1%, C₃A - 8%, and C₃S should not be less than 52%. The initial set of cement paste should occur no earlier than 1.5 hours after mixing with water. Cement used for the production of asbestos-cement products, does not usually contain mineral additives.

The industrial manufacture of asbestos-cement products was made possible by the invention of an Austrian engineer, Ludwid Hatschek. The Hatschek machine was a wet transfer roller. It was used to produce the initial asbestos-cement sheets (Fig. 13.1), while two other manufacturing processes - the Mazza process, and the Magnani semi-dry process were used for producing pipes and corrugated sheets respectively. After being formed, most products were steam cured to achieve the optimum microstructure for strength and durability. Asbestos-cement building products have many desirable material characteristics, such as being lightweight, impermeable to water, durable, tough, resistant to rot, termites, soiling, corrosion, warping, and fire, and easy to clean and maintain. Asbestos-cement also possesses low thermal conductivity and is therefore a good electrical insulator.

Processing of asbestos-cement products as the most widespread wet method of production involve: asbestos fission; mixing of fine fibers with the cement and water and formation of suspension; followed by formation of products on the sheet- and pipe-molding machines with subsequent thermal treatment. Molding is the formation from the suspension in meshed cylinder of a thin layer of asbestos-cement, their dehydration and compacting. The resultant products of the fibrous sheet are subsequently subjected to profiling, and pipes - turning. Composition of asbestos-cement mass can be the following: 13-17% asbestos, Portland cement 83-87% (content of asbestos, is higher at the production of pipes than the sheet materials). In some cases, finely powderedquartz sand (30-40%) can be added into the Portland cement. Corrugated sheets are made of the different profiles characterized by height and pitch of corrugations (Table 13.1). Corrugated sheets are manufactured presently usually with length up to 2.5 m, width of 1.15 m and thickness of 7.5 mm. These sheets are assigned for roofs of industrial and agricultural buildings and structures. They are supplied along with the details, which are used for arrangement of walls angles, deformation joints, etc.

Table 13.1

Characteristic of corrugated asbestos-cement sheets and details

*40 mm – height, 150 mm – pitch of corrugation; **54 mm – height, 200 mm – pitch of corrugation.

Roofs from corrugated asbestos-cement sheets are easily constructed and does not require placement of rigid foundation.

The indicator of the profile fabricability is the ratio between the internal radius of the corrugation and the sheet thickness.

Flat sheets may be pressed or not pressed. They normally measure about 3.5 m in length; have a width of 1.5 m, and thickness of 10 mm. Pressing improves the strength properties of products. The bending strength of pressed sheets is not less than 23 MPa, the average density - not less than 1800 kg/m³; whilst non-pressed sheets have average strength and densities of 18 MPa and 1600 kg/m³ respectively.

Flat sheets are intended mainly for construction of prefabricated structures of wall panels, sanitary cabins, as well as for arranging acoustical ceilings, air pits, etc.

Flat sheets are often incorporated into composite products. ‘Transitop’ is a typical composite board consisting of an integrally impregnated insulating board core, faced on both sides with asbestos-cement board. Waterproof adhesives are used to laminate the insulating core as well as to bond the noncombustible asbestos-cement faces to the core. This combination of materials provides for structural strength, high insulation values, and maintenance-free interior and exterior finishes in a single fire-resistant panel.

The painted sheets and the textured finish by the pattern, can be used for the external facing of walls and panels inside buildings. There is a widespread method of asbestos-cement sheets painting with perchlorovinylenamels. Decorative coatings, based on the silicate paints are also effective. Asbestos-cement sheets can be finished with wood veneer.

The asbestos-cement panels are widely used for the erection of the industrial buildings in particular for construction of their roofs. It is permitted to erect the walls and roofs of industrial buildings with dry and normal operating regimes from asbestos-cement panels. Porous gaskets can be used for construction of joints between panels.

The application of asbestos-cement panels for construction of walls and roofs decreases the weight of structures several folds, and reduces the labour-intensiveness of building works, in comparison with the reinforced concrete,

Asbestos-cement pipes are widely used in the construction. Depending on its purpose, they are divided into:- pressure pipes for arrangement of water-supply systems and non-pressure pipes for arrangement of ventilation and sewerage systems, cable laying, etc.

A maximum working pressure of between 6-15 MPa can be used for the construction of pressure conduits, and asbestos-cement pipes.

Different types of clutches in particular with rubber gaskets (Fig. 13.2) can be used for connection of asbestos-cement pipes.

Asbestos-cement clutches provide the containment of the connections between pipes. Pipes and clutches should be straight, cylindrical shape, without cracks. Hydraulic test of pipes and clutches should not reveal any signs of water infiltration on their surfaces.

Asbestos-cement pipes in comparison with the metal pipes are about 3-4 times lighter. Their imperfections include fragility and low acid resistance.

Date: 2015-12-18; view: 980