Longevity of the natural stone materials

Longevity is the generalizing (complex) construction property of natural stone. It serves as one of the important criteria in selection and evaluation the material, and combines a number of the physical, mechanical and chemical properties, which directly influence on the effectiveness of its use, namely: frost resistance, strength, porosity, water absorption, wearability, corrosion resistance. According to the modern classification the longevity is evaluated by the lifetime of stone (in the years) before the appearance of the first signs of destruction (initiation of destruction) and up to the final destruction. Depending on the value of these indices natural rock materials are classified into four groups of longevity (Table 5.4). According to the indices of longevity the most of the varieties of natural stone relates to the most stable rock materials. The fine-grained quartzites and quartz-like sandstones, and also fine-grained granites, which have been preserved in the earliest monuments of human culture, are especially long-lived.

Table 5.4

Classification of the rocks depending on the longevity

The destruction of stone in the process of operation can be appeared in the different forms: peeling of surface, appearance of cracks, crumbling of the separate particles and fragments of constructions, appearance of plastic deformations.

Climatic action (the so-called physical erosion) is one of the main reasons, which reduce the period of the service of stone: that is displacement and the evaporation of moisture with the dissolved in it substances and sharp temperature variations with the passages through the zero temperature. Negative action on the water-saturated stone has not so much the value of temperature of freezing, as frequency of transition through 0°C, i.e. alternation of the cycles of freezing and thawing. This action on the stone leads to the development of cracks in it, the break of the cross connections between the separate voids, slackening of the connection between the crystals, reduction in the mechanical strength. In this case the basic reason for destruction is the action of the freezing water. The freezing water is enlarged and exerts the strong pressure on the walls of voids. This pressure can be reached up to 210 MPa at a temperature - 20°C. As a result of this process a large mechanical stresses can be acted on the sections of stone in the closed space, which can lead to its destruction.

If the rock has predominantly large voids, which are not connected, the periodic action of minus temperatures leads to the breakage of the walls between the voids, reduction in strength, and sometimes even to the disintegration of the stone. However in the case if rocks have voids, surrounded by the capillaries which being communicated with it, then part of the water during the freezing wrings out into the small voids and capillaries. Such small voids and capillaries are reserve capacities, and have not harmful mechanical influence upon the skeleton of rocks. In that case the stone can be enough long-lived even even at low mechanical strength.

Although the temperature coefficient of linear expansion in natural stone is relatively low, the stone can be destroyed because the crystals of different rock-forming minerals have different coefficients of linear thermal expansion. As a result rocks under the action of sharp temperature drops can be covered with the network of the small cracks, which facilitate further destruction.

Significant negative effect on the longevity of stone have the mechanical factors, such as: impact actions, compressive and bending loads, abrasion, action of abrasive particles.

There are also important chemical factors, which in a number of cases can sharply reduce the period of the service of elements made of the natural stone (so-called chemical weathering). In the case of the insufficient waterproofing of rock constructions the infiltration of ground water into the stone with the subsequent deposit of salts is possible, which leads to the appearance of bloom and saltings, and sometimes to the destruction of insufficiently steadfast stone. The chemical factors, adversely affecting at the longevity of the stone, also include the significant atmospheric content of oxide SО₃, which during the moistening leads to formation of the sulfuric acid or sulfates in the stone, which are expanded during the hydration.

The process of the gradual destruction of rock materials in the structures can be prevented by the different design and chemical methods of protection, which facilitate the reduction in the action of factors enumerated above.

Design methods are evinced by the making of the smooth or polished surfaces of materials, which can not detain rain and melt water and pass aggressive matters inside the rock materials.

The chemical methods of protection provide for the conservation of stone by the sequential impregnation of its upper layer with some substances. The undissolved substances, which clogged the voids and microscopic cracks, are formed as a result of the reaction between these substances. The silicatization, fluating and also modification by polymeric compounds are belonged to such methods.

Silicatization consists in the impregnation of the upper layer of stone with the solution of liquid glass, and then - by the solution of chloride calcium. Formed as a result of reaction silicate of calcium CaO∙nSiO₂ is the practically undissolved compound.

The process of floating consists in impregnation of the limestones and other rocks, which contain calcite, by the water solutions of the fluorosilicate salts. These salts (fluates) can be added into chemical reactions with the soluble components of stone with the formation of fluoric salts of calcium, magnesium and silica, which not dissolved in the water. They condense at the surface of stone and make its inaccessible for the aggressive matters. For instance, silica and fluoric salts are formed at processing of limestone with fluosilicate magnesium:

MgSiF₆ + 2CaCO₃ = MgF₂ + 2CaF₂ + SiO₂ + 2CO₂­.

fluate limestone undissolved salts

Process of fluating can be used for the noncarbonate (acidic) rocks. For this purpose they preliminarily are impregnated with the solution of lime salt, which subsequently forms with the fluate protective layer from the compounds undissolved in the water.

Also it is possible to use of water solutions and emulsions, polymeric substances and water-polymeric dispersions. For instance, silicone resin and also the water solution of urea-formaldehyde resin are used for the hydrophobization of surface and voids of stone. Reliable waterproofing is created by some monomers with their subsequent polymerization by the thermocatalytic processing. This method of modification is most effective for processing of porous rocks.

Combination of design and chemical methods leads to a considerable increase of the longevity of natural rock materials in different structures.

Date: 2015-12-18; view: 1051