The rapid expansion of the diversity of stone in our time does not allow builders to accumulate experience in its use under various climatic and operational conditions. In such circumstances, assessing the quality of the stone and its suitability for stone cladding can be determined by the external inspection of an independent qualified specialist using non-destructive testing instruments.
More accurately and reliably, the suitability of stone for use in stone cladding under certain conditions can only be established based on laboratory studies of its physical and mechanical properties. It is also important to consider the integrity of the supplier as a whole. It is no secret that laboratory tests characterize, for example, Jurassic marbled limestone as a finishing material that withstands up to 250 frost resistance cycles when directly tested in fresh water. However, it should always be remembered that the selection of material for a specific project is entirely the responsibility of the supplier.
🔍Here is an example of the plinth of two public buildings. On the left, it is deteriorating, while on the right, it is in operational condition and will serve for many more decades.
GOST 9479-2011 "Blocks made of rocks for the production of facing, architectural-construction, memorial and other products" provides for the definition of 10 mandatory parameters, which include: average density, water absorption, compressive strength in dry condition, reduction of compressive strength after water saturation, abrasiveness, resistance to impact effects, atmospheric resistance (acid resistance, salt resistance), specific effective activity of natural radionuclides, decorativeness, presence of harmful impurities.
At the same time, for different groups of rocks, Table 3 of this GOST establishes different tolerances. Marble and marbleized limestone fall into one group of medium-strength rocks, for which the average density is set at no less than 2600 t/m3, water absorption is no more than 0.75%, the compressive strength in a dry state is at least 50 MPa, and the reduction in strength upon water saturation is no more than 30%.
But it should be noted that marble in its primary state was limestone. The layers of accumulated limestone, when the blocks of the Earth's crust moved, either rose upwards, in which case they were destroyed in the process of geological weathering, or sank deeper. The immersion could reach several kilometers or more. As is known, with depth, the temperature increases, on average by 10°C for every 100 meters.
Under the influence of high temperatures, immense pressure, and hydrothermal solutions, structural changes occur in limestone. Calcite [CaCO3], which is in a cryptocrystalline state (grain size less than 0.001 mm), recrystallizes, its grain size increases, and the grains fill the gaps between organic fragments, filling large organic debris. The nature of the stone's porosity has a huge impact on its physical and mechanical properties.
The tiniest pores, abundant in limestone, can have different properties regarding moisture and air depending on their origin and formation conditions. Some pores can be completely filled with water, while others have an isolated nature and are not filled with water. There are pores that are partially filled with moisture during water saturation, while part of the chamber continues to contain air. Some pores have the appearance of narrow channels, in which water does not freeze even at significant negative temperatures.
The quantitative ratio of different types of pores ensures the limestone's resistance to frost, its frost resistance. As is known, water expands when it freezes, breaking the stone. The more pores with non-freezing water and pores with air pockets there are in the stone, the more frost-resistant the stone is, since the voids not filled with water act as a damper, softening the destructive effect of freezing water.
For limestone, GOST 9479 provides a group of low-strength rocks where these rocks are divided into dense limestones and porous limestones. For limestones in this group, the average density and water absorption are not standardized. Not because low density and high water absorption are good, but because for this group of rocks such parameters are individual and have a large range of values. The relationship between the quality of limestone and its density and water absorption does not adhere to general rules; this relationship for this type of stone is strictly individual. The compressive strength limit for porous limestones is set at 10 MPa, and for dense limestones, it is 25 MPa, respectively. The requirements for reducing strength under water saturation for porous and dense limestones are the same and set at 35%.
Jurassic limestone is classified as limestone with low to medium degrees of marbleization. Depending on the layer, it provides different average values for the characteristics specified in GOST standards. It is very important to know that only layers 10-11; 14-17, and 21-25 of Jurassic stone are used for exterior finishing. With professional selection and grading of the stone, taking into account the achieved frost resistance of 250 cycles, the stone will not deteriorate throughout the entire projected service life of the building.
🔍The wall of the British Embassy in Moscow on Smolenskaya Embankment is made of Jurassic limestone from JuraLimestone GmbH. The lifespan of the stone is over 20 years.
During the process of recrystallization, the primary porosity characteristic of limestone decreases, and with complete recrystallization (marbleization) of the rock, it is completely destroyed. In marble, a different type of pores appears – these are the pores of the intergranular space. The relationship of such porosity with the quality of the stone has a different dependence; this dependence is of a general nature, therefore, requirements for density and water absorption have been introduced for marble.
Recrystallization of limestone alters other parameters of the physical and mechanical properties of such rocks. The faces of the growing carbonate grains meet each other, begin to grow into one another, and bend. In the absence of pores, this leads to a significant increase in the strength properties of the stone, primarily to an increase in the values of the compressive strength in the dry state and flexural strength.
We examined the two extreme members of the limestone-marble series. The intermediate link of this series is called marbled limestone. In this case, GOST 9479 does not take into account the degree of recrystallization (marblization). In nature, there is a vast variety of rocks in natural deposits that occupy the intermediate link between limestone and marble with varying degrees of recrystallization, from the first percentages to 80-90. As our research has shown, depending on the degree of recrystallization (marblization), the indicators of the physical and mechanical properties of such rocks can differ dramatically from primary limestones and marbles.
Thus, with an average density below the standard requirements, the compressive strength of some varieties of Jurassic limestones (JuraLimestone Jura Gold) can reach 181.7 MPa, which is higher than the strength of granite, with a density of 2700 kg/m3. This is due to the unique combination of grains of recrystallized limestone with other stone structures. For the marbled limestone JuraLimestone Jura Travertin from the quarry in Petersbuch, the compressive strength reaches 98.72 MPa (for gabbro, diabase, basalt, the norm according to GOST is 70 MPa), with a density of 2380 kg/m3 and water absorption of 2.54% (GOST requirements are 0.75%). At the same time, the frost resistance of the stone was more than 150 cycles of alternating freezing. According to petrographic studies, the stone belongs to marbled limestones of not high marbleization degree.
🔍In this regard, when assessing the quality of marbled limestones, it is necessary to determine the degree of their marbling (recrystallization) under a microscope, and for values of this parameter below 50%, the requirements for average density and water absorption for such stone are not regulated.
Therefore, when choosing stone for use in building finishes and interiors, especially marbled limestone, it is necessary to conduct laboratory studies, identify all possible negative factors affecting quality, and pay special attention to petrographic studies with a mandatory determination of the degree of its recrystallization (marblization).
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