Technical characteristics of stones (thermal, frost resistance)

Thermal properties

• Thermal expansion and contraction of rocks

Building stones, like metals, expand due to heat and contract due to cold, but do not return to their original dimensions. As a result of the above factors, that lovely, transparent and beautiful stone will no longer exist, and the hardness, resistance and other useful characteristics of the stone will gradually decrease. If the stones are compactly placed together without a joint, the wall will probably buckle due to the above factors, but if the joint is filled with mortar, it can tolerate some elongation. Even so, it is sometimes recommended to use elastic bands.

• Thermal conductivity

The ability of an object to transfer heat is called thermal conductivity. The coefficient of thermal conductivity is the amount of heat that passes through an object with an area of ​​one square meter and a thickness of one centimeter until the temperature difference at the two ends of the object reaches one degree Celsius. The coefficient of thermal conductivity depends on the amount of porosity, the shape and size of the body’s cavities, the amount of water, the composition of materials and the temperature.

Thermal conductivity is different in different stones. In porous rocks, thermal conductivity is lower than dense rocks. However, stones are often non-conductive and can be considered relatively insulating. . Stones such as pumice, perlite, and vermiculite have a low thermal conductivity and expand due to heat, and are therefore used as thermal insulation.

• Fire resistance

Many building stones are very sensitive to fire and break, especially the combined effect of fire and water greatly weakens them. Rock breakage due to fire may occur due to different stresses in the surface and inner part of the stone or due to initial heating and then sudden cooling by cold water flow during fire fighting. The best way to determine the fire resistance of building stone is to test it at high temperatures. Generally, rocks that are homogeneous in terms of grain size and their minerals formed at high temperature are more resistant to fire. It seems that sandstones have better resistance than granites. If the calcareous stones are heated to a temperature higher than 850 degrees, they will become calcined quicklime; But at lower temperatures, they are less damaged than other stones. Of course, dolomite stones are also damaged at lower temperatures.

In general, as a group of materials, building stones have little resistance to fire, especially if they cool quickly. Compared to baked clay products such as bricks and pottery, stones are in much lower stages in this regard.

Frost resistance

A good building stone should be frost resistant. Glacial rupture occurs due to the absorption of water into the cracks of the rock and its freezing. This issue is due to the change in water volume during freezing, which increases by about 9% of the initial volume, and as a result, an internal stress is created in the rock. In the same conditions, it is expected that a stone with high absorption will break much easier than a stone with low absorption; But this is not always the case; Because many factors affect this issue. Among them, we can mention the dimensions, shape, the way of distribution and the hardness of the stone. A rock with high porosity may have a high percentage of absorption, but it does not break down due to freezing; Because the water comes out quickly. On the other hand, rock with irregular shapes holds water for a longer period of time, which causes a lot of pressure to build up inside it when it freezes. Of course, stone destruction due to freezing is not always due to water absorption from the environment, but it can also be due to mine surface water.

Freezing test provides a measure of stone resistance to extreme weather changes. According to the standard, the saturated samples are placed in the freezing room with a temperature of -15°C for 6 hours, and then they are taken out of the room and placed in water at 20°C for the same period. Each period of freezing and warming is considered a cycle. The stone to be tested, after performing the specified number of periodic periods of freezing and thawing, should not suffer appreciable damage and show signs of apparent damage such as flaking, global cracking or color fading. If the stones suffer from cracks and small and microscopic crushing as a result of repeating the experiment and contraction and expansion, they will lose weight, which is determined by weighing; Therefore, at the end of all five freezing periods, the samples are weighed. To determine the effect of the freezing phenomenon on the rock strength, the rock compressive strength at the end of the test is determined and compared with the control rock compressive strength.

The freezing coefficient is defined as the ratio of the compressive strength after freezing to the compressive strength in water saturation conditions. Materials whose freezing coefficient is more than 0.75 will be resistant to freezing conditions.

When choosing a stone in terms of its resistance to frost, the following issues should be considered:

1. Layered rocks should not be mined in cold weather.

2.  Even if possible, you should choose stones that are known for their resistance to frost.

3. stones with high water absorption should not be used in cold and humid places.

4. Most igneous rocks have very high resistance to freezing.

5. The tests that are conducted to determine the resistance against freezing should be as close as possible to the conditions of the place of use in terms of the number of periods of freezing and thawing and the high and low temperature of the test.

6. When testing to check the resistance of stone against freezing, factors such as the percentage of dry weight loss, the percentage of compressive strength loss, the creation of cracks and the separation of stone grains should be taken into consideration. Another test to estimate the frost resistance of rocks is to saturate them in sodium sulfate solution and then dry them. In this process, a lot of internal tension is created due to the growth of sodium sulfate crystals inside the cracks of the stone. This process can be repeated several times. This test is much more intense than the normal freezing test and the method and interpretation of the results are different.