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Introduction and selection of stones

Parameters considered in selection of a good construction stone

Being cheap, hard, durable and naturally good looking, stones are often used in construction but keeping in view the variable properties of stones of different types, there must be some criteria for the selection of stones for construction. The criteria is based upon the following parameters.

1. Chemical composition of stone:
2. Strongness and hardness:
3. Durability:
4. Resistance to fire:
5. Bio-Deterioration:
6. Appearance:
7. Susceptibility to being quarried in large sizes:

1. Chemical composition of stones:

Using/selecting a stone for construction, its chemical properties and composition must be tested and verified because different elements and compounds in stones have different properties. For instance, Magnesium in Limestone causes it to be more stronger and is called Dolomite. Feldspar, in large quantities in stone is a source of weakness because CO₂ dissolves Potassium, Sodium, and even Calcium in the Feldspar leaving pure white clay behind.

Presence of Mica, even less than 2-3% makes stone unsuitable for building purposes. Stones with silicates as cementing materials are resistant to weathering.

2. Strongness and hardness:

The more compact grained and heavier a stone, the more stronger it is. A crystalline stone is superior to a non-crystalline texture. The specific gravity of good stone should be above 2.7.

Stones used for road metal, paving blocks, floor slabs and railway ballast have to withstand mainly abrasion or wear and tear. Stone wall subjected to vibrations of machinery and moving loads should necessarily possess toughness. Strongness and hardness itself depend on some factors:

Factors affecting strength, hardness and toughness

1. Hardness or softness of the components
2. Proportions of the hard and soft minerals
3. Size and shape of the minerals
4. Cohesion
5. Porosity
6. Density
7. Cementing material

a) Hardness or softness of the components:

The composition of the compounds determines its hardness or softness. Stones containing Si, Na, K are poor while that containing Mg, Ca, and Fe are good, as they are harder. If the stone is composed of soft and unhardened materials it will result in a soft materials and vice versa.

b) Proportion of hard and soft materials:

The amount of soft and hard material in a specific sample of stone also matters. Greater the amount of hard materials more will be the resistence to weathering.

c) Size and shape of the minerals is stones:

Crystalline solids are hard and compact, thus superioir to non-crystalline. Finer the crystals, stronger the stones and vice versa, This property i.e fineness reduces the pores in the stone.

d) Cohesion:

It is the property of atoms or particles to attract each other. The fine grains have more cohesive power than the coarser grains. Greater the cohesion in stone causes increase in the hardness, strongness and toughness of stones. The property of compactness also depends deeply on cohesion.

e) Porosity:

Stones in wet conditions and having pores in them allow a lower crushing strength than normal. Porosity can reduce the strength upto 30 - 40% e.g limestone and sandstone are affected by this property. Porosity is the property of a substance in which it contains pores i it. It also reduces the resistence to a concentrated (point) load.

f) Density:

If a stone is compact, dense, it would also be non-porous and strong , thus toughness also depends upon density.

g) Cementing material:

Stones with silicates as cementing material will be resistant to weathering than those with calcareous or ferruginous binding material. So, cementing material also affects the choice of stone selection.

3) Resistence to heat:

Resistence to heat means that the stone must have a very low amount of expansion due to large increase in temperature. Silicious materials are good at areas where resistence to fire is required.

4) Bio-deterioration:

Certain trees and creepers thrust theri roots in the joints of stones and have both mechanical and chemical adverse effects. Special microbes can grow on the surface and in minute fissures, their by-products cause flaking and discoloration.

5) Appearance:

The aesthetic aspect that is color, appearance and show of stones must also be considered when being used in a project. Appearance depends on the color and the ease with which the stone can be dressed, rubbed or polished.

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