Properties of Natural Stone for Facade Cladding

If you have decided to use only natural materials to finish your own house, it is worth knowing a little more about them. Natural stone is a thing of status and beauty, but it is rather demanding in its conditions of use and expensive. In this issue, we invite our readers to get acquainted with the aspects that must be kept in mind when choosing stone for finishing.

What makes a stone natural?

Stone is a rock, made up of minerals and their combinations. Natural stone is a durable, strong, wear-resistant material that holds up to external conditions. It differs from artificial stone in its origin, composition, environmental friendliness, and certain other qualities.

Origin

You don't need to memorize how one rock or another came to be, but you can greatly simplify your choice by knowing that, by origin, all rocks are divided into primary (igneous), secondary (sedimentary), and metamorphic (altered). These general categories define the groups of stones suited to facades or interiors, because their origin determines many of their properties.

Igneous rocks came directly from molten magma. Granites, syenites, gabbros, and labradorites are the result of magma cooling under high pressure within the Earth's crust; they are called deep or intrusive igneous rocks and are characterized by their density and fully crystalline structure. Extrusive rocks — those formed at low temperature and pressure on the Earth's surface — usually have a fine crystalline structure and high porosity. These are basalts, porphyries, volcanic tuffs, ashes, and pumice.

Sedimentary rocks are the result of the breakdown of primary rocks, of the drying up of bays, seas, and lakes, and of the decay of plant and animal remains. Common characteristics of these rocks are porosity, jointing, and solubility in water. This is how travertine and dolomite, used in finishing work, came to be. Organic sedimentary rocks include limestone. The clastic sedimentary rocks also widely used in finishing include sandstones, breccias, and conglomerates.

Metamorphic rocks are the result of the transformation (metamorphosis) of igneous and sedimentary rocks into a new kind of stone under the influence of pressure, high temperature, and chemical processes. These include crystalline marble and quartzite, as well as the slates gneiss and schist.

Properties of natural stone

Whatever a stone's origin, specialists will evaluate it against a certain set of criteria. They should be taken into account so that you are not disappointed over time, because stone cladding is for the long term.

The decorative effect of stone

The aesthetic appeal of natural stone, primarily its color and pattern. Even the most "unremarkable" stone can be made more decorative through machining.

Strength of stone

The property that gives the material its durability. Depending on the hardness of the minerals that make up their composition, stones are divided into three groups:

hard — granite, gabbro, quartzite, quartz sandstone;
of medium strength — marble, travertine, limestone;
low-strength — limestones, tuffs.

Abrasion resistance of stone

Wear of a stone surface at a foot-traffic intensity of 1 million people per year.

Groups of stones by abrasion resistance
Quartzites and granite-group rocks — less than 0.12 mm
Basalts, microcrystalline marble — 0.12-0.35 mm
Soft basalts, marble, sandstones, dolomites — 0.35-0.6 mm
Marbleized limestone, travertine, limestone, tuffs — 0.6-1.5 mm
Loose limestones — 1.5-2.5 mm

Density of stone

Depends on the rock's porosity and the minerals it contains. By density, stones are divided into two categories: light (up to 2200 kg/m³) and heavy (more than 2200 kg/m³); this also determines the weight of a piece — the higher the density, the heavier it is.

Porosity of stone

The presence in a rock of pores, cracks, cavities, and other voids, which directly affect its water absorption and its salt and acid resistance — and thus its durability — as well as its strength, thermal conductivity, polishability, and workability. As total porosity increases, the strength and weight of the stone decrease, but its workability improves.

Polishability of stone

The maximum luster a material can take is also a property that depends on its porosity. Every rock and material has a luster limit, beyond which the quality of the polished surface no longer improves. The luster limit of glass is taken as the standard, at 200 units.

Excellent (170-200 units) — fully crystalline marble, fine-grained granite, quartzite;
good (140-170 units) — granites, marbleized limestones;
medium (70-140 units) — basalts, limestones, dolomites;
poor (less than 70 units) — tuffs, loose limestone.

Water absorption of stone

Water absorption also depends on the porosity of the stone: the more natural voids and cracks there are, the more water seeps into the material. When water freezes, it expands and exerts strong pressure within the pores. In addition, after evaporating it leaves behind concentrated salt solutions from which crystals grow, creating strong crystallization pressure. At low porosity and high water absorption, this pressure creates cracks in the stone. A highly porous rock distributes the crystallization pressure evenly, and no new cracks form (limestone is a perfect example). A stone's acid, salt, and frost resistance depends on its water absorption and mineral composition.

Acid resistance of stone

The tendency of rocks and materials to react with various acids that can break down or transform the rock. For example, marble reacts to all acids, including food acids (citric and acetic); travertines, limestone, and dolomites, like marble, are destroyed by hydrochloric acid (it is not found free in nature, but the risk increases significantly in cities where chlorides are used for snow removal). Limestone and travertine, however, have high salt resistance, which means they are good for finishing plinths that interact with surface waters carrying salts. Granite also has this property, but salt movement can then run along the joints of the masonry.

Frost resistance of stone

The ability of water-saturated stone to withstand repeated cycles of freezing and thawing without showing signs of damage or significant loss of strength.

Water absorption and salt, acid, and frost resistance are the qualities that determine the performance of natural stone finishes. These are the first things to assess when choosing a material for exterior cladding.