Building Stone - A Technical Primer

by Frank R. Melka

Introduction

 

Stone, one of the first building materials used by man, is often taken for granted by any of us in the construction industry, as well as the average lay person. We design, specify, install and use stone because of its superior performance of function and beauty, but we pay little or no attention to the technical aspects of the chemistry and mineral composition of the product. This handbook was developed for the purpose of assisting architects and others involved with the use of building stone to be better informed, and hopefully, to answer questions that may arise.

It is not the intent of this booklet to cover the broad scope of building stone, since the subject can be extensive and complex. Furthermore, no attempt is made to discuss the application, limitations, detailing or aesthetics of stone; each one of these is a salient factor that has been reviewed in many volumes of books written by experts. The desired goal is to fill a need for a single source summary for many of the basic technical facts pertaining to the geological, chemical, and physical properties of building stone, as well as provide a glossary of the related terminology for construction stone products. This handbook is dedicated to the designers, specifiers, installers, and users of building stone.

 

Dave Andersen

 

 

Chapter 1: Rock

Rock is the major solid element, which exists in the earth. Our society has found man uses for rock over the years. Not only are rocks used in construction, but they are also used as fuels, ores, chemicals, and precious gems. Rock is everywhere - under the soil, oceans, and polar caps, as well as that which is exposed to our view. It was all formed thousands of years ago, and because it was naturally formed, rock exists in a totally unique structure and appearance. Once rock is quarried, it is thereafter referred to as stone. The natural incomparable beauty of this material is made apparent by various fabrication techniques.

There are three classes of rock which make up the earth's outer crust:

  1. Igneous

  2. Sedimentary

  3. Metamorphic

Igneous Rock was formed from the solidification of molten matter called magma which originated at high pressures and temperatures within the earth. The texture of igneous rock is determined by the rate of cooling which took place when the magma became a solid. If the cooling was slow, the result was a coarse-grained crystalline structure as present in granite, and if cooling was rapid, a fine-grained non-crystalline structure like glass resulted. The principal minerals in igneous rock are FELDSPAR, QUARTZ, MICA, and sometimes HORNBLENDE.

Sedimentary Rock was formed by the disintegration of other rocks and soils, along with the mineral remains of sea organisms which were deposited in seas, lakes, valleys, and deltas in reactively even thick beds of strata. A major characteristic of most sedimentary rocks is the layering or stratification present in their formation. However, a sedimentary rock like limestone can exhibit very little stratification and have a uniform structure in all directions.  Sedimentary rocks are formed chiefly from the minerals QUARTZ, KAOLINITE, CALCITE, and DOLOMITE.

Metamorphic rock was originally sedimentary or igneous rock, but the physical and/or chemical composition was changed through natural forces such as heat, pressure, and hydrothermal solutions which permeated the rock.  Since all kinds of rocks may be subjected to metamorphic action, there is a wide range of variation in the mineral composition of metamorphic rocks and there is no definitive distinction between many types of rock in this classification.  Generally they exist in a highly crystalline form, having a foliated or laminated structure similar to the stratified structure of sedimentary rocks.

 

 

Chapter 2: Building Stone

Quarried pieces of rock are called stone. The most common Building or Construction Stone products are:

 

Igneous Stones

Granite is a coarsely crystalline unstratified igneous rock composed of alkali feldspars, quartz, mica and hornblende. Since granite is highly speckled, it varies greatly in color depending on the range of the minerals present and the location of the quarry. When the mica in granite is white, it is known as muscovite, when it is dark it is biolite. Quartz and feldspar are always present in granite. In certain types of granite, hornblende is an important ingredient and it may be even more abundant than the micas. There are some granites in which mica is totally absent.

 

Sedimentary Stones

Limestone is a monomineralic sedimentary rock consisting predominantly of calcium carbonate, but magnesia, silica, alumina, and iron oxide are also present in varying amounts. Some limestones may show evidence of fossils, but most limestone will not show any trace of their origin due to the fine grinding to which the marine organisms were subjected after deposition. Color will range from gray to buff. In the building industry, limestones which can be polished are commonly classified as marbles.

Sandstone is a sedimentary rock which was formed by the consolidation of beds of sand deposited by water carrying sand in suspension. The cementing material of this rock may be clay, calcium carbonate, iron oxide, or silica. The strongest and most durable sandstones are those in which silica is the cementing material, but the degree of cementation can also be a factor. Pure sandstone is silicon dioxide and white in color. Various shades of yellow, brown and red sandstone are due to the presence of different iron oxides.

Brownstone is a dark-brown or reddish-brown sandstone, and the color is determined by the high content of iron oxides. This product was once a very commonly quarried stone in the northwestern United States, but supply is now limited or almost non-existent in this area.

Conglomerate is similar to sandstone, but in conglomerates the cemented particles of the rock are gravel rather than sand. Pudding Stone, with well rounded particles, and Breccia, with angular particles, are two examples of conglomerate rock.

 

Metamorphic Stones

Marble is a metamorphic rock derived from limestone, and composed chiefly of calcium carbonate in compact crystalline or crystalline-granular state. Limestone, however, in a very compact structure, or showing only traces of a crystalline structure, may be called marble if it is capable of taking a polish or if it is suitable or desirable for ornamental and decorative purposes. Building marble is classified into four types:

  1. Calcite - Stone composed of up to 5% magnesium carbonate.

  2. Dolomite - stone composed of greater than 40% magnesium carbonate.

  3. Serpentine - stone composed of hydrous magnesium silicate, usually a dark green color.

  4. Travertine - stone composed of crystalline calcium carbonate in either an aragonite or calcite formation.

Slate is a fine-grained metamorphic rock structured with tiny crystals of mica-like minerals and was derived from clays and shales. Slate is a foliated stone which exhibits excellent rock cleavage for splitting easily into thin, smooth slabs.

Bluestone is a dense, fine- to medium-grained metamorphic sandstone. It splits readily into thin slabs, but is very strong and durable.

Gneiss is a common metamorphic rock which is a coarse-grained laminated or foliated structure with a mineral composition similar to granite. Generally, gneiss is referred to as "Trade Granite".

Quarizitic Sandstone is a metamorphic sandstone similar to quartzite, but not as hard because it is in an intermediate state between a sandstone and a quartzite. the sedimentary structure of cross-bedding is preserved, thereby gibing the stone a banded appearance.

Quartzite is a very hard, durable, crystalline metamorphic rock formed from a compact granular rock composed of quartz sandstone. The fusing of the quartz grains is so complete that when broken, quartzite will split across these grains rather than between them. Usually quartzite is pink to red and occasionally gray, but pure quartzite is white.

 

 

Chapter 3: Rock Chemistry

 

The three chemical classes of rocks

Argillaceous rocks are composed primarily of Alumina (Al2O3)which is the chief component of clay.

Siliceous Rocks ate composed primarily of Silicon Dioxide (SiO2) which is the principal ingredient of quartz sand.

Calcareous Rocks are composed primarily of Calcium Carbonate or lime (CaCO3).

 

Common minerals in stone and the elements/compounds they contain:

Calcite - Calcium Carbonate

Dolomite - Calcium Magnesium Carbonate

Feldspars - Potassium, Sodium,, or Calcium Aluminum Silicates

Hornblende - Calcium Magnesium Silicate

Kaolinite - Hydrous Aluminum Silicate

Micas - Complex Hydrous Silicates of Aluminum with other elements as Calcium, Potassium, Magnesium, Sodium, Lithium, and Iron

Quartz - Silicon Dioxide

On the basis of chemical composition, rocks may be either acidic or basic. When calcium, iron and/or magnesium oxides are present in heavy proportions, the rock is basic, whereas if silica is plentiful, the rock is acidic. Limestone and Marble are basic: Granite, Sandstone, and Bluestone are acidic.

 

Chapter 4: Physical Characteristics of Stone

Values indicated are for an average comparison only, and the specific quarry involved should be consulted for accurate data.

  
 

% Absorption by Weight
Density PCF
Compressive Strength PSI
Modulus of Rupture PSI
Abrasion Resistance
   
Granite
.30
170
24,000
2,000
20
   
Marble
       
 
   
Calcite
.75
162
7,500
1,000
10
   
Dolomite
.75
175
7,500
1,000
10
   
Serpentine
.75
168
7,500
1,000
10
   
Travertine
.75
144
7,500
1,000
10
   
Limestone
       
 
   
I Low Density
12.0
110
1,800
400
10
   
II Med. Density
7.5
135
4,000
500
10
   
III High Density
3.0
160
8,000
1,000
10
   
Sandstone
       
 
   
I Sandstone
20.0
140
2,000
300
8
   
II Quartzitic Sandstone
3.0
150
10,000
1,000
8
   
Quartzite
1.0
160
20,000
2,000
8
   
Slate
       
 
   
I Exterior
.25
175
10,000
7,200
8
   
II Interior
.45
175
10,000
7,200
8
   
   

Chapter 5: Common Stone Finishes and Faces

 

The Selection of surface finish or face is a function of the type, grade, and/or geological structure of the stone under consideration.

 

Chat Sawed Finish:* Plane surface similar to the appearance of a sand blasted finish, but will sometimes show parallel saw marks.

 

Fine Rubbed or Smooth Finish: Smooth, free of scratches, and without sheen.

 

Honed Finish: Smooth, dull sheen without reflections or gloss.

 

Machine Tooled:

8 Cut Finish: Plane surface of interrupted, parallel, and concave grooves ranging from 1/32" to 1/16" deep, spaced not over 3/32" apart.

6 Cut Finish: Same as 8 cut finish except grooves are not more than 1/8" apart.

4 Cut Finish: Same as 8 cut finish except grooves are not more than 7/32" apart.

Natural Cleft Face: A natural split or cleaved surface, moderately rough, with some texture variation.

 

Plucked Finish:* Plane surface obtained by rough planing of the stone which causes the small particles to be broken or plucked out.

 

Polished Finish: Glossy mirror surface with sharp reflections.

 

Quarry Face: Surface of the stone as it comes from the quarrying operations or due to naturally formed seams. Seam faced stones area often highly colored by mineral deposits which have penetrated into the seams through the flow of water.

 

Rock or Pitch Face: The surface is one which is formed with a pitching chisel, but the perimeter edges are straight and lie in a single plane.

 

Rubbed Finish: Similar to a fine rubbed finish, but could contain occasional scratches.

 

Sand Blasted Finish: Plane surface with coarse, irregular finish produced by blasting with an abrasive. Coarseness will vary depending on the abrasive used and the grain structure.

 

Sawn Finish: Relatively plane surface with scouring texture consistent with the saw used. Gang saws produce parallel marks, and circular or rotary saws produce circular marks.

 

Shot Ground Finish:* Plane surface with pronounced circular markings that have no regular pattern.

 

Shot Sawed Finish:* Plane surface with coarse, uneven, ripped, and irregular parallel grooves.

 

Split Face: Surfaces formed by mechanically splitting the stone.

 

Thermal Finish: Plane surface with flame treatment applied in a uniform manner. Coarseness of surface will vary depending on the grain structure.

 

*These are listed only for historical reference, since they are no longer used in the industry due to advances in cutting machinery.

 
   

Glossary of Terms Related to the Building Stone Industry

A

Agglomerate - A man-made product fabricated to look like quarried stone. Usually composed of stone chips or fragments embedded in a matrix of mortar or thermosetting resins.

Alabaster - A mane given to a stalagmitic or stalactitic calcium carbonate and a calcium sulfate (gypsum). The calcium carbonate variety is a translucent stone of yellowish milky color, while the calcium sulfate has a fine granular quality. The latter was abundant in Tuscany and Piedmont, and when carved into statues and vases is frequently sold a "Florentine Marble".

Alumina - Aluminum oxide (Al2O3)

Anhydrite - A specific calcined gypsum rock whose chemical composition is anhydrous calcium sulfate (CaSO4). It is used in Portland Cement to control set.

Anhydrous - a hydrate which has given up all of its previously held water molecules.

Aragonite - a translucent white mineral found in calcium carbonate.

Argillaceous - Clay-bearing rock.

Arris - The edge of an external angle formed by the intersection of two surfaces.

Artificial Marble - Man-made marble. See Agglomerate.

Ashlar - Accurately shaped, rectangular-cut stone unit also referred to as "cut stone".

B

Basalt - A fine-grained, dense, igneous rock, but unlike granites contains little or no quartz or feldspars. The colors of basalts are very dark green to black and often sold as granites.

Bed Joint - A horizontal layer of mortar on which building units are laid.

Biotite - A dark, iron and magnesium-rich mica found in granite.

Bluestone - A blue to bluish-green argillaceous metamorphic sandstone of even texture and bedding, historically quarried neat the Hudson River.

Breccia - A name applied to both granite and conglomerate stone because of the coarse and angular particles present.

Brick - A man-made solid or hollow building unit which is made from clay or shale.

Brownstone - a Dark-brown or reddish-brown sandstone. A sedimentary rock.

Bullnose - A rounded outside corner or edge.

Butt Joint - An external corner formed with the meeting of two square-edged stones, either one overlapping the other.

C

Calcareous - Calcium-bearing rock.

Calcine - The removal of water molecules in hydrates by heat.

Calcite - The natural mineral form of calcium carbonate (CaCO3).

Calcite Marble - A crystalline variety of marble or limestone containing not more than 5% magnesium carbonate.

Calcium Oxide - Calcined limestone (CaO).

Carbonate - A salt of carbonic acid.

Carbonic Acid - A weak acid (H2CO3).

Cast Stone - A man-made stone. See Agglomerate.

Caulking - The application of a sealant in a joint or opening to prevent the passage of water, air, dust and noise.

Cement - A hydraulic mixture, without aggregate, consisting of a calcined mixture of clay and pulverized limestone.

Concrete - A composite material consisting of Portland Cement, aggregate, and water: when mixed together, will result in a chemical action that will set and harden into a rock-like mass.

Conglomerate - A stone similar to sandstone but the rock particles are rounded or angular gravel rather than sand.

Coping - A cap or covering course on top of a masonry wall. Designed to shed water, protect the top, and provide a finished, closed appearance to the wall.

Course - A continuous layer of building units running horizontally in a wall.

Cut Stone - See Ashlar.

D

Dimensional Stone - Natural building stone that has been selected, trimmed or cut to specific shaper and/or sizes.

Dolomite - a calcium magnesium carbonate [CaMg(CO3)2]Dolomite Marble - A crystalline variety of marble or limestone containing more than 40% magnesium carbonate.

Draft - The tooled strip or border of a faced stone, also known as a margin.

Drip - A slot cut into the bottom of a projected stone to interrupt the capillary action of water.

E

Efflorescence - A white powdery deposit of soluble salts of calcium, potassium, and sodium which forms on the surfaces of masonry. Efflorescence is caused by the release of excess "water of crystallization" and/or moisture penetration, thereby, causing the leaching of these salts to the surface when the water evaporates.

F

Face - The exposed surface of a stone with its specified finish.

Feldspar - Any of a group of crystalline minerals, all silicates of aluminum with either potassium, sodium, calcium, or barium. An essential constituent of nearly all crystalline rocks.

Fieldstone - An uncut, well-rounded stone found on the surface or in the soil of the earth.

Flagstone - A stone used for outdoor paving, fabricated from natural thin rock, or split from rock which cleaves easily.

Fleuri Cut - Unique to the marble industry, it is a cut parallel to the natural bedding plane, See Vein Cut.

Foliation - The layered or banded structure of rock in which the mineral grains became re-aligned at right angles to a directional force that existed during metamorphism.

Full Bed Mortar - The troweling of both the setting bed mortar and pointing mortar in one operation. See. Setting Bed and Pointing.

G

Gneiss - A metamorphic rock with a banded or coarsely foliated structure; often called "Trade Granite".

Granite - An unstratified igneous rock composed of coarse grains or crystals of quartz, feldspar, mica and sometimes hornblende.

Gravel - Composed chiefly of quartz but may contain granite, limestone, basalt and other rocks.

Grout - Mortar with a high water ratio used to fill open joints between installed stone or building units. Normally, grout is worked into these joints with a hard rubber float.

Gypsum - A hydrated calcium sulfate (CaSO42H20). It is formed naturally as the result of the reaction of sulfuric acid produced by the decomposition of pyrite upon the calcium carbonate of shells existing in clay. A sedimentary rock.

H

Halite - Rock salt; Sodium chloride (NaCl). A sedimentary rock.

Hemihydrate - A hydrate which contains one-half of a molecule of water compared to one molecule of the principal element or compound forming the hydrate.

Hornblende - A group of mineral including calcium, iron, magnesium, and aluminum silicates.

Hydrate - A mineral formed by the combination of water and some other elements or compounds.

Hydrous - Containing chemically combined water.

I

Igneous Rock - One of the three principal groups of rock that make up the earth's surface; formed by the solidification of molten matter. See Sedimentary Rock and Metamorphic Rock.

J

K

Kaolinite - A hydrous aluminum silicate mineral.

L

Limestone - A sedimentary rock composed predominantly of calcium carbonate. Limestone was formed by the decomposition of rock, soil, plant and animal life that was deposited at the bottom of lakes or seas. Any limestone may be called a marble if it is capable of taking a polish.

M

Marble - A metamorphic limestone in a more or less crystalline or crystalline-granular state. Any limestone capable of taking a polish can also be called a marble.

Margin - See Draft.

Masonry - A built-up combination or construction of building units or materials of clay, shale, concrete, glass, gypsum, stone or other similar products bonded together with mortar.

Metamorphic Rock - One of the three classes of rock making up the earth's surface; formed from igneous and sedimentary rocks that were changed by heat, pressure, and hydrothermal solutions. See igneous Rock and Sedimentary Rock.

Mica - Any of a group of mineral silicates in a multi-layered form.

Mortar - A plastic mixture of cement, water, and fine aggregates which combine together through a chemical process of crystallization to form a hardened solid that bonds building units together. See Water of Crystallization.

Mortar Bed - A troweled layer of mortar, in a plastic state, in which building units will be set.

Muscovite - A white, aluminum-rich mica found in granite.

N

Natural Bed - The horizontal stratification of rock as it was formed in the earth.

O

Onyx - A variety of quartz in crystalline form of calcium carbonate. It is characterized by a structure of parallel bands each differing in color or in the degree of translucency. Considered to be a marble because it can take a polish.

Oolitic Limestone - A limestone formed in the sea, consisting of spherical or sub-spherical particles of calcite-coated grains of sand.

P

Paver - A block or chunk of stone or brick used for paving.

Plaster of Paris - a calcined gypsum in a hemihydrate state (CaSO41/2H2O).

Pointing - the troweling of mortar into a building unit joint after the setting bed mortar has hardened. See Full Bed Mortar and Setting Bed Mortar.

Portland Cement - A hydraulic cement product obtained by pulverizing and calcining a properly proportioned mixture of three minerals - lime, silica, and alumina.

Pyrite - The natural sulfides of certain metals. The most common is iron pyrite, which is iron disulfide (FeS2), a brittle mineral that is a brassy yellow in color with greenish-black streaks.

Q

Quarry - The location of a mining operation where a natural deposit of rock is extracted from the earth as stone through an open pit or an underground mine.

Quartz - Silicon dioxide (Si02) occur