Natural Stone Classification

Stone Classification

Stones have been used in building construction since thousands of years. Its qualities like durability, strength, hardness, etc. has made it one of the effective materials for construction purposes.

Stones are derived from rocks which are classified as:-

1. Geological origin: Sedimentary, Igneous and Metamorphic
2. Chemical composition: Siliceous, Argillaceous and Calcareous
3. Physical form: stratified, unstratified and foliated

Geological classification of Rocks

Sedimentary Rocks 

• They are formed as the sediments deposited over a period of time mostly at the bottom of sea and oceans. They include minerals and remains of plants and animals. E.g. Limestone, Sandstone

Metamorphic Rocks

• They are formed when the already existing rocks undergo changes due to intensive heat and    pressure. E.g. Marble, Slate

 

Igneous Rocks

• These stones are formed when the molten magma from the earth cools inside the earth or on the earth surface and solidifies. E.g. Granite

 

 Physical Properties Classification

 

Chemical classification of Rocks

 

Siliceous Rocks 

• They are sedimentary rocks which mainly contain silica. They are hard and durable. E.g. Flint, Chert

Argillaceous Rocks

• They are sedimentary rocks with the main constituent as argil i.e., clay. These stones arehard and durable but they are brittle. They cannot withstand shock. E.g. Mudstone, Claystone, Slates and laterites.

• Calcareous Rocks

• They basically are sedimentary rocks with main constituent as calcium carbonate. Limestone is acalcareous rock of sedimentary origin while marble is a calcareous rock of metamorphic origin.

Physical classification of Stones

Stratified 

• Stratified rocks show a layered structure and can be easily split up into slabs. Sedimentary rocks are stratified rocks.E.g. sandstone, limestone, slate, etc.

Unstratified

• They cannot be easily split into thin layers. Igneous rocks are unstratified rocks. E.g. Granite, trap, marble, etc.
• Metamorphic rocks may be either stratified or unstratified

Components of Rocks: Crystals and Grains

Crystals

• They are individual minerals that have grown in place to form a rock. In igneous rocks they are minerals that crystallize from the solidifying magma/lava. In metamorphic rocks, they are minerals that grow within the solid rock through chemical reactions between neighbouring minerals. Thus identifying crystals in a rock means that the rock must be either igneous or metamorphic.

 Grains

• They are pieces of pre-existing rock or organic material (e.g., shells) that have been cemented together to form a sedimentary rock. Thus identifying grains in a rock means that the rock must be sedimentary.

 The list of rocks suitable for building stone is summarized in the table below :-

Rock Family	Rock Name	Common Colors	Common Features
Sedimentary	Sandstone	Reddish Brown, Beige, White, Grey	Grainy; composed of rounded grains up to 2 mm in diameter. “Sandy” feel. Bedding is common.
	Limestone	Grey, Cream, Tan, Pink	Grainy. Fossils are common. Bedding is common. Stylolites are common.
Igneous	Granite	White, Pink, Speckled	Crystalline. Crystals large enough to see. Light colored
	Granodiorite	“Salt and Pepper”	Crystalline. Crystals large enough to see. Mix of light and dark crystals.
	Gabbro	Black, Green, Dark Grey	Crystalline. Crystals large enough to see. Dark colored
Metamorphic	Marble	White, Pink	Crystalline. Wispy, “marbled” textures are common. No fossils.
	Gneiss	Pink, Black and White	Crystalline. Bands of distinctly different color. Bands may be irregular and folded

Manufacturing Processes

The process of manufacturing engineered stone can be broken down into:-

1. Creating/gathering the material
2. Forming the block/slab using industrial machine
3. Processing the finished block/slab into tiles or other products

Some companies import boulders themselves to crush into agglomerates (stone powders) of various grain size for their products, others simply buy already crushed stone powders.

After the block / slab is formed and cured (this usually takes between three and seven days depending on products and weather conditions), the stone can be processed in basically the same manner as its natural counterpart.

Detailed manufacturing process includes:

1.      Selection of the right Quarry and the Big Block

2.      Cutting down the Block to Slabs

3.      Sizing of the Slabs

4.      Filling Process

5.      Polishing or Honing

6.      Final Quality Check and Shade Selection

Manufactured Stone

Manufactured Stone is made up of concrete mix which is moulded and colored to give a realistic look and feel of natural stone. They are widely used in home decorations and public buildings.

 

Advantages

• Manufactured Stones are more lightweightand flat back. This allows for its easy and faster to install as compared to natural stones
  
• It is reusable, eco-friendly, non-toxic and new green interior decoration material
  
• Easy to clean
  
• Heat, fire and Impact resistant
  
• Repairable and used long term

 Disadvantages

• Manufactured stones are costly as compared to readily for freely available natural stones
  
• To avoid improper installation hiring a professional is a necessity. Flaws in the installation are apparent if not done properly
  
• They do not give a realistic look like that of natural stones
  
• Certain limitations like outdoor use/swimming pool as they may get affected by sunlight/chlorine

References:

http://academic.brooklyn.cuny.edu/geology/powell/courses/eesc1101/EESC%201101%20-%20Lab%204%20-%20Brooklyn%20College%20Building%20Stone.pdf

http://www.eschooltoday.com/rocks

http://www.shutterstock.com

What is Crystallization?

What is Crystallization?

Crystallization is a process to chemically alter the surface of the stone. In this process a steel wool pad is used in combination with a weighted floor machine and acid solution to bring a polish to stone floors. The most common ingredients of crystallization chemicals are acid, magnesium fluorosilicate and water. 

In this reaction the magnesium salts are primarily left on the surface of the stone and removed during the next cleaning of the surface, and the calcium fluorosilicate (CaSiF6) is bonded to the underlying stone and is now the layer we walk on. 

The surface of the stone has now been chemically altered and there is no way to reverse the process. Note that this new surface of the stone is not a coating but is now part of the stone itself. The only way to remove a crystallized layer is through mechanical action such as diamond honing with diamond discs or the SRDS’ (scratch removal discs). Chemical strippers commonly used to remove acrylics will not remove crystallization The resulting layer of calcium fluorosilicate formed on the surface of the stone is harder, glossier, and more stain resistant than the original stone surface.

The crystallization process consists of spraying a liquid onto the marble floor and buffing it in with steel wool under a standard speed floor machine. The steel wool generates heat through abrasion and the chemical reacts with the marble, producing a new compound on the surface of the stone.

Almost all crystallization chemicals contain two main ingredients: acid, fluorosilicate compounds and sometimes, waxes or acrylic polymers. Crystallization can only react with calcium-based stones such as marble and limestone.  Although the process can work on non-calcium based stones such as granite, the reactions are entirely different. In the chemical reaction, acid attacks the calcium carbonate of the stone, leaving an etch mark on the stone surface.  When the crystallization liquid is sprayed on a marble surface, the acid attacks the calcium carbonate. The fluorosilicate compound then attaches itself to the calcium ion, forming a new compound called calcium fluorosilicate.

Simply put, the crystallization process works by forcing one ion from one molecule to another in the cement matrix that holds the crystals together. This forms a new cement matrix that can be harder than the original cement matrix of the stone. The newly hard and the preexisting softer structures form two layers, and thus a layer of separation is between them. In many stone varieties, especially those that contain carbon elements, this causes the stone to delaminate. In others, especially many low and medium density limestone, this causes iron (from the steel wool) to enter into

the stone’s chemistry. All stones that undergo this process have dramatic changes in their element construction. In order for this reaction to take place, frictional heat must be generated. This is the reason for using steel wool on the buffing machine. 

The process must be performed by trained operators’ who are familiar with the techniques of this process. Excessive moisture in the stone can hamper the crystallization reaction and cause problems.

WHEN AND WHERE TO USE

To begin with, there is no one product or process that is the be-all and end-all to maintaining polished marble & terrazzo. Crystallization is simply one method of many that may or may not be the application of choice depending on the floor. 

Obviously, you would not want to use crystallization on a honed, non-reflective surface. This process is meant only for polished surfaces. If you are confronted with maintaining a large polished marble or terrazzo floor with high foot traffic, an excellent way for maintaining this polish, more times than not, will be crystallization. 

Keep in mind that, today, some crystallizers contain wax. This can create a build-up on the stone surface over time. When this happens, you will need to use an alkaline detergent, recommended by a professional in the industry, to remove this build-up. If the alkaline detergent does not remove the build up, then you will need to use diamond abrasives to remove the build-up before continuing the maintenance program. There are proven crystallization products in the market place that contain zero wax. 

This is a maintenance process, not a restoration process. By overusing a no-wax crystallizer and allowing it to build up on the surface, you’re pushing the product to remove deeper scratches in the surface that it cannot remove. This type of damage should be removed by restoring the floor with diamond abrasives.  Another popular method of maintaining polished marble is the use of powder/paste compounds, which contain oxalic acid or potassium oxalate, among other ingredients. Here, you’re attempting to recreate what happens in the stone processing factories around the world. The factories polish most marble in a polishing line, running the material through a series of abrasive stones. The final polish is achieved by using something such as a 3200 grit; this grit’s main ingredient is oxalic acid or potassium oxalate. 

The factory polish is created by chemically transforming the marble surface with the acidic slurry from the 3200 grit. This transformation changes microns of the surface from calcium carbonate to calcium oxalate. 

Maintaining polished stone surfaces with oxalic acid compounds usually provides great results and is a widely accepted method. However, these types of products may have adverse effects on green and maroon stones. 

These compounds will also begin to build up on the surface if overused.

In many cases, the buildup occurs quicker than crystallization. Also, when compared to crystallization, the process can be slow and messy.