The word Stones is what we call natural rocks after quarrying them out of the earth’s crust .According to the Building Stone Institute of America(BSI) and others like Arthur Lyons rocks can be put in three geologic categories :(1)Sedimentary rocks (2)Igneous rocks and (3)Metamorphic rocks.Below we shine alight on a few details relating to each category.
1.Sedimentary rocks-are formed by weathering and erosion of older rocks.This action by water,ice and wind breaks the rocks into progressively smaller pieces and are carried by rivers to oceans and other water bodies.These deposits are laid down as sedimentary beds of mud or sand,which build up layers,become compressed and are in the end cemented together by minerals like calcium carbonate ,quartz,iron oxide or by dolomite found in underground water.
The bedding planes that are natural in sedimentary rock are put to use when quarrying the stone and to obtain maximum durability in masonry building.Quarried sedimentary rock contain quarry sap allowing easy carving and work on them;however after exposure to the open air,they become hard like nails to work with. Example include Sandstone, limestone, and dolomite
2.Igneous rocks-Formed when magma (hot liquid rock within the earth) cools. Depending on whether solidification happens slowly within the earths crust or rapidly at the surface the Igneous rocks can be defined as Plutonic or Volcanic respectively. Plutonic rock’s slow cooling allowed large crystals formation which can be seen in granite. Volcanic rocks like basalt and pumice are fine grained.Examples are Granite, Syenite, Diorite, Gabbro, Andesite and Basalt.
3.Metamorphic stones-formed from sedimentary or igneous rocks by action of heat and pressure.To put it differently,they are formed by the crystallization of older rocks by the action of intense heat and or pressure within the earths’s crust. Clay is changed into slate,limestone to marble and sandstone to quartizite.
A building under construction using Nairobi building stones
Building stones in Kenya
Having dealt with the classification of Rocks,allow me shift to something that is equally interesting, and that is building stones used in Kenya and beyond.They are quite a number and include the following:
- They are dense and hard impermeable to water and resistant to impact damage.Granite can be sawn,rough punched,picked ,fined tooled,honed ,polished or flamed to give them a perfect finish.They are either fine ,medium or coarse grained igneous rocks composed of quartz, feldspar, and Colors range from pink, red, gray, blue, green, tan, brown, black and every color and shade between.
- Because they are expensive granite is use most often as cladding material(40mm externally or 20mm internally).Or it can be cast directly onto concrete cladding units.
- Granite can also be used for flooring,hard landscaping pavements etc.Finally polished granite can be used as kitchen counter tops.
- Its a fine grained stone that is hard and can melt at 2400°C.This can be cast into tile units which are deep steel grey in color.
- Larger cast units for worktops ,either hone or polished can be cut to size.
- This is limestone rocks that have been changed by the action of intense heat and pressure in the earth’s crust.
- The calcium carbonate or Serpentine is recrystallized into a mosaic of near equal-sized calcite crystals
- Calcites is white ,so a pure marble is white and translucent.The colors and veins in a typical marbles are due to impurities within the original limestone.These range from red,pink,brown green ,beige,cream,white,grey and black.
- Its attached by acids ,therefore should be honed not polished for external uses.
- They are hard and dense but fissures and veins may require filling
- Can be used for cladding and fixing clamps and hooks used need be in stainless steel phosphor bronze or copper
- Floor slabs should be a minimum of 30mm,laid on minimum 25 mm bed.
- Marble wall and bathroom floor tiles should be between 7mm to 10 mm in thickness
- They are fine to medium grained sedimentary of rock having at least 60% free Silica. Colors range from grey, yellow to brown.
- They can be sawn,split faced,clean rubbed or tooled to give them an attractive finish.
- To use them for cladding-75mm to 100mm thickness is used and fixed with cramps and corbels
A finished building constructed with natural stones
- It’s a sedimentary rock composed of classic and sand-sized grains of calcites, fossils or shell .
- Composed mainly of calcium carbonate either crystallized from solution as calcite or formed from accumulation of fossilized shell deposits by various sea organism.
- Colors range from light grey, tan to light brown.
- They can be fine-rubbed,fine dragged,split faced or tooled to desired finish.
- Externally limestone must not be mixed with or located above sandstone, because they may negatively affect the sandstone.
- Its fine grained metamorphic sandstone.
- Colors range from shades of blue grey, green, and red.
- Formed from fine grained sand free clay sediments
- They are fine grained rocks which split easily along its cleavage
- As a result slate can be split into thin sections(4-10mm) for roofing slates
- Slate can be sawn, sanded,fine rubbed,honed,polished,flamed or bush hammered.
- Spain is world’s largest producer of slate
- Its strong, acid and frost resistant,lasting up to 400 years as roofing material.
- The minimum recommended pitch for slate roofing is 20° under sheltered exposures and 22.5° under severe exposure.
- Fixing nails should be of copper or aluminium
- It can be used for flooring ,cladding,copings,cills and stair threads
- Colors range from grey, purple ,red and brown.
- These are a foliated metamorphic quartz –feldspar containing rock.
- They often have mica or chlorite. Schist split readily along planes of foliation.
- This is sandstone that has metamorphosed because of intense heat and pressure.
- The grains of sandstone are recrystallized into a matrix of of quartz to form a a hard to wear stone used as a flooring material.
- It has mica which allows the stone to be split along smooth cleavage planes.
- Exported by Norway and South Africa.
- A naturally occurring gypsum or calcium sulfate
- Used in making carved monuments and ornaments
- In pure form its white and translucent
- Traces of iron oxide may give it light brown,orange or red color