1. General Information[edit | edit source]
1.1 Geological description of limestone[edit | edit source]
Limestone is a sedimentary rock composed mainly of calcium and magnesium carbonate. It is formed by the deposition either of the skeletons of small creatures and/or plants (organic limestones), or by chemical precipitation, or by deposition of fragments of limestone rock, on the beds of seas and lakes. Limestones are contaminated to a greater or lesser extent by the deposition of sand" or clay which are the source of the impurities found in them. Usually there is a difference in quality in a deposit from one layer to the next. The purest carbonates and the most suitable from the production point of view tend to be the thick bedded type. Carbonate deposits may be found in horizontal layers as deposited, or at an angle from the horizontal due to earth movements. They will vary in density, hardness and chemical purity.
1.2 Chemical composition of limestone[edit | edit source]
Limestone is made up of varying proportions of the following chemicals with calcium and magnesium carbonate being the two major components.
|
Calcium carbonate |
CaCO3 |
|
Magnesium carbonate |
MgCO3 |
|
Silica |
SiO2 |
|
Alumina |
Al2O3 |
|
Iron oxide |
Fe2O3 |
|
Sulphate |
SO3 |
|
Phosphorus |
P2O5 |
|
Potash |
K2O |
|
Soda |
Na2O |
The two main impurities are silica and alumina with iron as the third.
For a general purpose lime, a limestone with an SiO2 content of up to 3.5 % and Al2O3 content of up to 2.5 % may be used where purer stone is not available, whereas lime for building or road construction purposes may have an SiO2 content of up to 10% (perhaps slightly more) and an Al2O3 content of 5 %. An Al2O3 proportion of greater than 5% will produce a semi-hydraulic or hydraulic lime.
1.3 Physical characteristics of limestone[edit | edit source]
The colour of most limestones is varying shades of grey and tan. The greyness is caused by the presence of carbonaceous impurities-and the tan by the presence of iron.
It has been found that all limestones are crystalline but with varying crystal sizes, unit formity, and crystal arrangement. This ret suits in stone with a corresponding variance in density and hardness (Boynton p. 21). For lime production purposes there are two factors related to limestones crystallinity and crystal structure which are of specifc interest:
Density or porosity is determined as the percentage of pore space in the stone's total volume. It ranges from 0.3% - 12%. At the lower end are the dense types (marble), and at the upper the more porous (chalk). Generally, the finer the crystal size, the higher the porosity but there are anomalies which suggest that each case be considered separately. A high porosity makes for a relatively faster rate of calcination and a more reactive quicklime.
Limestone varies in hardness from between 2 and 4 on Moh's scale with dolomitic lime being slightly harder than the high calcium varieties. Limestone is in most cases soft enough to be scratched with a knife. Marbles and travertines have the highest compressive strength whilst chalk has the lowest.
Due to the variance in porosity, the bulk densities of various limestones range from 2000 kg/m3 for the more porous to 2800 kg/m3 for the most dense.
The specific gravities of limestones range from 2.65-2.75 for high calcium limestones and 2.75-2.9 for dolomitic limestones. Chalk has a specific gravity of between 1.4 and 2.
1.4 Classification of limestone[edit | edit source]
High calcium limestone is composed primarily of the minerals calcite or Aragonite (CaCO3) with a total oxide (CaO + MgO) content of over 95 % It can be a fine to a coarse "rained stone of varying porosity and hardness. Chalk is a soft, fine "rained, highly porous limestone. The pure, white chalks can have up to 99 %calcium carbonate whereas the grey variety can have up to 20 % impurities i.e. only 80 % CaCO3. Dolomitic and magnesian limestones, in addition to the CaCO3, contain a relatively large proportion of MgCO3. Usually limestones containing 20 % to 44 % MgCO3 are referred to as "dolomite" or as dolomitic limestone, and those containing between 5 -20 % MgCO3, as magnesian limestone. They both vary in purity, density, hardness and colour. Marble is a metamorphosed limestone. It is either high calcium or dolomitic and highly crystalline, dense and hard, and varies in purity. Oolitic limestorne is a chemically precipitated limestone of high purity. Travertine is a hard limestone formed by chemical precipitation in hot springs. Tufa has the same derivation but is softer and more porous. Hydraulic limestones, sometimes referred to as argillaceous limestones have a high proportion of clay and silicia ( 15-20 %) and can be either high calcium or magnesian. When fired a lime is produced which can set under water (hydraulic lime).
There are several other types of limestones (less common and of limited value) such as coral limestones, shell limestones, marl, cherty limestones and stalagmites and stalactites.
1.5 Chemical reactions in the production of lime[edit | edit source]
Calcination
A. High calcium limestones:
CaCO3 + heat -> CaO + CO2
B. Magnesian limestones:
CaMg (CO3)2 + heat -> CaCO3 + MgO + CO2 (at around 750 °C)
CaCO3 + heat -> CaO + CO2 (at around 1100 °C)
Hydration
CaO + H2O -> Ca(OH)2 (water)
MgO + H2O -> Mg(OH)2 (water)
Simply stated, limestone plus heat produces quicklime and quicklime plus water produces lime hydrate.
1.6 Common uses of lime in developing countries[edit | edit source]
1.6.1 Use of lime in building construction
Lime is used in buildings in one of two forms, either as dry slaked lime powder or as putty. The choice of whether to use one or the other form depends on the preference of the builders, the availability of water at the production site and the available means of transport. There is little difference in terms of quality between the two.
The properties which make lime an excellent cementitious building material are the shape and fineness of the particles, which provide the plasticity necessary for a good workable mortar or plaster, and its chemical properties which are the mechanical, strength giving characteristics.
Historically, lime was used for building in such places as ancient China and Greece and, although since the discovery of portland cement its relative use has decreased it is used in mortars and plasters in combination with cement in most countries of the world.
Good mixing proportions are:
|
lime |
cement |
sand |
|
1 |
1 |
6 |
|
1 |
2 |
9 |
The first mix is used where greater strength and resistance to weather are required and the second where these properties are unnecessary.
In situations where the sand to be used is very fine and/or not well graded, a mix of 1:1:7 can be used. Where the use of cement is limited in one way or another a small proportion of cement will suffice or none need be used at all. The use of such mortars or plasters is however limited since they are susceptible to weathering. They are best used on internal walls, external walls which are well protected from the rain, and in dry climates. The mix suitable under such circumstances is 1:2/3, i. e. 1 part lime to 2 or 3 parts sand. The same mix proportions are used for mortars and plasters. Special care must be taken where the' lime is of low quality (relatively low hydroxide content) and the sand badly graded and fine.
Limewash as an external and sometimes internal wall coating is used in many countries today. Its advantages are that it is relatively cheap, easy to apply and gives a clean white finish, whilst its disadvantage is that it rubs off easily.
The durability of limewashes can be improved by adding small amounts of common salts or stearates. The recipes given by Bessey are:
|
"Quicklime |
1 kg |
|
Tallow |
60 g |
|
Water |
|
Quicklime is broken into small lumps and tallow is placed in shreds over it. Enough water is then added to slake the quicklime. Once slaking is complete the remaining water is added and then the slurry is screened to remove any lumps and remaining shreds of tallow.
If only hydrated lime is available, powdered calcium stearate and water are added to it to produce a slurry. The recipe is:
|
"Hydrated lim |
1 kg |
|
powdered calcium stearate |
50 g |
|
Water as required" |
Lime in combination with pozzolanic material can be used as an alternative cementitious material to portland cement. This type of cement can be used successfully for most purposes other than structural concrete work, i.e. in plaster, mortar, production of building blocks, screeds etc. Some types of pozzolanic materials are: pulverized fuel ash, blastfurnace slag, volcanic ash, diatomatious earth, under-fired clay bricks, porcellanite and others. Each has a particular form in which it is most effective as a pozzolana.
1.6.2 Soil stabilization in road construction
The construction of roads in tropical and sub-tropical regions in areas where the soil is clay or laterite will require the use of a soil stabilizer. Lime, like portland cement, reacts chemically with certain constituents of the clay and acts as a binder. The effectiveness of lime as a soil stabilizer is increased in warm climates where its rate of strength development will be faster than in cooler areas. 2 - 5 % hydrated lime is mixed with soil from the road and spread over the road surface for compaction. The quantities required will depend on the compaction density, width and thickness of sub-base or base designed, and also on the proportion of available lime in the lime hydrate. For a road 11 m wide, sub-base 200 mm thick and a compaction density of 1500 kg/m3, where 4 % lime is to be added, 130 tonnes lime per kilometer of road will be required.
For an existing limework with a low output (up to 15 tonnes), the implications of such a demand are:
- A long period of warning will be required to develop stockpiles, the development of which will require special financing arrangements since the stockpiling will cause excessive strain on cashflow.
- The accent will be placed on the quantity rather than the quality produced. It should be noted however, that the higher the quality the lower the quantities of lime required. The economic effects on transport cost, sales prices, etc. need to be carefully considered.
- A dry hydrate needs to be produced.
The possibility of installing a small limeworks near the road works, if possible in a situation where its operation will be continued after the completion of the road, must be considered as it may be more economic than transporting large quantities over long distances.
1.6.3 Lime in agriculture
Lime and limestone are used in agriculture to neutralize the acidity of the soil and to promote effective use of added fertilizers. Crushed limestone, dolomite, chalk, etc. are preferred as they are slower acting than lime and therefore last longer.
Other uses are:
- water purification,
- sugar refining,
- tanning,
- neutralizing acid mine water,
- oil well drilling,
- wire drawing,
- paper and pulp production,
- sewage treatment,
- metallurgical processing,
- petroleum refining,
- calcium silicate brick production,
- pains productionand others.
