Calcium aluminate cements
Encyclopedia
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
s consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement" and "Ciment fondu" in French. They are used in a number of small-scale, specialized applications.
History
The method of making cement from limestoneLimestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
and low-silica bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
was patented in France in 1908 by Bied of the Pavin de Lafarge Company
Lafarge
Lafarge is a French industrial company specialising in four major products: cement, construction aggregates, concrete and gypsum wallboard. In 2010 the company was the world's second-largest cement manufacturer by mass shipped behind Holcim.-History:...
. The initial development was as a result of the search for a cement offering sulfate resistance. The cement was called in the French "Ciment fondu". Subsequently, its other special properties were discovered, and these guaranteed its future in niche applications.
Composition
The main active constituent of calcium aluminate cements is monocalcium aluminateMonocalcium aluminate
Monocalcium aluminate is one of the series of calcium aluminates. It does not occur in nature, but is important in the composition of calcium aluminate cements.-Properties:...
(CaAl2O4, CaO · Al2O3, or CA in the cement chemist notation
Cement chemist notation
Cement chemist notation was developed to simplify the formulas cement chemists use on a daily basis. It is a "short hand" way of writing the chemical formula of oxides of calcium, silicon, and various metals.-Abbreviations of oxides:...
). It usually contains other calcium aluminates
Calcium aluminates
Calcium aluminates are a range of minerals obtained by heating calcium oxide and aluminium oxide together at high temperatures. They are encountered in the manufacture of refractories and cements....
as well as a number of less reactive phases deriving from impurities in the raw materials. Rather a wide range of compositions is encountered, depending on the application and the purity of aluminium source used. Constituents of some typical formulations include:
| Oxide/Mineral | General purpose | Buff | White | Refractory |
|---|---|---|---|---|
| SiO2 | 4.0 | 5.0 | 2.7 | 0.4 |
| Al2O3 | 39.4 | 53.0 | 62.4 | 79.6 |
| Fe2O3 | 16.4 | 2.0 | 0.4 | 0 |
| CaO | 38.4 | 38.0 | 34.0 | 19.8 |
| MgO | 1.0 | 0.1 | 0.1 | 0 |
| Na2O | 0.1 | 0.1 | 0 | 0 |
| K2O | 0.2 | 0 | 0 | 0 |
| TiO2 | 1.9 | 1.8 | 0.4 | 0.1 |
| Monocalcium aluminate Monocalcium aluminate Monocalcium aluminate is one of the series of calcium aluminates. It does not occur in nature, but is important in the composition of calcium aluminate cements.-Properties:... |
46 | 70 | 70 | 35 |
| Dodecacalcium hepta-aluminate Dodecacalcium hepta-aluminate Dodecacalcium hepta-aluminate occurs rarely in nature as the mineral mayenite. It is an important mineral phase in Calcium aluminate cements and is an intermediate in the manufacture of Portland cement... |
10 | 5 | 0 | 0 |
| Monocalcium dialuminate | 0 | 0 | 17 | 30 |
| Belite Belite Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 .... |
7 | 5 | 0 | 0 |
| Gehlenite Gehlenite Gehlenite, , is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.The type locality is in the Monzoni Mountains, Fassa Valley in Trentino in Italy, and is named after Adolf Ferdinand Gehlen by A.J... |
4 | 14 | 11 | 1 |
| Ferrite Calcium aluminoferrite Calcium aluminoferrite is a dark brown crystalline phase commonly found in cements. In the cement industry it is termed ferrite... |
24 | 5 | 2 | 0 |
| Pleocroite | 1 | 1 | 1 | 0 |
| Wüstite Wüstite Wüstite is a mineral form of iron oxide found with meteorites and native iron. It has a gray color with a greenish tint in reflected light. Wüstite crystallizes in the isometric - hexoctahedral crystal system in opaque to translucent metallic grains. It has a Mohs hardness of 5 to 5.5 and a... |
7 | 0 | 0 | 0 |
| Corundum Corundum Corundum is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red... |
0 | 0 | 0 | 33 |
The mineral phases all take the form of solid solutions with somewhat variable compositions
Manufacture
The cement is made by fusing together a mixture of a calcium-bearing material (normally limestoneLimestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
) and an aluminium-bearing material (normally bauxite
Bauxite
Bauxite is an aluminium ore and is the main source of aluminium. This form of rock consists mostly of the minerals gibbsite Al3, boehmite γ-AlO, and diaspore α-AlO, in a mixture with the two iron oxides goethite and hematite, the clay mineral kaolinite, and small amounts of anatase TiO2...
for general purposes, or refined alumina for white and refractory cements). The liquified mixture cools to a basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
-like clinker
Clinker
Clinker may refer to:* Clinker , construction method for wooden boats* Clinker , waste from industrial processes* Clinker , a kilned then quenched cement product* Clinker brick, rough dark coloured bricks...
which is ground alone to produce the finished product. Because complete melting usually takes place, raw materials in lump-form can be used. A typical kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
arrangement comprises a reverberatory furnace
Reverberatory furnace
A reverberatory furnace is a metallurgical or process furnace that isolates the material being processed from contact with the fuel, but not from contact with combustion gases...
provided with a shaft preheater in which the hot exhaust gases pass upward as the lump raw material mix passes downward. The preheater recuperates most of the heat in the combustion gases, dehydrates and de-hydroxylates the bauxite and de-carbonates the limestone. The calcined material drops into the "cool end" of the melt bath. The melt overflows the hot end of the furnace into molds in which it cools and solidifies. The system is fired with pulverized coal or oil. The cooled clinker ingots are crushed and ground in a ball-mill
Cement mill
A cement mill is the equipment used to grind the hard, nodular clinker from the cement kiln into the fine grey powder that is cement...
. In the case of high-alumina refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
cements, where the mix only sinters, a rotary kiln
Cement kiln
Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates...
can be used.
Reaction with water
The hydration reactions of calcium aluminate cements are very complex. The strength-developing phases are monocalcium aluminate, dodeca-calcium hepta-aluminate and beliteBelite
Belite is an industrial mineral important in Portland cement manufacture, a name for dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 ....
. Calcium aluminoferrite, monocalcium dialuminate, gehlenite
Gehlenite
Gehlenite, , is a sorosilicate, Al-rich endmember of the melilite complete solid solution series with akermanite.The type locality is in the Monzoni Mountains, Fassa Valley in Trentino in Italy, and is named after Adolf Ferdinand Gehlen by A.J...
and pleochroite contribute little to strength.
The reactive aluminates react with water initially to form a mixture of:
CaO · Al2O3 · 10 H2O,
2 CaO · Al2O3 · 8 H2O,
3 CaO · Al2O3 · 6 H2O, and Al(OH) 3 gel,
the amounts of each depending upon the curing temperature.
The first two hydrates subsequently decompose to a mixture of 3 CaO · Al2O3 · 6 H2O, Al(OH)3 gel and water, this process being called "conversion". Because of the loss of water, conversion causes an increase in porosity, which can be accompanied by a decrease in strength. This need not be a problem in structural concrete provided that a sufficiently high cement content and a sufficiently low water/cement ratio are employed.
Applications
Because of their relatively high cost, calcium aluminate cements are used in a number of restricted applications:- in construction concretes, rapid strength development is achieved, even at low temperatures.
- in construction concretes, high chemical resistance is possible.
- in refractory concretes, strength is maintained at high temperatures.
- as a component in blended cement formulations, various properties such as ultra-rapid strength development and controlled expansion can be obtained.
Problems
Incorrect use of calcium aluminate cements has led to widespread construction problems, especially during the third quarter of the 20th century when this type of cement was used because of its faster hardening properties. After several years some of the buildings and structures collapsed due to degradation of the cement and many had to be torn down or reinforced. Heat and humidity accelerate the degradation process called "conversion"; the roof of a swimming pool was one of the first structures to collapse in the UK.In Madrid, Spain, a large housing block nicknamed Korea (because it was built to house Americans during the Korean War
Korean War
The Korean War was a conventional war between South Korea, supported by the United Nations, and North Korea, supported by the People's Republic of China , with military material aid from the Soviet Union...
), built 1951~1954 was affected and had to be torn down in 2006. Also in Madrid the Vicente Calderón soccer stadium was affected and had to be partially rebuilt and reinforced.