Calcium aluminate cements [1] are cements 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.[ citation needed ]
The method of making cement from limestone (CaCO3) and low-silica bauxite (Al2O3) was patented in France in 1908 by Bied of the Pavin de Lafarge Company. The initial development was as a result of the search for a cement offering sulfate resistance. The cement was known as "Ciment fondu" and "Ciment électro-fondu" in French. [2] [3]
As indicated by Bied (1922), [2] who was the inventor of this type of cement, the terms "Ciment fondu" ("fused cement") and "Ciment électro-fondu" ("electro-fused cement") refer only to the manufacturing process involving the melting of the base materials (CaO obtained after the decarbonation of CaCO3, and Al2O3). This is because there is no temperature range in which it is possible to observe the gradual softening and clinkerization of these materials, as is the case with Portland cement at around 1450 °C. In the absence of a softening temperature, calcium aluminates are obtained directly by fusion of the precursor materials, and Bied (1922) clearly indicated his preference for the appellation "ciment alumineux" ("aluminous cement") referring to its composition rather than to a manufacturing process. [2]
Subsequently, its other special properties were discovered, and these led to its future in niche applications.[ citation needed ]
By the 2010s, the product was found in the US market under the name FONDAG cement (FOND Aluminous Aggregate), sometimes referred to as ALAG (ALuminous AGgregate). FONDAG cement is a mix of up to 40 percent alumina, and is stable at high temperatures and thermal cycling from −184–1,093 °C (−300–2,000 °F; 89–1,400 K; 160–2,500 °R) [4]
CAC cement invented in 1908 by Bied [2] is sulfate-free and hardens to give mainly hydrated calcium aluminates or carboaluminates (AFm phases: Aluminium Ferrite mono-substituted phases), sometimes accompanied with C–S–H as a minor component, while Ca(OH)2 (portlandite) is absent. [5] So, CAC cement must not be confused with calcium sulfo-aluminate (CSA) cement containing calcium sulfate and invented later in 1936. [6]
The main constituent, and also the most reactive phase, of calcium aluminate cements is the monocalcium aluminate (CaAl2O4 = CaO · Al2O3, also written as CA in the cement chemist notation). It usually contains other calcium aluminates 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. [7] 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 | 46 | 70 | 70 | 35 |
| Dodecacalcium hepta-aluminate | 10 | 5 | 0 | 0 |
| Monocalcium dialuminate | 0 | 0 | 17 | 30 |
| Belite | 7 | 5 | 0 | 0 |
| Gehlenite | 4 | 14 | 11 | 1 |
| Ferrite | 24 | 5 | 2 | 0 |
| Pleocroite | 1 | 1 | 1 | 0 |
| Wüstite | 7 | 0 | 0 | 0 |
| Corundum | 0 | 0 | 0 | 33 |
The mineral phases all take the form of solid solutions with somewhat variable compositions.[ citation needed ]
The cement is made by fusing together a mixture of a calcium-bearing material (normally calcium oxide from limestone) and an aluminium-bearing material (normally bauxite for general purposes, or refined alumina for white and refractory cements).
The melting of the mixture is achieved at 1600 °C and is energy demanding. [8] [9] The more elevated temperature explains a part of its higher production costs than for the clinker of ordinary Portland cement sintered at 1450 °C.[ citation needed ]
The liquified mixture cools to a vesicular, basalt-like clinker 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 arrangement comprises a reverberatory furnace 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. In the case of high-alumina refractory cements, where the mix only sinters, a rotary kiln can be used.[ citation needed ]
CAC cements gain strength more rapidly than ordinary Portland cement (OPC). Sometimes, a retarder is needed to ensure a longer workability.[ citation needed ]
In contrast to Portland cements, calcium aluminate cements do not release calcium hydroxide (Ca(OH)2, portlandite, or lime) during their hydration. [5]
The hydration reactions of calcium aluminate cements are very complex. The strength-developing phases are monocalcium aluminate (CA), dodeca-calcium hepta-aluminate (C12A7), and belite (C2S), a dicalcium silicate. Calcium aluminoferrite (C4AF), monocalcium dialuminate (CA2), gehlenite, and pleochroite contribute little to the concrete strength.[ citation needed ]
During the cement setting, the reactive aluminates react with water initially to form a mixture of hydrated phases expressed hereunder in normal oxide notation and also abbreviated in the more compact cement chemist notation (CCN) (CaO = C; Al2O3 = A; H2O = H; and SiO2 = S):
the amounts of each is depending upon the curing temperature.[ citation needed ]
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 concrete 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. [10]
The inappropriate use of calcium aluminate cements as a common construction material without special precautions has led to structural stability problems in buildings.
On 8 February 1974, the roof of a swimming pool collapsed in the UK. [11] In 1984, the roof of a factory building in Uherské Hradiště in Czechoslovakia (built 1952) collapsed, killing 18 people. [12] In Madrid, Spain, a large housing block nicknamed Korea (because it was built to house Americans during the Korean War), 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. [13]
Because of their relatively high cost and delicate implementation, calcium aluminate cements are used in a number of restricted applications where performance achieved justifies costs: [3] [ citation needed ]
The biogenic corrosion resistance of calcium aluminate cements is used today in three main applications:[ citation needed ]
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.
Portland cement is the most common type of cement in general use around the world as a basic ingredient of concrete, mortar, stucco, and non-specialty grout. It was developed from other types of hydraulic lime in England in the early 19th century by Joseph Aspdin, and is usually made from limestone. It is a fine powder, produced by heating limestone and clay minerals in a kiln to form clinker, grinding the clinker, and adding 2 to 3 percent of gypsum. Several types of portland cement are available. The most common, called ordinary portland cement (OPC), is grey, but white portland cement is also available. Its name is derived from its resemblance to Portland stone which is quarried on the Isle of Portland in Dorset, England. It was named by Joseph Aspdin who obtained a patent for it in 1824. His son William Aspdin is regarded as the inventor of "modern" portland cement due to his developments in the 1840s.
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the rest being a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be further purified before it can be refined into aluminium.
Cement chemist notation (CCN) was developed to simplify the formulas cement chemists use on a daily basis. It is a shorthand way of writing the chemical formula of oxides of calcium, silicon, and various metals.
Sodium aluminate is an inorganic chemical that is used as an effective source of aluminium hydroxide for many industrial and technical applications. Pure sodium aluminate (anhydrous) is a white crystalline solid having a formula variously given as NaAlO2, NaAl(OH)4 (hydrated), Na2O·Al2O3, or Na2Al2O4. Commercial sodium aluminate is available as a solution or a solid.
Other related compounds, sometimes called sodium aluminate, prepared by reaction of Na2O and Al2O3 are Na5AlO4 which contains discrete AlO45− anions, Na7Al3O8 and Na17Al5O16 which contain complex polymeric anions, and NaAl11O17, once mistakenly believed to be β-alumina, a phase of aluminium oxide.
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2(SO4)3(OH)12·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system. The prismatic crystals are typically colorless, turning white on partial dehydration. It is part of the ettringite-group which includes other sulfates such as thaumasite and bentorite.
Ye'elimite is the naturally occurring form of anhydrous calcium sulfoaluminate, Ca
4(AlO
2)
6SO
4. It gets its name from Har Ye'elim in Israel in the Hatrurim Basin west of the Dead Sea where it was first found in nature by Shulamit Gross, an Israeli mineralogist and geologist who studied the Hatrurim Formation.
Alite is an impure form of tricalcium silicate, Ca3SiO5, sometimes formulated as 3CaO·SiO2, typically with 3-4% of substituent oxides. It is the major, and characteristic, phase in Portland cement. The name was given by Törnebohm in 1897 to a crystal identified in microscopic investigation of Portland cement. Hatrurite is the name of a mineral that is substituted C3S.
Belite is an industrial mineral important in Portland cement manufacture. Its main constituent is dicalcium silicate, Ca2SiO4, sometimes formulated as 2 CaO · SiO2 (C2S in cement chemist notation).
Cement clinker is a solid material produced in the manufacture of portland cement as an intermediary product. Clinker occurs as lumps or nodules, usually 3 millimetres (0.12 in) to 25 millimetres (0.98 in) in diameter. It is produced by sintering limestone and aluminosilicate materials such as clay during the cement kiln stage.
Tricalcium aluminate Ca3Al2O6, often formulated as 3CaO·Al2O3 to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates. It does not occur in nature, but is an important mineral phase in Portland cement.
Calcium aluminates are a range of materials obtained by heating calcium oxide and aluminium oxide together at high temperatures. They are encountered in the manufacture of refractories and cements.
Dodecacalcium hepta-aluminate (12CaO·7Al2O3, Ca12Al14O33 or C12A7) is an inorganic solid that occurs rarely in nature as the mineral mayenite. It is an important phase in calcium aluminate cements and is an intermediate in the manufacture of Portland cement. Its composition and properties have been the subject of much debate, because of variations in composition that can arise during its high-temperature formation.
Monocalcium aluminate (CaAl2O4) is one of the series of calcium aluminates. It does occur in nature, although only very rarely, as two polymorphs known as krotite and dmitryivanovite, both from meteorites. It is important in the composition of calcium aluminate cements.
Calcium aluminoferrite is a dark brown crystalline phase commonly found in cements. In the cement industry it is termed tetra-calcium aluminoferrite or ferrite. In cement chemist notation (CCN), it is abbreviated as C
4AF meaning 4CaO·Al
2O
3·Fe
2O
3 in the oxide notation. It also exists in nature as the rare mineral brownmillerite.
An AFm phase is an "alumina, ferric oxide, monosubstituted" phase, or aluminate ferrite monosubstituted, or Al2O3, Fe2O3 mono, in cement chemist notation (CCN). AFm phases are important hydration products in the hydration of Portland cements and hydraulic cements.
Friedel's salt is an anion exchanger mineral belonging to the family of the layered double hydroxides (LDHs). It has affinity for anions as chloride and iodide and is capable of retaining them to a certain extent in its crystallographical structure.
Concrete degradation may have many different causes. Concrete is mostly damaged by the corrosion of reinforcement bars due to the carbonatation of hardened cement paste or chloride attack under wet conditions. Chemical damages are caused by the formation of expansive products produced by various chemical reactions, by aggressive chemical species present in groundwater and seawater, or by microorganisms. Other damaging processes can also involve calcium leaching by water infiltration and different physical phenomena initiating cracks formation and propagation. All these detrimental processes and damaging agents adversely affects the concrete mechanical strength and its durability.
The pozzolanic activity is a measure for the degree of reaction over time or the reaction rate between a pozzolan and Ca2+ or calcium hydroxide (Ca(OH)2) in the presence of water. The rate of the pozzolanic reaction is dependent on the intrinsic characteristics of the pozzolan such as the specific surface area, the chemical composition and the active phase content.
AFt Phases refer to the calcium Aluminate Ferrite trisubstituted, or calcium aluminate trisubstituted, phases present in hydrated cement paste (HCP) in concrete.
ALAG (ALuminous AGgregate) is a synthetic calcium aluminate aggregate manufactured by the fusion of bauxite and limestone into a partially re-crystallized aggregate of approximately 40% alumina. It is essentially Fondu clinker, crushed and sized into grades commonly needed by concrete and castable formulators.
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