Calcium aluminate mixtures

Portland cement is classified as a hydrauHc cement, ie, it sets or cures in the presence of water. The term Portland comes from its inventor, Joseph Aspdin, who in 1824 obtained a patent for the combination of materials referred to today as Portland cement. He named it after a grayish colored, natural limestone quarried on the Isle of Portland, which his cured mixture resembled. Other types of hydrauHc cements based on calcium materials were known for many centuries before this, going back to Roman times. Portland cement is not an exact composition but rather a range of compositions, which obtain the desired final properties. The compounds that make up Portland cements are calcium siHcates, calcium aluminates, and calcium aluminoferrites (see ). 

Cement. Portland cement, a mixture of calcium siUcate and calcium aluminate minerals, is produduced by the calcination of argillaceous limestone or mixtures of limestone and clay (see Cement). Although other clays can be used, ka olin is preferred because of its alumina and siUca content and low level of impurities. It is especially desirable in the manufacture of white cement and other types requiring careful control of chemical composition. Air-floated ka olin, because of its low cost, is usually used. 

The catalysts used in the process are essentially nickel metal dispersed on a support material consisting of various oxide mixtures such as alumina, silica, lime, magnesia, and compounds such as calcium aluminate cements. When the catalyst is made, the nickel is present as nickel oxide which is reduced in the plant converter with hydrogen, usually the 3 1 H2 N2 synthesis gas ... 

The dry product is ground to a powder and then a little calcium sulfate (CaS04) is added to slow down the setting rate of the cement. When water is added to the mixture, slow complex chemical changes occur, resulting in the formation of a hard interlocking mass of crystals of hydrated calcium aluminate and silicate. 

The reactors are packed with a mixture of the catalyst and the carbon dioxide sorbent. The catalyst is Nickel supported on calcium aluminate and the carbon dioxide sorbent is dolomite. The particle sizes for the catalyst and dolomite are around 1 to 7 mm. 

Monosulphate is troublesome to prepare. Mylius mixed 500 ml of a supersaturated calcium aluminate solution (649 mg CaO and 969 mg Al203l ) at 18°C, with shaking, with 2924ml of saturated CH (1.25gCa01" ) and 334ml of saturated gypsum (1.93gCaS041 ). After 30 min, crystallization was complete the mixture was filtered, washed four times with a little water followed by 96% ethanol and ether, and dried over CaClj and soda lime without evacuation. Monosulphate may also be prepared hydrothermally (A8). 

Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in Si02 and often also in AI2O3, and low in CaO they are sufficiently reactive that mixtures of them with water and CaO produce C-S-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI2O3, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of CH and decreased Ca/Si... 

Mixtures of calcium aluminate and Portland cements, sometimes also with slaked lime (CH) or additional gypsum, can be used to make rapid setting mixtures for grouting and similar applications. The strengths obtainable (e.g. 25 MPa at 5 h) (G73) are relatively high for cements of this type. 

Mixtures with calcite, slag, gypsum or Portland cement Cussino and Negro (C54) reported that loss of strength in calcium aluminate... 

Various methods have been used to obtain cements that set and harden rapidly. They include the use of Portland cement with admixtures and of mixtures containing both Portland and calcium aluminate cements, described in Sections 11.5 and lO.I.IO, respectively. Another approach has been the manufacture of clinkers containing either CuAy CaF, or C 4A, S. both of which hydrate rapidly under appropriate conditions with the formation of ettringite. 

The authors [23] demonstrated that a substantial effect could be achieved by adding a mixture of separately activated Al(OH)j and Ca(OH)2, taken at a molar ratio of 1 1, to ordinary cement instead of calcium aluminates. It was also found that the mechanical activation of only AI(OH)j was enough, since Ca(OH)2 exhibits substantial solubility in water. 

Portland cement before treatment with water consists of a mixture of calcium silicates, mainly Ca2Si04 and CagSiO, and calcium alumi-nate, Ca3Al20. . When treated with water the calcium aluminate hydrolyzes, forming calcium hydroxide and aluminum hydroxide, and these substances react further with the calcium silicates to produce calcium aluminosilicates, in the form of intermeshed crystals. 

Portland cement is typically composed of about 25% P-dicalcium silicate (lamite), and 50% tricalcium silicate with the balance made up of various calcium aluminates and calcium iron aluminate (brownmillerite). Setting occurs when the cement is hydrated all the components show varying degrees of reactivity with water, but the most significant hydraulic activity is associated with the tricalcium silicate, which forms a cohesive mixture of calcium hydroxide and calcium silicate hydrate (C-S-H)... 

A further synthesis method is self-propagating combustion synthesis [33, 36, 37], In this alternative approach, nitrate starting powders are dissolved in H20 and urea (CH4N20) is added. When this mixture is boiled, dehydrated, and dried, it forms a hygroscopic precursor to calcium aluminates, which can be crystallized by heating in dry air between 250 and 1,050°C. The gaseous decomposition products of the precursor mixture are NH4 and HCNO, which ignite at 500°C, locally the temperature in the dried foam increases to 1,300°C, which promotes crystallization of the CAC phase. 

The mixture of calcium silicate and calcium aluminate that remains molten at the furnace temperature is known as slag. 

The basic constituent, the clinker, is obtained from the pyroproeessing of calcareous and argillaceous rock. Portland types of cement include those that are mixtures of clinker and pozzolana, blast furnace slag and fly ash, in a range of combinations. In addition, non-Portland calcium aluminate cements (formerly called high alumina cement or HAC) are manufactured by pyroproeessing bauxite and lime. 

The overall glass composition is by far the most important factor in controlling the batch-free time. Simple oxide mixtures, such as those used to produce calcium aluminate glasses, often form eutectic mixtures which melt directly with very short batch-free times. Many non-silicate melts are very fluid at any temperature above the melting point of their components and rapidly dissolve all batch particles. Borate, phosphate, and germanate melts can be formed at much lower temperatures than are typically required for silicate melts. As a result, it is usually easier to decrease their viscosity by increases in temperature, e.g., an increase in temperature from 1000 to 1200 C is more easily attained than an increase from 1400 to 1600 C. 

There is a significant number of metastable calcium aluminate lydrates. Then-identification is difficult because of a laige number of polymorphs and high susceptibility to the formation of caiboaluminates under the influence of CO2, although they occur as well ciystallized hydrates. In cement paste they can also form the nanometric mixtures with the C-S-H phase. Therefore it will be convenient to begin the discussion of calcium aluminate hydration from the presentation of the CaO-Al203-H20 system (Fig. 3.34). 

In calcium aluminate clinkers the lower or higher Cj2A, amounts are present, which causes set acceleration of cement paste. Sometimes in the sintered clinkers even the CjA phase can be present, due to the local inhomogeneity of the raw mixture, in which the larger limestone grains can occur. Calleja [16] proposed the simplified formulae for the calculation of calcium aluminate cement phase composition. 

Usually in the process of CA hydration the mixture of the two hexagonal hydrates together with the colloidal aluminum hydroxide are formed. It is all the more probable that calcium aluminate cement contains always some amount of C,2A2 phase, which hydrating gives at once these two hexagonal phases ... 

As it was shown by Mehta [46], the rate of ettringite formation in the mixture of anhydrous calcium aluminates, calcium hydroxide and gypsum was the highest in the case of CA. The reaction in the mixture with C4 AjS was slower, while the reaction of C3A was hampered, because of the impermeable ettringite layer formation (see Sect. 3.3). The C,2A.7 is also not a good source of ettringite, because it reacts... 

Fig. 9.15 Expansion of calcium aluminate cement and gypsum mixture measured (continuous line) and calculated (dashedline). (according to [76])...

A important experimental support of in situ hypothesis was presented by Cottin [76]. The mixture of calcium aluminate cement and gypsum is hydrating with too low water content. The change of volume calculated from the phase composition of the paste corresponds very well to the measured value. 

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