Synthesis of Calcium Aluminates

CAC require large industrial facilities, similar to those used to make ordinary Portland cement. The raw materials for CAC are typically bauxite and limestone, which are ball – milled and mixed together to form a feed of appropriate composition, which is fed into rotary kilns to form a calcium aluminate clinker. The clinker is ball-milled to produce the cement. Analysis for composition and mineralogy at various stages of manufacture are essential to ensure a consistent product, see for example Chakraborty and Chattopadhyay [32] for a discussion of the bulk processing of high alumina CAC.

For high purity calcium aluminate compositions, solid-state synthesis is still the norm [33, 34]. Most CAC compounds are made by solid-state reactions between ground powders of calcium carbonate and purified alumina. The sintering tempera­tures depend on alumina content. More recently attempts have been made to synthesize CA compounds using processes with temperatures less than 900°C. These latter methods include sol-gel synthesis and precipitation and are important for production of high – purity homogenous powders with small grain size.

Amorphous calcium aluminate powders have been synthesized chemically by Uberoi and Risbud [35] by sol-gel methods. These materials were made from calcium nitrate (Ca(NO3)2) and by using aluminum di-sec-butoxide acetoacetic ester chelate (Al(OC4H9)2(C6H9O3) ) as the source of alumina.

A further synthesis method is self-propagating combustion synthesis [33, 36, 37]. In this alternative approach, nitrate starting powders are dissolved in H2O and urea (CH4N2O) 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.