Calcium Aluminate Cements
Calcium aluminate phases are used as cements in refractory and other specialized applications [1, 2, 22, 23]. The ceramics in the calcium aluminate (CaO-Al2O3) system are closely related to Portland cements and have similar properties in terms of rapid hardening and setting times . Their phase equilibria are closely related to that of Portland cements as and are formed in the binary CaO-Al2O3 of the ternary CaO-Al2O3-SiO2 phase diagram. The binary phase diagram (Fig. 1) shows that calcium aluminate cements (CACs) have a wider range of compositions than Portland cements, but are dominated by the monocalcium aluminate (CaAl2O4, also referred to as CA).
Fig. 1 The CaO-Al2O3 phase diagram [25, 26]
CACs were developed in response to the need for cements resistant to groundwater and seawater attack and are the only cements, other than Portland cement, that are in continuous long-term production . The property of CAC that was most important in their commercial development is the resistance to sulfate attack, which contrasted with the poor-sulfate resistance of contemporary Portland cements , and CAC was first patented in 1908 . Most early applications, in construction projects following the First World War, were in structures exposed to seawater, such as harbor pilings. Because CAC hardens rapidly, it was adopted for prestressed concrete beams in the post World War II construction boom, with some unfortunate results. Poor understanding of the material properties of CAC and incorrect water to cement ratios led to the collapse of several buildings, and the use of Portland cements, which are cheaper, has replaced CAC in prestressed concrete beams.
There are, however, several important niche applications for CAC. Most notably, CACs are used as linings to sewers and mine tunnels. Calcium aluminate cements are resistant to chemical attack from sulfate-producing bacteria that thrive in sewer systems (especially in warmer climates), and sprayed concrete linings to sewers have been shown to resist degradation for periods up to 30 years. The high impact and abrasion resistance of CAC also makes it suitable as a lining material for ore tunnels in mines and because CAC sets rapidly, it can be sprayed onto tunnel walls (as “shotcrete”) and even used as a tunnel lining.
Additional specialist applications include castable refractory ceramics and use as bioceramics, which are discussed later.