Expansive cements calcium aluminate cement-based

As aforementioned expansive cements produced industrially are based on the formation of ettringite. The technologies differ orrly with the type of calcium alu-minates and of the matrix material. The types of expansive cements are shown in Table 9.3. Two types of matrix are used Portland cement and calcium aluminate cement. The following phases are r ed as the sotrrce of aluminum ions CjA, CA (as calcium aliuninate cement), C4A3S and the calcitrm aliuninate hydrates C AH or CAHjq. The second technology is the oldest one, and was implemented by Dossier in forties XXc [60]. 

Concrete based on calcium aluminate cement performs rather well if exposed to sulfate solutions, especially if made with a low water/ cement ratio and high cement content. However, cases of expansion and cracking have also occasionally been reported (Scrivener and Capmas, 1998). The reasons for the good sulfate resistance of this type of cement are not obvious. It is mostly attributed to a surface densification of the hardened material, resulting in a very low permeability of the formed surface layer, and/or to the absence of calcium hydroxide in the system (Scrivener and Capmas, 1998). Unlike Portland cement and related binders, magnesium sulfate solutions are less aggressive to calcium aluminate cement than alkali sulfate solutions. This is due mainly to the absence of the C-S-H phase in the hardened calcium aluminate cement pastes, which is particularly sensitive to the action of magnesium sulfate. 

In the most widely used expansive cements the cementitious component consists of ordinary Portland cement, but other binders such as calcium aluminate cement may also be employed. Most commonly, the expansive stresses in the course of hardening are created by the formation of ettringite (C3A.3CS.32H). This phase is formed in the reaction of a constitnent containing alutninnm oxide with calcinm snlfate, calcinm oxide or hydroxide and water. Less common than expansive cements based on ettringite... 

In expansive cements based on ettringite formation, ettringite is formed in the reaction of a suitable aluminum-bearing phase with calcium sulfate, calcium hydroxide, and water. Suitable aluminate phases include CA, CjA, and especially C4A3S. The most... 

Figure 7. Effect of expansive additives based on different calcium aluminate hydrates or HAC on free expansion of cement paste...

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