Very rapidly hardening cements

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. 

Regulated-set cement and jet cement are modified Portland cements in which the normal aluminate phase is replaced by CuA CaF, through the use of a raw mix containing CaF,. Uchikawa and Tsukiyama (U2I) gave chemical (Table 10.4) and phase compositions of two jet cements. Botli contained approximately 60% of alite, 20% of C, i A CaF,. 1% of belite and 5% of ferrite. Admixtures are required to control the rate of reaction of the C, 1 A CaFj and the nature of the products. One of the cements included a proprietary retarder based on citric acid, togetlier with 2 /o of CaC O, . The other contained 2.5% of hemihydrale. In each case, Na,S04 (T o) and anhydrite were also present. The specific surface areas were around 550 m kg . 

CiiA CaFj and jet cements, including the effects of various admixtures. A wide variety of methods was used. For the two cements mentioned above, hydrated in pastes at 20 C, the main product at 3 h was ettringite. At this stage, 40-45% of the C,iA7 CaF2 had reacted. SEM showed needles of ettringite, 0.5-1.0 pm thick, growing from the particles of clinker and 

In most of the cements described in this section, ettringite is an important hydration product, but, as in supersulphated cements, its formation does not 

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The production of alinite clinker, which has the properties of rapid hardening cement, due to high hydraulic activity of alinite, is, however, linked with two problems. The first one is the HCl emission from the rotary kiln HCl is formed in reaction of chlorine with water vapour, contained in the kiln gases. Nudelman [154] proposed the closed chlorine cycle with the calcittm chloride recovery, in reaction between HCl and CaCOj. However, it wottld complicate seriously the functioning of kiln installation. Another problem relates to the very high chlorine content in alinite clinker and consequently in cement, which will cause the corrosion of reinforcement in concrete. These questions are discussed in Chap. 6. However, the alinite cement can be applied in special cases, in the concreting works at lowered temperature, principally in permafrost area. 

This mass very rapidly hardens and after ten minutes is already fairly hard, and after three hours harder than any other in the course cf time, however, it is exceeded in hardness and solidity by the cement prepared according to Experiment 9,... 

High alumina cement is used in mortars for refractory brick walls and in refractory concretes when mixed with special aggregate, like corundum or fire clays. At present the application of high alumina cement is, in general, very restrained it is limited to non-structural elements and to refractory structures where there is no risk of high humidity. Other applications are in repair where rapid hardening is required (Barnes and Bensted 2001). 

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