Special Portland cements

Tricalcium silicate is an essential constituent of all Portland clinkers. In its crystalline lattice it contains Ca cations in combination with 8104 and anions in a ratio of 3 1 1. Upon heating or cooling tricalcium silicate undergoes a series of reversible phase transitions  

The crystal stmctures of the individual tricalcium silicate phases are similar, and differ mainly in the spatial orientations of the Si04 tetrahedra. 

Tricalcium silicate is thermodynamically stable only between about 1250°C and 2150° C. It melts incongmently above 2150°C, and becomes thermodynamically unstable with respect to dicalcium silicate and calcium oxide below 1250°C. Thus, to produce a clinker containing tiicalcium silicate, the raw mix has to be heated to a temperature that is higher than the lower limit of thermodynamic stability of this phase, and has to be cooled rapidly enough to prevent a noticeable decomposition of tricalcium silicate in the course of cooling. Such a conversion of C3S to C2S and C may take place especially at temperatures slightly below 1250°C, whereas at ambient or only moderately elevated temperatures tricalcium silicate remains preserved for an indefinite time. 

In industrial clinkers the existing tricalcium silicate is doped with foreign ions, also present in the raw mix. These dopants prevent a conversion of tricalcium silicate to its Tj modification, which is the one that is metastable at ambient temperature, and stabilize one of the high-temperature modifications of this compound, usually Mj or M. Such a doped form of tricalcium silicate is usually called alite. 

Upon contact with water, tricalcium silicate undergoes hydration, yielding an amorphous calcium silicate hydrate phase called the C-S-H phase (or just C-S-H) and calcium hydroxide as products of hydration. The rate of hydration will depend on the quality and quantity of dopants incorporated within the crystalline lattice, on the cooling rate in the production of the chnker, on the fineness of the cement, and on other factors. 

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Fig 7.26 Relative strength (strength of samples cured in sea water to the strength of samples cured in standard conditions) (according to [171]). Dashed area corresponds to 6 cements PC 55, cements PC 45 with additives marked with crosses PC 55 - Portland cement without additives PCM 55 special Portland cement without additives used in the sea environment prise mery, PCL... 

In ordinary Portland cement these ratios vary typically in the following ranges LSF — SK) 95, SM — 2.2—2.6, AM — 1.5—2.5 Xhey may deviate significantly from these ranges in special Portland cements. 

Special Portland cements 17 detail of type S expansive cement see section 21.3.3.)... 

To avoid expansion and concrete damage even in the presence of alkali-sensitive aggregates the content of equivalent NaO (defined as Na20g= Na2O+0.66K2O) in the concrete mix should not exceed 4 or even 3 kg/m. To meet this criterion a special Portland cement with particularly low alkali content must be employed. In such a cement the equivalent Na20 content should not exceed 0.6%. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

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