Sulphate composite cements

Table 9.5 Phase composition of sulphate-aluminate cements and the strength of mortars...

In the papers [129-137] many examples of the sulphate-aluminate cements compositions one can find, very similar to those given in Table 9.5. 

The strength development of sulphate-aluminate cement mortars is strongly influenced by the SOj/AljOj ratio. This ratio affects the phase composition of paste, as well as the rate of hydration products formation Klein and Mehta [136] exammed the phase composition of hydration products, formed in the mixture C4A3S+CaSO +CaO+H20. The results are shown in Fig. 9.29. 

Fig. 9.30 a Effect of the three main phases proportions on the physical properties of cements (according to [132]) 1 rapid hardening cements field, II slightly expansive cements field. III field of expansive cements and cements applied in the self prestiessed concretes production the number correspond to the approximate 3C2S/C4A3S ratio, b Three-components system showing the composition of sulphate-belite cements, (according to [138])... 

Portland cement reacts with jarosite in the presence of water to form various Ca-Al-Fe silicate-sulphate-hydrate phases (Figure 2) and Ca-Al-Fe oxide ( Ca4Al2Fe08.5 or Ca2(Al,Fe)205). Among the various cement reaction products, the Ca-Al-Fe silicate-sulphate-hydrate and Ca-Al-Fe silicate-hydrate phases appear to be the most common. Table II shows the average electron microprobe-determined compositions of the calcium silicate phase in the Portland cement and of the Ca-Al-Fe silicate-sulphate-hydrate cement reaction products. As the compositions of these reaction products vary widely, the values in Table II are only indicative. 

Table II - Average Electron Microprobe-Determined Compositions of the Calcium Silicate Phase in the Portland Cement and the Ca-Al-Fe Silicate-Sulphate-Hydrate Cement Reaction Products (wt%) ...

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

Of the pfa characteristics that influence reactivity, the glass content appears to be much the most important, but specific surface area, glass composition and the effect of stress in the glass caused by the crystalline inclusions may also be relevant (U17). Of external factors, the RH, temperature (C43) and alkali content of the cement are probably the most important. Sulphate ion may also enhance reactivity by promoting the removal of AF from the glass (U17). The rates of the pozzolanic reaction and of strength development are more sensitive to temperature than are those of hydration and strength development for pure Portland cements (e.g. Ref. H52). 

Several concretions in the Pliocene, Mamoso-arenacea and Loiano formations possess calculated or whole-rock 6 C values more C depleted than -10%o. These have higher IGV values than other cemented samples (e.g. 31% for more C-depleted carbon samples vs. 14% for more C-enriched carbon samples in the Loiano), which indicates that they were cemented at shallower depths than most samples. But these earlier-phase concretions still formed after significant compaction had occurred, which would also be below the depth of sulphate survival. Mass balance calculations indicate that from 20 to 30% of the carbon in these more C-depleted cements must have been derived from the oxidation of methane, assuming that methanic carbon has a 6 C composition of at least -40%o (Curtis, 1977). 

The early marine cements on both basin margins have relatively wide variations in carbon isotopic composition compared with the basin centre (Fig. 10). Carbon isotopes in the basin centre have S CpDB values between +5%o and -10%o, whereas basin margin cements commonly have values between +10%o and -20%o values as low as -30%o and as high as +20%o also occur. Presumably, the overall shallower depositional conditions at the basin margin and/or greater fluid mobility compared with the basin centre have resulted in different reactions, producing dissolved carbon within tens of metres of the sediment-water interface. Negative S Cp B values presumably represent bacterial oxidation or sulphate reduction, and positive values are from bacterial fermentation (Irwin et ai, 1977). 

Cement is a binder that sets and hardens by itself or binds other materials together. The most widely known application of cements is in construction a second one is the area of bone cements. Cements used in construction are characterized as hydraulic or nonhydraulic and mostly for the production of mortars and concrete. Hydraulic cements set and harden after combining with water. Most construction cements are hydraulic and based on Portland cement, which consists of calcium silicates (at least 2/3 by weight). Nonhydraulic cements include the use of nonhydraulic materials such as lime and gypsum plasters. Bone cements and bone cement composites refer to compounds that have a polymer matrix with a dispersed phase of particles. For instance, polymethylmethacrylate (PMMA) is reinforced with barium sulphate crystals (for radio-opacity) or with hydroxyapatite... 

Resistance of concrete to external attack is only limited to the corrosion of sulphate water solution, in this connection only the CjA content is normalised, respectively additionally the sum 2C3A+C4AF. Exceptionally in France were normalised the requirements concerning cements designed to sea constmctions (ciments prise mer ). The composition of these cements should fulfil the following condition ... 

Table 1.2 Requirements eonceming the ehemieal composition of cements. (Content of Cr(VI) cannot exceed 2 mg/kg in the case of higher chromium content it must be redueed by addition for example iron(ll) sulphate)...

The complexity of any individual binder reaction with water is well illustrated on the example of gypsum hemihydrate hydratioa The hydration process in the case of multiphase material as Portland cement is, must be much more complex. The reactions of individual cement phases with water occur simultaneously and interfere the presence of minor components, first of all alkalis and sulphates, modifies further the composition of the liquid phase. Therefore the rate of hydration of basic cement phases is strongly affected. 

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