Studies of cement formation

An equally simple chemical study was carried out on phytic acid-aluminosilicate cements (Prosser et al., 1983). Phytic acid, myo-inositol hexakis(dihydrogen phosphate), is a naturally occurring substance found in seeds, and it is a stronger acid than phosphoric acid. Cements were prepared using aqueous solutions of phytic acid, concentrated to 50 wt%, and with 5 wt % zinc dissolved in the acid to moderate the rate of reaction with the glass powder. Discs of cement were prepared and these were 

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Crisp, S., Prosser, H. J. Wilson, A. D. (1976). An infra-red spectroscopic study of cement formation between metal oxides and aqueous solutions of poly(acrylic acid). Journal of Materials Science, 11, 36-48. 

A final point needs to be made. Theory has indicated that AB cements should be amorphous. However, a degree of crystallization does sometimes occur, its extent varying from cement to cement, and this often misled early workers in the field who used X-ray diffraction as a principal method of study. Although this technique readily identifies crystalline phases, it cannot by its nature detect amorphous material, which may form the bulk of the matrix. Thus, in early work too much emphasis was given to crystalline structures and too little to amorphous ones. As we shall see, the formation of crystalUtes, far from being evidence of cement formation, is often the reverse, complete crystallinity being associated with a non-cementitious product of an acid-base reaction. 

Hornsby, P. R. (1977). A study of the formation and properties of ionic polymer cements. Thesis for PhD, Brunei University, Middlesex, England. 

In 1968 Wilson published an account of his early search for alternatives to orthophosphoric acid as a cement-former with aluminosilicate glasses. Aluminosilicate glasses of the type used in dental silicate cements were used in the study and were reacted with concentrated solutions of various organic and inorganic adds. Wilson (1968) made certain general observations on the nature of cement formation which apply to all cements based on aluminosilicate glasses. 

A major contribution came when Ono (1957) pub lished a microscopical study of the formation of port land cement clinker, tracing the changes of phase chem istry and crystal morphology as functions of raw material fineness, mixing, and heating rate. He concluded with a prophetic statement that the microscopical information might be useful in the control of clinker manufacture. 

In the DTA and TG studies of cement containing different amounts of TEA, evidence was obtained for the formation of lower amounts of Ca(OH)2 in the presence of the admixture. In Fig. 15 the amount of lime (DTA estimation) formed in the cement paste in the presence of TEA is given.I ] Comparison of the results at the same degree of hydration has revealed that TEA promoted the formation of C- S-H with a higher C/S ratio. 

Nicholson, J. W., Brookman, P. J., Lacy, O. M., Sayers, G. S. Wilson, A. D. (1988a). A study of the nature and formation of zinc polyacrylate cement using Fourier transform infrared spectroscopy. Journal of Biomedical Materials Research, 22, 623-31. 

In a classic study, Kingery (1950b) examined a large number of oxides for cement formation with orthophosphoric acid. He observed three types of reaction no reaction, violent reaction with crystallization, and controlled reaction with cement formation. 

The setting reaction of dental silicate cement was not understood until 1970. An early opinion, that of Steenbock (quoted by Voelker, 1916a,b), was that setting was due to the formation of calcium and aluminium phosphates. Later, Ray (1934) attributed setting to the gelation of silicic acid, and this became the received opinion (Skinner Phillips, 1960). Wilson Batchelor (1968) disagreed and concluded from a study of the acid solubility that the dental silicate cement matrix could not be composed of silica gel but instead could be a silico-phosphate gel. However, infrared spectroscopy failed to detect the presence of P-O-Si and P-O-P bonds (Wilson Mesley, 1968). 

The following account is based mainly on the studies of Wilson and coworkers, with some re-interpretation of experimental data. The composition of the cement used is given in Table 6.9. In brief, the reaction takes place in several overlapping stages extraction of ions from the glass, migration of cations into the aqueous phase, precipitation of insoluble salts as pH increases, leading to formation of an aluminium phosphate gel. 

There have been a number of studies aimed at understanding the chemistry of the curing and setting of magnesium oxychloride cements and at identifying the phases that are present in the final material. Investigations in the first half of the twentieth century revealed that cement formation in the MgO-MgCla-HaO system involves gel formation and crystallization of... 

X-ray diffraction has been applied to certain AB cements. For example. Crisp et al. (1979), in a study of silicate mineral-poly(acrylic acid) cements, used the technique both to assess the purity of the powdered minerals employed and to monitor mineral decomposition in mixtures with poly(acrylic acid), in order to indicate whether or not cement formation had taken place. They employed Cu radiation passed through a nickel filter for most of the samples, a seven-hour exposure time was found to be adequate for the development of a discernible diffraction pattern. Samples were identified by reference to published powder diffraction data. 

In another study, oscillating rheometry was used to examine the effect of adding various simple metal salts to glass-ionomer cements (Crisp, Merson Wilson, 1980). It was found that cement formation for certain glasses which react only slowly with poly(acrylic acid) could be accelerated significantly by certain metal salts, mainly fluorides such as stannous fluoride and zinc fluoride. Some non-reactive glasses could be induced to set by the addition of such compounds. 

Ettringite has been widely studied in cement and concrete due to the deleterious expansion, which has been attributed to its formation after the concrete has hardened (Lea 1971 Mehta 1973 Dunstan 1980 Cohen 1983 Monteiro Mehta 1985 Tikalsky 1989 Day 1992 Tishmack et al. 1999). Ettringite also causes expansion in some types of high-S CCPs (see below). 

The formation of beachrock will be examined as an example of carbonate cement formation, because it has been extensively investigated and because it represents a chance to study carbonate cement emplacement under conditions where the rate of cement precipitation is relatively rapid and the associated solutions can be analyzed directly. It also differs from the cementation process in our model in that carbon dioxide can be degassed to the atmosphere, resulting in major changes in the saturation state of the cementing solution. 

In the first effective studies of hydrated cements by DTA, Kalousek and co-workers (K27,K32,K33) observed the early formation of ettringite and its subsequent replacement by what was termed a solid solution, but which was probably a mixture of AFm phases. They found no CjAHg or other hydrogarnet phases. Quantitative or semiquantitative determinations of gypsum and ettringite indicated that less than half the total SO3 present could be accounted for by these phases. The authors concluded that the solid solution was eventually replaced by a product which they termed Phase X , and which was possibly a gel containing all the oxide components of the original cement. 

X-ray microanalyses of CjS-pozzolana pastes showed a gradual decrease in Ca/Si ratio on passing from regions near the iinreacted CjS to ones near the pozzolana (021). TMS studies of pastes of C3S, (3-C,S and cement with and without natural pozzolanas (U9,M44) showed that, in the presence of the latter, formation of polymeric anions is accelerated and their mean molecular weight is increased. TMS results and determinations of combined water showed that the hydration of P-CjS is almost completely suppressed in the presence of a pozzolana and that, in pastes with CjS, 16-29% of the pozzolana had reacted in 180 days (M44). The eflect on P-CjS hydration is similar to that found using QXDA for pfa (D12). Chemical extraction showed 10-45% of the pozzolanas in pastes with C3S to have reacted in 28 days, compared with under 10% for a pfa (U9). 

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