摘要:

IntroductionCement clinker compounds are produced from mixtures of minerals in high temperature solid state reactions and contain a variety of phases where the main compounds are calcium silicates, calcium aluminates, and calcium aluminoferrites. The hydration properties of cement have been investigated by several research groups and results of such investigations are plentifully available in the literature on the hydration of Portland cement1,2on the effects of gypsum in cements,3and on the hydration of calcium aluminate cements.4Investigations of chemical reactions in synthesis and hydration of cements were typically carried outex-situon reaction products after the reaction has been stopped in some way. There are obvious advantages to making measurements using on-line techniques. An example concerning the production of cements is the development of an X-ray powder diffractometer for on-line diffraction of Portland cement5which combined with a full pattern profile analysis6,7gives a continuous on-line phase analysis of the cement.5Before these types of phase analyses were developed it was more realistic to study pure phases of the cement components rather than the complex heterogeneous mixture of phases of cement. The earliestin-situneutron powder diffraction investigation of the reactions of calcium aluminates with water (D2O) is an example of this approach.8The powder neutron diffractometer D1B at the Institute Laue–Langevin, Grenoble, was used with 2.517 Å neutrons and the instrument was ideal at that time for thesein-situinvestigations.Synthetic calcium aluminates were used in the investigation instead of industrially produced cements, as it was estimated that the neutron powder diffraction patterns of samples with several phases present would have too many overlapping reflections to make quantitative phase analysis possible. The rate of reactions between water (D2O) and the calcium aluminates decreased in the sequence C3A, C5A3, CA to CA2, where all compounds gave Ca3Al2(OD)12(C3AD6) as the final end product at temperatures at and over 30 °C.8(For cement chemical nomenclature see refs.1 and 4). The synthetic calcium silicate C3S reacted faster with D2O than β-C2S,9and in these cases the crystalline reaction product was Ca(OD)2. Brownmillerite reacted with D2O forming Ca(OD)2and C3AD6,9and C4AF reacted faster than CA but slower than C3A and C5A3at the temperature 63 °C.8,9The effect of the additive CaSO4·2D2O on the hydrolysis of C3A10and C12A7,10,11with D2O at 27 °C, and of the addition of CaSO4·0.5D2O on the hydrolysis of CA10at 80–120 °C was investigated, and in all these cases a precursor phase was formed prior to the formation of ettringite, C6AS&cmb.macr;3D32. Ettringite starts to form when the precursor phase is at its maximum in quantity and it is then depleted rapidly in the growth of ettringite. The precursor has a powder pattern which was indexed on a hexagonal unit cell witha= 11.19(1) andc= 21.42(1) Å, which is comparable to the unit cell of ettringite,a= 11.23,c= 21.44 Å. It was suggested that the precursor was a metastable form of ettringite and that the differences in the intensities of the Bragg reflections could derive from a less ordered to an ordered arrangement of the deuterium atoms which would be observed in the neutron diffraction case and not in the X-ray diffraction patterns of the precursor and ettringite.The effect of the additive CaCO3on the hydrolysis of CA10with D2O at 80 °C resulted in formation of the compound Ca4Al2(OD)12CO3·5D2O, and the effect of the additives CaCl2and CaBr2on the hydrolysis of C12A7with D2O at temperatures up to 108 °C gave the final reaction products α-Ca2Al(OD)6Cl·2D2O and Ca2Al(OD)6Br·2D2O.12At temperatures up to 40 °C these two compounds were formed from a precursor phase which had an X-ray (and neutron) powder pattern that suggests the precursor phase to be C4AD19.When synchrotron X-ray radiation became readily available in the early 1980s, techniques known from neutron scattering could be applied to X-ray experiments which could be made in energy dispersive as well as in constant wavelength modes. The X-ray synchrotron radiation was orders of magnitude more intense than the known neutron sources, so experiments could be performed faster and with smaller samples than with neutrons or with X-rays from sealed tubes. An early example of this reduction in time is the measurement of a well resolved powder pattern of barium titanate recorded in 20 s in the energy dispersive diffraction mode.13The energy dispersion diffraction mode was used in a series ofin-situstudies of the hydration processes of alumina- and Portland-cement components.14The sample holder was a cylinder of approximately 10 mm diameter which could be rotated to randomize the particle distribution and in which the solid could be mixed with water during the diffraction experiment.15,16Studies were made on the formation of ettringite15–17where an increase in the unit cell parameteraof ettringite was observed during the growth froma= 11.14 Å toa= 11.22 Å. Ettringite formation starts almost immediately as a carbonate and hydroxy ettringite, and when sulfate ions become available by the dissolution of calcium sulfate then sulfate ettringite is formed which was observed as a peak shift of the 112 reflection during the early part of the hydration process. The hydration reaction of calcium alumina cement at 50 °C shows that CA is converted to CAH10, and C2AH8is then formed as two intermediate phases α-C2AH8and β-C2AH8. The final compound C3AH6is starting to appear when α-C2AH8has been converted to β-C2AH8, which is then simultaneously depleted during the formation of C3AH6.18The hydration of C3A with water was investigated in the temperature range 26–36 °C.19C3A reacts with water to an intermediate phase, and C3AH6does not begin to form until the intermediate phase has reached its maximum level. The intermediate phase hadd-spacings at 10.7 and 5.36 Å, which fits C2AH8and C4AH19, but the intensity ratios of the two reflections fits best for C2AH8. A small peak atd= 3.58 Å supports the conclusion that the intermediate phase was C2AH8.19,20In contrast to the energy dispersive diffraction mode and the use of samples with large volumes as described above, the constant wavelength mode and the use of samples in capillaries is another approach toin-situinvestigations using X-ray synchrotron radiation.In-situstudies of catalysis have been made using this approach,21andin-situstudies of phase transitions22and hydrothermal synthesis23have been performed. Counters such as imaging plates,24INEL position sensitive detectors,25or MAR area detector diffractometers26have been used. As well,in-situinvestigations could be made using sealed X-ray tubes and appropriate position sensitive detectors.A fair number of investigations have been made on the components of alumina cements and Portland cements, and these components have in most cases been synthesized as pure phases. Limited work has been made on the hydrolysis of industrially produced cements by thein-situmethods outlined above. The different phases in industrially produced cement may well take other routes in the hydrolysis process than the pure synthetic phases described above, because the cement phases made from minerals maya priorinot be chemically pure. For this reason, it is important to study the hydrolysis of industrially produced cements, and the results of such investigations are reported below.

ISSN:1477-9226    

DOI:10.1039/b301095n

出版商:RSC    

年代:2003

数据来源: RSC