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  • Author or Editor: F. Wilburn x
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Abstract  

The effect of sample mass, heating rate and partial pressure of carbon dioxide on TG, DTG and DTA curves for the decomposition of some common carbonates has been investigated. These variables gave a marked effect, similar in magnitude for both DTG and DTA. The effect of sample mass, or depth of undiluted sample, is shown to be due to an increase in the partial pressure of carbon dioxide within the reacting powder. This effect is most pronounced in nitrogen but is much reduced in carbon dioxide. Inert diluents have little effect on the curves since they do not increase the partial pressure of CO2. The first stage of the decomposition of dolomite (CaMg(CO3)2) varies with increasing partial pressure of carbon dioxide in an anomalous manner and hence the effects of these procedural variables (except heating rate) are not similar to those observed for magnesite (MgCO3) and calcite (CaCO3). The second stage is, however, strongly dependent on these variables and behaves in a manner that would be predicted for a sample of calcite diluted with magnesite.

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Abstract  

A theoretical approach has been used to show that, except for certain types of reaction mechanism, the ease with which it is possible to distinguish the form of the reaction mechanism by the reduced-time plot method depends particularly on the rate of transfer of heat into the sample. The original reduced-time plots [1] were calculated from model equatioons which assume that the sample is, from the outset, at a fixed temperature and remains under isothermal conditions throughout the reaction. The variations produced in the appearance of reduced-time plots when the sample is programmed to rise to a given fixed temperature through various temperature schedules have been investigated. It is shown that even relatively rapid temperature rises can produce distortion of the reduced-time plots for various reaction equations. If the reaction mechanism is known, however, fairly accurate values of the activation energy for the reaction can be determined, even when the furnace used has relatively poor heat-transfer characteristics.

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Abstract  

The stability of Supersulphated Cement (SSC) is investigated at 95°C when subjected to relative humidities of 100, 53 and 11% of water vapour. Previously [1] investigations at 25, 50, 75°C under the same conditions of humidity reported the stability of ettringite, one of the initial hydration products. At 95°C, decomposition of ettringite, is found at all humidities and is rapid at 100% relative humidity. The hydration products of cement pastes at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). The formation of the hydragarnet, plazolite is recorded during the decomposition/dehydration process enhanced by possible carbonation. Rehydration studies on the products after storage for up to 9 months were carried out using distilled water and the samples tested for ettringite content. It is concluded that ettringite in SSC is inherently unstable at 95°C.

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Abstract  

The stability of supersulphated cement (SSC) is investigated. The hydration products of cement pastes prepared at a water cement ratio of 0.27 were determined by thermogravimetry (TG) and X-ray diffraction (XRD). Ettringite, one of the initial hydration products, is shown to be stable under conditions of storage at 25, 50 and 75°C and when subject to relative humidities of 100, 53 and 11% of water vapour in each case. The effect of drying on ettringite stability at the higher temperatures is discussed in relation to the relative humidity.

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Abstract  

One of the principal uses of supersulfated cement has been for structures exposed to sea water and sulfate bearing ground waters. The resistance to such environments has been related to the absence of calcium hydroxide and the combination of much of the free alumina into ettringite during hydration. This paper reports the resistance of SSC to sulfate solutions in which ettringite has been decomposed. Prism samples were subjected to initial water storage at 25°C for both 28 days and 6 months. Samples were also cured for 6 months at 95°C and at both 11% and 100% R.H. The control samples of 28 days were compared with the 6 months samples of a more mature undecomposed SSC paste. After curing the prisms were measured and all the samples were immersed in three sulfate solutions (0.7M Na2SO4 , 0.7M MgSO4 and saturated CaSO4), and water at the same time. Measurements of linear expansion over 6 months were carried out. Core and surface material following immersion was examined by DTG and XRD. The study indicated that SSC is resistant to sodium and calcium sulfate solutions. Strong magnesium sulfate solutions decomposed the samples under all conditions. A possible mechanism for this attack is suggested.

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TG, DTG and DTA curves of magnesite are dependent on procedural variables, especially sample mass, heating rate and partial pressure of carbon dioxide, in a similar manner to those of calcite [1], although the magnitude of the effect is less for magnesite. The first stage of the decomposition of dolomite varies with increasing partial pressure of carbon dioxide in an anomalous manner and hence the effects of these procedural variables (except heating rate) are not similar to those observed for magnesite and calcite. The second stage of the decomposition of dolomite is, however, strongly dependent on these procedural variables and behaves in a manner that would be predicted for a sample of calcite diluted with magnesia. A 1∶1 molar mixture of magnesite and calcite also behaves as would be predicted from the behaviour of the single carbonates but differently from that of dolomite.

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The effect of procedural variables, including sample mass, heating rate, particle size and partial pressure of carbon dioxide, on TG, DTG and DTA curves for the decomposition of A. R. calcium carbonate and limestone has been studied. Such variables have a marked effect, similar in magnitude for both DTG and DTA. The effect of sample mass, or depth of undiluted sample, is shown to be due to an increase in the partial pressure of carbon dioxide within the reacting powder and has been called the bed-depth effect. This effect is most pronounced in nitrogen but is much reduced in carbon dioxide. Inert diluents have little effect on the TG curves but changing the composition of the inert carrier gas causes variations which are correlated with the thermal conductivity of the gas. Water vapour causes a lowering of the DTG and DTA peak temperatures.

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