Heat Evolution Curves Research Articles

An examination of interference in waste solidification through measurement of heat signature

The hydration of cementing materials is accompanied by heat evolution which is closely related to their structure development. The presence of wastes usually interferes with the hydration of cementing materials. This study examined their interference in waste stabilization/solidification processes through the measurement of adiabatic heat evolution using a computerized Quadrel™ system. Two cementitious materials, an alkali-activated blast furnace slag binder and an ASTM Type I portland cement were used to solidify an electric arc furnace (EAF) dust, which has high concentrations of B, Cr, Hg, Pb, Ni and Zn. The EAF dust contents were 0, 30 and 60% by mass. Different mixing conditions were also examined. The interference of EAF dust with the hydration of cementing materials was described using several parameters derived from the heat evolution curves: equivalent initial time of setting (equivalent time at 20°C); total heat evolution at initial time of setting; equivalent final time of setting, total heat evolution at final time of setting and total heat evolution at equivalent time of 28 and 90 days. Experimental results indicated that the Quarel™ system was a useful tool to examine the interference in waste stabilization/solidification and to assist with the selection of cementing materials.

Quantitative study of yeast growth in the presence of added ethanol and methanol using a calorimetric approach.

Using a calorimeter with 24 sample units the heat evolved during incubation of yeast cultures at 30 degrees C was detected in the form of growth thermograms. Ethanol and methanol added to the culture medium produced changes in the growth thermograms that could be analyzed to calculate the 50% inhibitory concentration (Ki) and minimum inhibitory concentration (MIC). Correlation of the heat evolution curves with the number of cells and the turbidity of the culture was found to be very good. It was found that addition of ethanol and methanol up to 7.65% had clear effects of inhibition on growth of all yeast strains studied, reducing the growth rate constant and delaying growth. However, the amounts of ethanol produced from the nutrients available in the culture vial was only little affected by the initial addition of up to 7.65% (v/v) of ethanol or methanol in the medium.

Reactions of octacalcium phosphate to form hydroxyapatite

Hydrolysis reactions of octacalcium phosphate (OCP) to form hydroxyapatite (HAp) were studied. OCP was mixed with the proportions of tetracalcium phosphate (TetCP) or calcium hydroxide needed to form stoichiometric HAp. The relationships between the rates of hydrolysis, microstructural evolution, and variations in solution chemistry were examined during the transformations of OCP to HAp. Isothermal calorimetry at 25, 38 and 60°C and variations in solution chemistry showed that conversion of OCP to HAp occurred more rapidly at higher temperatures. Moreover, its transformation to HAp was more rapid when TetCP was present. Reactions involving TetCP and OCP showed heat evolution curves ( dQ dt versus t) characterized by the presence of two peaks, one due to mixing of the reactants and a second due to the conversion reaction. The rate of OCP hydrolysis to HAp was reduced when calcium hydroxide was present, if not entirely stopped. Calorimetric curves showed mixing peaks but the absence of reaction peaks when OCP hydrolyzed in the presence of Ca(OH) 2. The only reaction observed was the dissolution of Ca(OH) 2. There was an absence of any notable reaction with OCP once it had dissolved. The direct hydrolysis of OCP in the absence of other solids occurred the most slowly. This is consistent with the hydrolysis behavior of other calcium phosphates. Lastly, OCP reactivity depended on the manner in which it was produced. Although more crystalline, OCP made using calcium phosphate precursors was more reactive than OCP produced by precipitation from calcium rmacetate/sodium dihydrogen phosphate solution.

Calorimetric studies of special cements

The kinetics and even the mechanism of cement reaction with water can be successfully investigated by use of microcalorimetry. In this study this method was applied to follow the hydration of the new family of portland cements containing C12A7 * and C11A7·CaF2 addition as well as special cement with C3A replacement by calcium sulphoaluminate. It has been found that C11A7·CaF2 acted as hydration retarder. The heat evolution curves for C12A7 containing samples without CaF2 are very similar to those for the reference portland cement samples. XRD and SEM studies confirm the results described above, relating to the retardation of alite hydration. The process is positively modified by the addition of anhydrite. In the presence of calcium sulphoaluminate (4CaO·3Al2O3·SO3) the hydration at early stage occurs with the rapid formation of large amount of the ettringite phase. The calcium fluoride acts as a set retarder. The full compatibility of calorimetry with SEM and XRD results should be underlined.

Hydration of type K expansive cement paste and the effect of silica fume: I. Expansion and solid phase analysis

The hydration reactions of Type K expansive cement pastes and the effect of silica fume were studied. Part I reports on the expansion characteristics of pastes and the corresponding changes occuring in the quantities of solid phases associated with expansion. Results of the rate of heat evolution curves of the pastes during the first 48 hours are also reported. Part II ∗∗ present results of the pore solution analysis and a proposed hydration mechanism. In pastes, with and without silica fume, expansion was associated with the formation of ettringite. Expansion continued as long as ettringite was forming. The expansion rate and magnitude at early age (the first two days of hydration) was larger when silica fume was added to the expansive cement paste. Solid phase analysis and the rate of heat evolution curves of the pastes also indicated that the addition of silica fume accelerated the hydration of the paste. These suggest that the silica fume resulted in a more efficient use of ettringite that formed during the early age. Silica fume also reduced the duration of expansion. This reduction was deemed advantageous because delayed expansion and, therfore, microcracking and degradation of the hardened expansive cement paste could be prevented with the use of silica fume.

Hydration and setting time of MgO-type expansive cement

The hydration and setting time of MgO-type expansive cement was investigated. The results show that the addition of MgO retards the initial hydration of the cements and increases the setting time. The causes for the retardation of hydration can be attributed to: (1) since the solubility product constant of Mg(OH) 2 is far less than that of Ca(OH) 2, and when MgO is added and Mg(OH) 2 is formed, the ionic product for calcium and hydroxyl ions [Ca 2+][OH −] 2, or Ca(OH) 2 saturation ratio, the ratio of Ca(OH) 2 concentration to saturated Ca(OH) 2 concentration under the same ionic strength is reduced in the liquid phase of the hydrating cement. And the time when the Ca(OH) 2 saturation ratio reaches the maximum is increased, the initiation of the second peak on the heat evolution curve of cement, i.e. the end of the induction period, is delayed; (2) Mg(OH) 2, which is formed by MgO hydration in a high alkali medium such as the liquid phase of hydrating cements, precipitates with a tiny crystallite size on the cement grains and forms a “protective layer”, which retards further hydration of the cements.

Application of a deconvolution method to kinetic studies with conduction type microcalorimeters

A deconvolution method has been developed using a microcomputer to correct the inevitable thermal inertia of a heat conduction type calorimeter. The numerical algorithms based on fast Fourier transform (FFT) were designed and constructed assuming that the convolution equation holds for the artificial stepwise heat evolution curve. This heat was generated electrically by a resistor inserted into the sample cell of the calorimeter. The algorithms reproduced correctly the original rectangular heat evolution curve, i.e. the curve without thermal inertia. However, the Tian-Calvet equation which was also tried, failed to reproduce the curve. The value of the activation energy for the alkaline hydrolysis of ethyl acetate obtained by the new deconvolution method is in good agreement with that in the literature obtained by chemical titration.

The effect of temperature upon the setting behaviour of refractory calcium aluminate cements

A setting time parameter has been determined from the heat evolution versus time curve measured by means of a thermocouple embedded in the cement paste as well as by the Gillmore needle. The setting time increases with increasing temperature, until it reaches a maximum between 26 and 30°C, for four out of the five refractory calcium aluminate cements studied. This anomalous retardation is not, therefore, restricted to ciment fondu, as reported elsewhere. The rate of consumption of CA during the first 24 hours of hydration as determined by X-ray diffraction also indicated a retardation at 30°C compared with 20°C for Secar 51 and Secar 71. The cause of the anomaly cannot be the presence of C2S, which is virtually absent in most of the cements studied. An alternative explanation must be found, which should also explain why Secar 80 behaves differently from the other cements investigated.

Thermal effect of oscillating solid state reactions

Solid state reactions with an intermediate show oscillating behaviour (ref. 1, 2). The dependence of heat evolution during reaction has been derived for isothermal and dynamic conditions using the relaxation kinetic model (ref. 3) and an Arrhenius rate constants extrapolation. Simulation of ideal heat evolution curves show effects to be taken in account evaluating quantitatively DTA or DSC signals.

A DSC study of structural relaxation in metallic glasses prepared with different quenching rates

Sets of amorphous Fe-Ni-Cr-P-B, Fe-Ni-Mo-B, Fe-B-C alloys have been prepared by melt spinning using different rotation speeds of the wheel. The enthalpy of relaxation, the glass transition temperature Tg, the crystallization temperature are studied as a function of quenching rate. Structural relaxation takes place continuosly in a wide temperature range from 420–450 K up to the Tg region (650–670 K) and it is governed by a spectrum of activation energy described by the heat evolution curve at constant heating rate. The chemical and topological contribution to structural relaxation are separated by means of suitable thermal treatments.