Dehydration kinetics of Portland cement paste at high temperature
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Portland cement paste is a multiphase compound mainly consisting of calcium-silicate-hydrate (CSH) gel, calcium hydroxide (CH) crystal, and unhydrated cement core. When cement paste is exposed to high temperature, the dehydration of cement paste leads to not only the decline in strength, but also the increased pore pressure in the paste. In this article, the dehydration kinetic was characterized in term of the combination of kinetics of CSH and CH. The dehydration kinetics data of cement paste at different heating rates was collected by thermogravimetry. The influence of temperature on the reaction rate is analyzed by Arrhenius equation. The Arrhenius parameters of CSH and CH, activation energy, and pre-exponential factor are determined by isoconversional method. The calculated kinetics parameters were validated by further experimental data finally.
KeywordsKinetics Dehydration Cement paste High temperature TG Isoconversional method
The dehydration of Portland cement paste plays an important role in the concrete structure explosive spalling at fire. Calcium silicate hydrate (CSH) and calcium hydroxide (CH) are the main hydration phases which dominate the properties of Portland cement paste. The temperature range corresponds to the dehydration of CSH is about from 105 to 1,000°C . The temperature range corresponds to the dehydration of CH is about from 400 to 550°C. The dehydration of cement paste, together with evaporation of capillary water, has a great influence on the build-up of high pore pressure, which can result to explosive spalling of concrete.
The dehydration analysis of cement paste can be facilitated by the use of thermal analysis techniques, including thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The dehydrations of different cement pastes [1, 2, 3, 4] were investigated by TG at given heating rates. The chemical meanings of peaks of TG/DTG curve were qualitatively described [2, 3, 4]. The dehydration kinetics of cemetitious materials were usually characterized by the modified Arrhenius equation , where the cemetitious materials were considered as pure substance.
However, the dehydration kinetics of Portland cement paste is too complex to be described by a single Arrhenius equation. The dehydration of Portland cement paste is comprised by dehydration of CSH and CH . Moreover, the dehydration of CSH itself is a multi-step reaction [6, 7], in which the Arrhenius parameters (activation energy and pre-exponential factor) vary with dehydration degree. Therefore, it’s difficult to use a single pair of Arrhenius parameters to reveal the chemical mechanism and to predict the dehydration process of cement paste accurately.
In this study, the dehydration kinetics of Portland cement paste was characterized as the combination of two parallel kinetics processes, i.e., kinetics of CSH and CH. To confirm that these two reactions dominate the dehydration process, the crystalline composition of cement paste sample was characterized by X-ray diffraction. The dehydration mass losses (TG curves) of cement paste at different heating rates were collected by thermogravimetry for the parameters determination. The TG curves of cement paste were separated into the TG curves of CSH and CH by a graphic method. The dehydration kinetics parameters of CH and CSH were computed, respectively, based on the TG data. The isoconvertional method  was performed to calculate the reaction-degree-dependent activation energy of CSH and CH. This kinetics model and calculated parameters were verified by further experimental data.
The experiment were carried out by using an ordinary Portland cement (OPC) CEM I 42.5N. The water/cement ratio was 0.5. The cement paste was cast in small plastic bottles. After curing in closed bottles for 28 days, the hardened paste were removed from the plastic bottles. Before measurement, the cement paste samples were stored in the oven at 105°C for 24 h to remove the free water. The cement paste samples were milled into powders (<50 μm) for thermogrametic analysis.
Crystalline phase identification
Portland cement paste is a complex material that might contain some minor hydration products and calcium carbonate (CaCO3). These minor phases can effect the total mass loss of cement paste at high temperature. Therefore, before thermal analysis, the crystalline composition of cement paste must be identified to make sure that the minor phases are negligible in samples, and that dehydration of CSH and CH is the main reaction during heating. The mineral composition of cement paste sample was measured by powder X-ray diffraction (XRD). The X-ray source of Cu Kα radiation (λ = 1.54056 Å) was used. The scan step size was 0.02° in the range 2θ from 5° to 70°.
Mass loss data collection by thermogravimetric analysis
The simultaneous thermogravimetry analyzer was used to measure the TG curves. The purge gas was nitrogen, and the gas velocity was 20 ml/min. The sample was heated from 30 to 1,100°C with different rates: 1, 2, 5, 10 and 30°C/min.
Separation of TG curve
Determination of kinetics parameters by isoconversional method
Results and discussion
It was also found that the temperature range corresponding to the dehydration of CH varies with the heating rate (Fig. 3c). For the samples with different heating rate, the dehydration starts at the same temperature, but ends at different temperature. This is because the samples with higher heating rate need less time to reach target temperature, and its dehydration time is less.
Discussion on variation of Ea with progress of reaction
The activation energy E a is the average excess energy that a water molecule must possess to release . A variation of E a with reaction process implies that systematic change of reactivity takes place during the progress of a reaction. The possible reasons of a variation of E a were discussed by Galwey .
The variation of E a of CH with α is very small (relative standard deviation = 3.85%). This can be explained that the dehydration of CH is a single step reaction. During the reaction process, the energy barrier to bond redistribution of CH almost keep the same. The small variation of E a might be caused by computational error.
In order to characterize the kinetics of the dehydration of different phase in cement paste, a global kinetics of cement paste was considered as the combination of kinetics of CSH and CH. The phases in cement paste was identified by powder XRD. Isoconversional methods [8, 10] were performed to study the dehydration kinetics of CSH and CH. The E a and the lnA of the dehydration of CSH and CH were determined, respectively. The following conclusions can be drawn:
In the fresh Portland cement paste that has been oven dried at 105°C, CSH and CH are the main hydration products. The minor crystalline phase (like CaCO3) can be neglected during the dehydration.
The E a of the dehydration of CH is about 151.82 kJ/mol. The variation of E a of the dehydration of CH is very small.
The E a of the dehydration of CSH varies from 83.69 to 371.93 kJ/mol. The variation of E a of the dehydration of CSH is explained by its complex globulous structure at high temperature.
The global kinetics model of cement paste and its measured parameters were verified by comparing the simulated mass loss with experimental data.
The authors thank the Dutch National Science Foundation (STW) for financially supporting the research project on “Explosive spalling of concrete: towards a model for fire resistant design of concrete elements” (code no. 07045).
This article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.
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