A dehydration kinetic model of calcium silicate hydrates at high temperature

Abstract

Dehydration of calcium silicate hydrates, the primary binding phase of cement paste, contributes significantly to the degradation of cementitious materials at high temperatures. Kinetics of the dehydration process is usually parameterized as the product of the classic Arrhenius equation and a kinetic expression, calibrating the influence of the temperature and the reaction degree, respectively. By means of the Exp-function approximation, an improved kinetic expression is proposed and the differential equation is solved by utilizing the Mean-value Theorem for integrals. The established dehydration model is validated by comparing with independent experimental measurements, concerning different heating regime and molar ratios of calcium-to-silicon. Both the temperature history and the calcium-to-silicon molar ratios influence the kinetics and the final status of the dehydration process. The established model provides access to further investigation of the microstructural evolution of heated concrete, containing supplementary cementitious materials.

Transient Nonlinear Heat Conduction in Concrete Structures: A Semi-Analytical Approach

“…The initial temperature is set equal to and the sudden temperature increase is set as , representing a moderate fire load [ 7 , 41 ]. As the investigated temperature is below , the dehydration process is not stimulated [ 5 , 8 ]. Therefore, both the mass density and the specific heat capacity are considered to be constants for normal concrete, which follows [ 9 ] as respectively.…”

“…Concrete structures are frequently subjected to daily temperature cycles [ 1 , 2 ], sometimes also to extreme weather events such as hail showers [ 3 , 4 ], and occasionally to accidental scenarios such as fire disasters [ 5 , 6 ]. The resulting thermomechanical loading raises not only macroscopic and microscopic thermal stresses [ 7 ] but also thermal decomposition of materials [ 8 ], which is a challenge to the safety, service performance, and long-term durability of concrete structures.…”

“…For example, following the simplified “ isotherm method” recommended by Eurocode 2 [ 9 ], concrete with a temperature over is ignored, while that with a temperature below is considered to maintain its full strength, in quantification of the bearing capacity of concrete members. More sophisticated prediction of the structural performance is related to calibration of the thermal degradation of concrete materials [ 8 , 10 ] and thermal stresses [ 11 , 12 ], as a result of the temporal and spatial evolution of temperature fields. Furthermore, the heat transfer process is also influenced by the mass transfer [ 13 , 14 ], as a result of vaporization and dehydration, and the cracking [ 15 , 16 ], as a result of mechanical response.…”

“…Both the mass density and the thermal conductivity of concrete decrease with increasing temperature, for example, the empirical functions of these properties recommended by [ 9 ]. This is attributed to the water loss [ 5 ] as a result of evaporation and dehydration [ 8 ] when heated. Apart from temperature, the thermal conductivity of concrete significantly depends on the moisture content, aggregate type, and measurement technique, leading to a fluctuation of experimental results reported in literature [ 13 , 20 ].…”