C-A-S-H Thermoelastic Properties at the Molecular and Gel Scales

Abstract

Data on C-A-S-H (calcium aluminate silicate hydrate) thermoelastic properties is a critical input for multiscale approaches aiming at understanding the thermo-mechanical behavior of cement systems with partial substitution of ordinary Portland cement with supplementary cementitious materials. Molecular simulations at finite temperature are deployed to compute the elastic constants, coefficient of thermal expansion, and heat capacity of C-A-S-H. The heat capacity is calculated following a recently proposed hybrid approach well-suited for hydrated minerals. With this approach, quantum corrections are added for the dry mineral, whereas confined water heat capacity is accounted for through full classical computations. The coefficient of thermal expansion of C-A-S-H is estimated for the first time. Using homogenization theory, the effective properties are computed at the gel scale as a function of the packing density using as input the properties computed at the molecular scale. Our results show good agreement with available experimental data.

Thermal properties of ASR products

“…Identified bands (I-V) in the VDOS correspond to various vibrations: Na-O, K-O, Si-O, and Ca-O vibrations for Band I, Si-O and O-H vibrations for Bands II and III, and intramolecular water vibrations for Bands IV and V. Hydrogen vibrations play a significant role in all bands, primarily contributing to Band V. Between bands II and IV and IV and V, the VDOS vanishes. These findings align with experimental spectra from other silicates, including C-(A)-S-H [16,31,36,63].…”

“…The estimate above is semi-classical since the VDOS is calculated using classical methods, but quantum corrections are introduced. The same approach was successfully used for the heat capacity of various solid phases in cement-based materials [31,36,[38][39][40].…”

“…These new results show that thermal expansion is positive for all Cartesian directions, indicating that the previous results for Na-Shlykovite are likely related to simulation imprecision. These values can be compared with the volumetric thermal expansion coefficients reported for C-(A)-S-H: 1.2-6.2 × 10 −5 /K according to various C-(A)-S-H models and Ca/Si ratios [31,36,60,61], since ASR products are also layered calcium silicate hydrates. Siliceous aggregates, clinker minerals and hdyrates such as portlandite and AF-phases have volumetric thermal expansion in the range of 2-4 × 10 −5 /K [62].…”

“…No experimental data, theoretical estimates, or empirical calculations have been proposed for the thermal conductivity and thermal expansion of all ASR products, particularly nanocrystalline and amorphous products. Molecular dynamics (MD) simulations have been successful in providing missing data on thermal properties of cement systems, including C-(A)-S-H [31,36], portlandite [31,37], clinker minerals [31,38,39], and AF-phases [40,41]. Thermal conductivity, thermal expansion, and heat capacity have been calculated for these phases.…”