Modeling the instantaneous phase composition of cement pastes under elevated temperatures

doi:10.1016/j.cemconres.2020.105987

Cement and Concrete Research

Volume 130, April 2020, 105987

https://doi.org/10.1016/j.cemconres.2020.105987 Get rights and content

Abstract

This paper models the instantaneous phase composition of cement pastes under elevated temperatures (up to 1200 °C). First, a hydration model was introduced. Then kinetics models for dehydration of cement hydrates, including C-S-H, CH and aluminate hydrates, and evaporation of free water were proposed. Subsequently, based on the kinetics models, the thermal decomposition model for cement pastes was proposed, which could predict the solid phase, porosity and water compositions of cement pastes under arbitrary temperature history. The thermal decomposition model was then validated by multiple experiments. Finally, a parametric study was conducted to investigate the effects of heating rate on the instantaneous phase composition of cement pastes. It was found that the phase compositions of cement pastes at the same temperature differ greatly from one another among different heating rate cases. This demonstrates the needs to consider temperature histories when analyzing real-world concrete structures under fires.

Introduction

Concrete undergoes complex coupled multiphase thermal, hydraulic, mechanical and chemical (THMC) processes when subjected to high temperatures caused by accidental fires. These coupled processes have drawn significant attention because they can lead to (explosive) spalling of concrete materials [1] and can cause additional loads on concrete members/structures [2]. To better understand concrete behavior under high temperatures, two different groups of experiments have been widely conducted. The first group focused on concrete under steady-state or pseudo steady-state (with very low heating rates) high temperatures, which measured physical and mechanical properties, including free thermal strains (or coefficient of thermal expansion (CTE)) [[3], [4], [5]], load-induced thermal strains (LITS) [[3], [4], [5]], strength [6,7], elastic modulus [7], specific heat [8,9], thermal conductivity [[9], [10], [11]], permeability [[12], [13], [14]], diffusivity [15], etc., for concrete at constant high temperatures. The second group focused on concrete under continuously elevated temperatures, which simultaneously reported time- and space-variant temperatures and pore pressures [[16], [17], [18], [19], [20], [21], [22], [23]]. Based on temperature-dependent mathematical models for physical and mechanical properties that were developed from, or calibrated with, experimental results from the first group, many researchers have developed thermo-hydraulic [[24], [25], [26], [27], [28]], thermo-mechanical [29] and thermo-hydro-mechanical [[30], [31], [32], [33], [34], [35]] models for concrete under elevated temperatures, which were compared with experimental results from the second group for verification. However, in realistic fire events, concrete usually undergoes rapidly rising temperatures so that the permissible chemical and physical changes of concrete at each elevated temperature do not have enough time to finish. Accordingly, at the same elevated temperature, concretes with different previous temperature history will have different phase compositions and hence different physical and mechanical properties.

To eliminate the inconsistency between steady-state physical and mechanical properties and non-steady-state thermal, hydraulic and mechanical processes, the chemical process should be incorporated; i.e., the fully coupled THMC model should be established for concrete under elevated temperatures, as illustrated by Fig. 1. With the previous temperature history, the thermal decomposition model predicts the instantaneous phase composition of cement paste at a certain time t. Then based on the instantaneous phase composition of cement paste, the physical and mechanical property models predict the instantaneous physical and mechanical properties for heated concrete. Subsequently, with the instantaneous physical and mechanical properties, the thermo-hydro-mechanical model predicts the temperature, pore pressure, stress and strain responses in concrete during the time interval Δt. Hence, through repeating the abovementioned steps, the thermo-hygro-mechanical model predicts the responses during the entire heating process. In such a way, the chemical process is coupled with thermal, hydraulic and mechanical processes, yielding a fully coupled THMC model for concrete under elevated temperatures, as illustrated by Fig. 1.

As highlighted by the dashed box in Fig. 1, the thermal decomposition model for cement pastes is a prerequisite for the fully coupled THMC model for concrete under elevated temperatures. Both Ulm et al. [36] and Pont & Ehrlacher [37] proposed variables or parameters describing dehydration degree evolution in their respective thermo-chemo-mechanical and thermo-hydro-chemical models for concrete under elevated temperatures. However, these variables or parameters considered the dehydration of cement paste as a whole, which did not distinguish that different cement hydrates had different behavior of dehydration kinetics. Zhao et al. [38] proposed a thermal decomposition model which considered the dehydrations of C-S-H, CH and ettringite, separately. But Zhao et al. ignored dehydrations of other aluminate hydrates and assumed dehydration of C-S-H to start at 600 °C which was not consistent with the experimental results done by Harmathy [9] and Zhang & Ye [39].

In this paper, a thermal decomposition model for cement paste was proposed, which combined the hydration model developed by Papadakis et al. [40,41] (Section 2), dehydration kinetics of C-S-H, CH and aluminate hydrates (Section 3), evaporation kinetics of free water (Section 3), and calculation formulas for the instantaneous solid phase, porosity and water compositions under arbitrary temperature history (Section 4). The proposed thermal decomposition model was compared with multiple experiments collected from the literature for validation (Section 5). Subsequently, a parametric study was conducted to investigate the effects of heating rate on the instantaneous phase composition of cement pastes under elevated temperatures (Section 6). Finally, several conclusions were drawn (Section 7).

Section snippets

Hydration model

Before analyzing dehydration kinetics of cement paste under elevated temperatures, it is necessary to know the phase composition of the hydrated cement paste. In this paper, Papadakis et al.'s hydration kinetics model [40,41] was adopted. The stoichiometry of Papadakis et al.'s hydration model is shown by Eqs. (1), (2), (3), (4), (5), (6). ${2C}_{3} S + 6H \to C_{3} S_{2} H_{3} + 3CH$ ${2C}_{2} S + 4H \to C_{3} S_{2} H_{3} + CH$ $C_{4} AF + 2 CH + 2 C \bar{S} H_{2} + 18 H \to C_{8} AF {\bar{S}}_{2} H_{24}$ $C_{3} A + C \bar{S} H_{2} + 10 H \to C_{4} A \bar{S} H_{12}$ $C_{4} AF + 4CH + 22 H \to C_{8} {AFH}_{26}$ $C_{3} A + CH + 12 H \to C_{4} {AH}_{13}$

The right-hand sides of Eqs. (1),

Stoichiometry of dehydration reactions and free water evaporation

As Papadakis et al.'s hydration model was used, the dehydration of a cement paste resides in dehydration of hydrates in the right-hand sides of Eqs. (1), (2), (3), (4), (5), (6). These hydrates are assumed to decompose following Eqs. (7), (8), (9), (10), (11), (12). $C_{3} S_{2} H_{3} \overset{T_{onset} = 110 ° C}{\to} C_{3} S_{2} + 3H (g)$ $CH \overset{T_{onset} = 430 ° C}{\to} C + H (g)$ $C_{8} AF {\bar{S}}_{2} H_{24} \overset{T_{onset} = 40 / 200 ° C}{\to} C_{4} AF + 2 CH + 2 C \bar{S} + 22 H (g)$ $C_{4} A \bar{S} H_{12} \overset{T_{onset} = 40 / 200 ° C}{\to} C_{3} A + C \bar{S} + 12 H (g)$ $C_{8} {AFH}_{26} \overset{T_{onset} = 70 ° C}{\to} C_{4} AF + 4CH + 22 H (g)$ $C_{4} {AH}_{13} \overset{T_{onset} = 200 ° C}{\to} C_{3} A + CH + 12 H (g)$

Dehydrations of aluminate hydrates (Eqs. (9),

Modeling the thermal decomposition of cement paste

Through combining the hydration model (Section 2), dehydration and evaporation kinetics models (Section 3), the thermal decomposition model of cement paste can be developed as illustrated in Fig. 5. The thermal decomposition model is able to predict the time-varying molar and mass concentrations (see Appendix A for details) and time-varying weight and volume fractions (see Appendix B for details) for the constituents in a unit volume of cement paste that is uniformly heated at arbitrary rates.

Validation of the thermal decomposition model of cement paste

Two different groups of tests collected from the literature are chosen to validate the thermal decomposition model. One group conducted thermogravimetry (TG) tests on ordinary Portland cement pastes, which yielded the residual mass of cement pastes under elevated temperatures. The other group carried out mercury intrusion porosimetry (MIP) tests on ordinary Portland cement pastes after high temperature exposure, which reported porosities of cement pastes after heating and cooling.

Basic information

A parametric study is designed and conducted to explore the effects of heating rate (HR) on the instantaneous phase compositions of cement pastes subjected to elevated temperatures. The cement pastes are assumed to be made up of ordinary Portland cement with water-cement ratio 0.5. The mineral composition of the cement is taken as the same as that used by Monteagudo et al. [49] shown in Table 3. The cement pastes are supposed to be cured at 95% relative humidity and 20 °C for 28 days. The

Conclusions

This paper first introduced a hydration model and then proposed kinetics models for dehydration of hydrates and evaporation of free water in hardened cement pastes. Based on the kinetics models, the thermal decomposition model for cement pastes was established, which could predict the instantaneous phase composition of cement pastes under arbitrary temperature history (up to 1200 °C). The thermal decomposition model was validated by multiple experiments collected from the literature. Based on

CRediT authorship contribution statement

Chao Jiang: Writing-original draft, Conceptualization, Methodology, Software, Investigation, Funding acquisition. Jing Fang: Validation. Jun-Yu Chen: Software. Xiang-Lin Gu: Funding acquisition, Supervision.

Declaration of competing interest

None.

Acknowledgements

This research work was supported by the National Natural Science Foundation of China (Grant No. 51908417). The first author appreciates the financial support of the International Postdoctoral Exchange Fellowship Program 2017 issued by the Office of China Postdoctoral Council (Approval No.: 32nd Doc. Of OCPC, 2017). The authors appreciate the great help and support from Prof. Yunping Xi and his excellent graduate students in Department of Civil, Environmental and Architectural Engineering at

References (58)

M. Petkovski et al.
Strains under transient hygro-thermal states in concrete loaded in multiaxial compression and heated to 250 °C
Cem. Concr. Res.
(2008)
Z. Xing et al.
Influence of the nature of aggregates on the behaviour of concrete subjected to elevated temperature
Cem. Concr. Res.
(2011)
R. Cerný et al.
Effect of compressive stress on thermal and hygric properties of Portland cement mortar in wide temperature and moisture ranges
Cem. Concr. Res.
(2000)
P. Kalifa et al.
High-temperature behavior of HPC with polypropylene fibers from spalling to microstructure
Cem. Concr. Res.
(2000)
Y. Pei et al.
The effects of high temperature heating on the gas permeability and porosity of a cementitious material
Cem. Concr. Res.
(2017)
Y. Shen et al.
Experimental study of the gas diffusion coefficient of heated concrete
Constr. Build. Mater.
(2018)
F. Lo Monte et al.
Ground-penetrating radar monitoring of concrete at high temperature
Constr. Build. Mater.
(2017)
P. Kalifa et al.
Spalling and pore pressure in HPC at high temperatures
Cem. Concr. Res.
(2000)
M.R. Bangi et al.
Effect of fibre type and geometry on maximum pore pressures in fibre-reinforced high strength concrete at elevated temperatures
Cem. Concr. Res.
(2012)
R. Felicetti et al.
A new test method to study the influence of pore pressure on fracture behaviour of concrete during heating
Cem. Concr. Res.
(2017)

N. Toropovs et al.

Real-time measurements of temperature, pressure and moisture profiles in high-performance concrete exposed to high temperatures during neutron radiography imaging

Cem. Concr. Res.

(2015)

J.C. Mindeguia et al.

Temperature, pore pressure and mass variation of concrete subjected to high temperature - experimental and numerical discussion on spalling risk

Cem. Concr. Res.

(2010)

J.H. Chung et al.

Numerical modeling of transport phenomena in reinforced concrete exposed to elevated temperatures

Cem. Concr. Res.

(2005)

M.B. Dwaikat et al.

Hydrothermal model for predicting fire-induced spalling in concrete structural systems

Fire Saf. J.

(2009)

M. Beneš et al.

Hygro-thermo-mechanical analysis of spalling in concrete walls at high temperatures as a moving boundary problem

Int. J. Heat Mass Transf.

(2015)

D. Gawin et al.

What physical phenomena can be neglected when modelling concrete at high temperature? A comparative study. Part 2: comparison between models

Int. J. Solids Struct.

(2011)

D. Gawin et al.

What physical phenomena can be neglected when modelling concrete at high temperature? A comparative study. Part 1: physical phenomena and mathematical model

Int. J. Solids Struct.

(2011)

R. Tenchev et al.

An application of a damage constitutive model to concrete at high temperature and prediction of spalling

Int. J. Solids Struct.

(2005)

S. Dal Pont et al.

Numerical and experimental analysis of chemical dehydration, heat and mass transfers in a concrete hollow cylinder submitted to high temperatures

Int. J. Heat Mass Transf.

(2004)

J. Pourchez et al.

Kinetic modelling of the thermal decomposition of ettringite into metaettringite

Cem. Concr. Res.

(2006)

A. Morandeau et al.

Investigation of the carbonation mechanism of CH and C-S-H in terms of kinetics, microstructure changes and moisture properties

Cem. Concr. Res.

(2014)

S.M. Monteagudo et al.

The degree of hydration assessment of blended cement pastes by differential thermal and thermogravimetric analysis. Morphological evolution of the solid phases

Thermochim. Acta

(2014)

S.C. Kou et al.

Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures

Cem. Concr. Compos.

(2014)

A.V. Soin et al.

A combined QXRD/TG method to quantify the phase composition of hydrated Portland cements

Cem. Concr. Res.

(2013)

G.H.A. Van Der Heijden et al.

Fire spalling of concrete, as studied by NMR

Cem. Concr. Res.

(2012)

V.K.R. Kodur et al.

Effect of high-temperature transient creep on response of reinforced concrete columns in fire

Mater. Struct.

(2017)

G.A. Khoury et al.

Transient thermal strain of concrete: literature review, conditions within specimen and behaviour of individual constituents

Mag. Concr. Res.

(1985)

D. Cree et al.

Thermal behaviour of unstressed and stressed high strength concrete containing polypropylene fibers at elevated temperature

J. Struct. Fire Eng.

(2017)

E.S.S. Lam et al.

Direct tensile behavior of normal-strength concrete at elevated temperatures

ACI Mater. J.

(2014)

Cited by (23)

Preference of curing regimes based on fire resistance of ultra-high performance cementitious materials (UHPCM) mixed with fly ash
2024, Journal of Building Engineering
The cement-fly ash (FA) binary system was prepared as the ultra-high performance cementitious materials (UHPCM), and the effect of curing regimes on the evolution of microstructure and compressive strength of UHPCM at elevated temperatures was investigated. The results showed that multi-stage combined curing (MC) could remarkably improve the compressive strength of UHPCM at ambient temperature, which attributed to the further hydration of cement and the pozzolanic reaction of FA to produce more stable hydrates. However, steam curing (90 °C for 7 d) was more conducive to the residual compressive strength growth and retention of UHPCM exposed to elevated temperatures. The mechanism of this phenomenon was revealed by XRD and SEM analysis. The further hydration and conversion reactions occurred mainly inside the steam cured specimens when exposed to fire temperature, and a large number of C–S–H gels were gradually transformed into crystalline hydrates with high strength, excellent thermal stability and different forms, which made the microstructure denser and thus had better fire resistance. Comparatively, a large number of crystalline hydrates were generated prematurely in dry-hot air cured specimens, which made the decomposition reactions dominate under fire, resulting in coarsening of microstructure and reduction of macroscopic mechanical strength of UHPCM continuously.
Experimental study on thermo-hydro-mechanical responses of concrete plates subjected to in-plane compressive loading and one-side heating
2024, Fire Safety Journal
This paper investigated experimentally the thermo-hydro-mechanical responses of pre-dried and saturated concrete plates subjected to in-plane compression and one-side heating. Three concrete plates for each of three grades were designed: one was saturated and then heated; one was saturated, loaded and then heated; the remaining one was pre-dried, loaded and then heated. During the heating and cooling process, the force, strain, temperature, pore pressure and cracking responses were measured and analyzed. Test results showed that compressive loads could increase fire spalling risks of saturated concrete by inducing principal cracks parallel to the exposed surface. However, the indispensable contributions of moistures to the formation of principal cracks indicated by crack responses were not consistent with those indicated by measured negligible pore pressures. For the same aggregate proportion, the thermal field was hardly affected by water-cement ratio or concrete grade/strength. During the heating process, for any given initial in-plane load, thermal dilation and softening competed causing load increases or decreases, but eventually softening outweighed thermal dilation. In addition, longitudinal strain distributions evolved on average toward the less compressed direction; meanwhile, time-strain curves showed jumps toward the less compressed direction, which might be caused by the local swellings due to clogged moisture/water layers.
Thermal behaviors of cement and mortar under microwave treatment and the influencing factors: An experimental study
2024, Construction and Building Materials
Microwave-assisted breakage of cementitious materials has emerged as a promising green and low-carbon method for dismantling cementitious structures. To understand the mechanism of microwave-induced cracking in structures, it is essential to study the thermal behaviors of cement and mortar under microwave treatment. In this paper, the effects of mixture ratio, water content, and microwave power on the microwave heating characteristics of cement and mortar specimens were investigated through real-time temperature and mass monitoring. In particular, the influence of microwave power on the bursting effect of cement and mortar specimens was analysed, and the mechanism of specimen bursting was explained. The results show that moisture content, water-cement ratio, and microwave power significantly impact the heating rate and mass loss of cement and mortar during microwave treatment. Based on the heating rate, the heating process of cement and mortar under microwave treatment can be divided into three to four stages, depending on the moisture content. The primary reasons for variations in the heating rate are water evaporation and the decomposition of hydrolysis products. The thin-wall ball model based on steam pressure theory can explain the bursting of cement and mortar specimens. Our findings suggest that increasing microwave power and water content in mortar specimens can lead to microwave-induced cracking.
An improved micromechanical model for the thermal conductivity of multi-scale fiber reinforced ultra-high performance concrete under high temperatures
2023, Materials and Design
An improved micromechanical model to estimate the thermal conductivity of multi-scale fiber reinforced ultra-high performance concrete (MSFUHPC) subjected to elevated temperatures is formulated in this study. This model considers the contributions of microstructure, multiscale fibers, water/cement ratio, thermal dehydration of hydrates, thermal cracking, interfacial thermal resistance, heating rate and temperature dependent thermal conductivity of each phase. To verify the proposed model, comparisons with experimental data on MSFUHPC specimens are carried out. Accordingly, factors affecting the thermal conductivity of MSFUHPC are discussed through the proposed model. Results suggest that blending multi-scale fibers and increasing the sand content can significantly enhance the thermal conductivity of MSFUHPC. While adding cenosphere particles can reduce the thermal conductivity. Overall, the thermal conductivity of MSFUHPC exhibits a decreasing trend with temperature. Meanwhile, thermal cracking occurs beyond 400 °C can remarkably decrease the thermal conductivity of MSFUHPC. A higher interfacial thermal resistance between sands and the matrix can result in a lower thermal conductivity. It is also found that thermal conductivity mainly depends on the maximum temperature experienced rather than the heating rate.
Development of high-strength and durable coal char-based building bricks
2023, Journal of Building Engineering
Pyrolysis coal char is a byproduct of the coal industry and causes undesirable environmental issues from conventional coal combustion and gasification. This paper presents a novel approach to utilize coal char for manufacturing new and environmentally friendly building bricks. The effects of sand, silica fume (SF), superplasticizer (SP), air-entraining admixture (AE), and graphene oxide (GO) on the properties of char-based bricks, including heat of hydration, thermogravimetric and differential thermal analysis, microstructures, density, and compressive strength, are investigated. A new treatment method using hydrophobic liquid in a vacuum condition is proposed to improve the durability of char bricks, which is evaluated based on water absorption and freeze-thaw (F-T) cycles. The experimental results indicate that the new bricks with 40% char content have a good compressive strength performance of 49.2–52.5 MPa, which compares favorably to conventional clay bricks with compressive strength of 10–20 MPa. Adding AE and GO slightly increases and decreases the compressive strength at 28 days, respectively. Moreover, the proposed treatment method significantly improves the durability of char bricks. The treated char bricks exhibit a boiling water absorption of ≤2.3%, which is 90% lower than that of untreated char bricks. The untreated and treated char bricks can achieve 22 and 120 F-T cycles, respectively. Overall, this study presents a promising approach to repurpose coal char and produce sustainable building bricks with high compressive strength and durability.
The role of calcium aluminate cement in developing an efficient ultra-high performance concrete resistant to explosive spalling under high temperatures
2023, Construction and Building Materials
Conventional ordinary Portland cement (OPC)-based UHPC is vulnerable to explosive spalling under high temperatures. This study aims to classify the role of calcium aluminate cement (CAC) as the substitution for OPC in UHPC under high temperatures and finally develop an efficient UHPC without explosive spalling. It was found that CAC-based UHPC (UHPC-CAC) and OPC-based UHPC (UHPC-OPC) reached their peak strength both at 250 °C, but the peak strength (153.8 MPa) of UHPC-CAC was higher than that (137.5 MPa) of UHPC-OPC. Explosive spalling was found in UHPC-OPC after reaching 500 °C, while no spalling was observed in UHPC-CAC even at 1000 °C. The effectiveness of CAC in reducing the mechanical degradation and preventing the spalling of UHPC was attributed to three potential mechanisms: (1) CAC had less physically bound water to cause the vapor pressure in UHPC; (2) The main hydration product (i.e., calcium aluminosilicate hydrates) decomposed to a lesser extent, compared to the hydration product (i.e., calcium silicate hydrates) of OPC, as proved by X-ray diffraction (XRD) analysis, Thermogravimetric analysis (TGA), and Fourier-transform infrared (FTIR) analysis; (3) there exists pores coarsening in UHPC-CAC, as evidenced by micro-XCT, which could release the water pressure in UHPC and thus prevent the spalling at elevated temperatures.

View all citing articles on Scopus

View full text

Modeling the instantaneous phase composition of cement pastes under elevated temperatures

Abstract

Introduction

Section snippets

Hydration model

Stoichiometry of dehydration reactions and free water evaporation

Modeling the thermal decomposition of cement paste

Validation of the thermal decomposition model of cement paste

Basic information

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgements

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Constr. Build. Mater.

Constr. Build. Mater.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Cem. Concr. Res.

Fire Saf. J.

Int. J. Heat Mass Transf.

Int. J. Solids Struct.

Int. J. Solids Struct.

Int. J. Solids Struct.

Int. J. Heat Mass Transf.

Cem. Concr. Res.

Cem. Concr. Res.

Thermochim. Acta

Cem. Concr. Compos.

Cem. Concr. Res.

Fire spalling of concrete, as studied by NMR

Cem. Concr. Res.

Effect of high-temperature transient creep on response of reinforced concrete columns in fire

Mater. Struct.

Transient thermal strain of concrete: literature review, conditions within specimen and behaviour of individual constituents

Mag. Concr. Res.

Thermal behaviour of unstressed and stressed high strength concrete containing polypropylene fibers at elevated temperature

J. Struct. Fire Eng.

Direct tensile behavior of normal-strength concrete at elevated temperatures

ACI Mater. J.