Thermodynamic properties of Portland cement hydrates in the system CaO–Al2O3–SiO2–CaSO4–CaCO3–H2O
Introduction
One of the unsolved problems in the application of Portland cement is to quantify the performance lifetimes of concrete constructions. Quantification is important to evaluate the performance of nuclear waste containments and, increasingly, long-lived infrastructure developments, where quantification has failed to keep pace with the expectation of stakeholders.
Although we have a wealth of empirical evidence on the performance of historic concretes, information on their formulation, emplacement and exposure history is often incomplete and, moreover, the nature of cements supplied today will almost certainly have changed since the original construction. Empirical studies and historic examples have however yielded much useful qualitative information on the aggressivity of various service environments. Numerous tests and test methods have been used as indicators of durability but they do not yield generic conclusions and their predictive capabilities are limited. As a consequence, designers of long-lived constructions have at present to rely on perceived wisdom, as interpreted by experts and incorporated into codes of practice.
The changing nature of cements is also of concern. Cement producers are under pressure to lower the specific energy requirements of cement production and reduce gaseous emissions. These goals are presently addressed by a combination of methods; partly by optimisation of process technology, including the use of alternative fuels and raw materials (the impacts of which are beyond the scope of this study), and partly by reliance on supplementary cement materials to lessen the need for energy-rich cement. Although the use of supplementary materials is generally regarded as beneficial in terms of strength and durability as, for example, highlighted by developments in cement and concrete standards, long term performance is not fully understood. Supplementary materials presently in use include industrial by-products such as slag, fly ash, silica fume, etc. as well as natural materials such as ground limestone, natural pozzolanic and semi-synthetic pozzolans such as metakaolin. Each of these materials has a complex but distinctive chemistry, mineralogy and granulometry. Moreover, each type of material ranges in composition and performance. Studies of their behaviour in blended cements under controlled conditions are confined to selected compositions and short term (1–5 years) laboratory measurements, perhaps supplemented by observations on actual constructions, for which conditions may not be well controlled.
The complexity of blended cement systems and the wide-ranging nature of supplementary cementing materials has meant that guesses – sometimes well informed – have to be made at the outset about what aspects of behaviour should be studied. But the number and complexity of the resulting systems are such that results are often confined to measurement of a few of the many parameters affecting performance. Arguably the most serious question arising from the results of empirical testing is how to extend or extrapolate the results to other compositions and formulations, or to conditions other than those measured, or both. At present we cannot address these issues, except qualitatively.
If quantification of performance is to be achieved, a new paradigm is needed and a key to the development of a successful paradigm must be to concentrate on generic approaches. Thermodynamics provides a consistent framework for the analysis of complex systems. Given an adequate database to support calculations, its strength lies in its generic nature; user-defined compositions and conditions can be selected for calculation. This realisation is not new although previous attempts to apply thermodynamics have had only limited success owing partly to deficiencies in the database to enable calculations. In this presentation we concentrate on development of the database.
From an industrial point of view it could be argued that thermodynamic approaches to cement durability are too theoretical and the calculations too difficult to perform. However, the latter is no longer true: geochemists, faced by similar problems of treating complex systems, have developed and validated computer codes capable of being implemented on a PC. Many reliable code packages are available in the public domain. Furthermore codes can be coupled to fluid mass transport modelling modules; in this case, thermodynamic datasets supply important physical properties, for example, molar volumes of solid and liquid phases including solid solutions.
Thermodynamics is most readily applied to isochemical systems, i.e., to systems having a constant composition, whereas many cement deterioration reactions involve transport of species into or out of the matrix (or both). In computer-based calculations, such processes also enable more complex conditions to be imposed on the system.
We do not argue that the primary output achieved by application of thermodynamic methods necessarily enables the durability and performance of cements and concretes to be quantified. But we argue that, in the search for quantification, a sound understanding of cement paste mineralogy, and of the ability to calculate features and processes arising from the interaction of cement with potentially aggressive agents introduced from the environment, together with additional possibilities for calculating physical functions and the introduction of kinetic variables, constitutes a great step forward. Other necessary links to develop integrated models of cement performance will be anticipated in discussion.
Section snippets
Historical development of cement databases of thermodynamic properties
A thermodynamic database will include many substances for which standard compilations already provide adequate data: it is not necessary to start totally afresh. For example, the thermodynamic properties of water and of many aqueous ions and complexes are well known. Database development focussing on cements is therefore mainly concerned with the properties of solids that are abundant in cements but uncommon or absent in nature. Thus a comprehensive database can be compiled by focussing on
Experimental
A focussed programme of data acquisition was undertaken. This involved synthesis, characterisation, analysis and data integration. New data were obtained through synthesis of phase pure substances with subsequent solubility measurements.
Software and standard databases
Chemical thermodynamic modelling consists of calculating the chemical speciation (i.e. amounts or concentrations of chemical components in all phases present in equilibrium state) from total bulk composition of the system and thermodynamic data for components. In the GEM (Gibbs free energy minimisation) method and GEMS-PSI code [18], the total Gibbs energy of the system is minimised at given temperature and pressure; accordingly, for each component, the standard molar Gibbs energy at the
Results
Table 2 summarises all thermodynamic data obtained in this study as well as thermodynamic data of supplementary phases needed to estimate the data of the cement hydrates. The following paragraphs explain in detail the determination of standard molar thermodynamic properties of the individual cement hydrates.
Data accuracy
The data for individual substances vary in quality. The reasons for this have been developed in the text but it is still difficult to establish error limits. Firstly, the quality of synthesis is variable: it is difficult if not impossible to establish the absolute phase purity of the preparations. Only the use of single crystals would enable a high confidence in the absolute purity of the preparations used. Secondly, the substances themselves vary in crystallinity with as yet unknown
Nomenclature in cement chemistry
- C
CaO
- A
Al2O3
- S
SiO2
- s
SO3
- c
CO2
- H
H2O
- A
Debye–Hűckel solvent parameters dependent on the dielectric constant of water and temperature (A = 0.5114 at 25 °C)
- αi
parameter dependent on the size of species, i, taken from Kielland's table (cited in [24])
- B
Debye-Hűckel solvent parameters dependent on the dielectric constant of water and temperature (B = 0.3288 at 25 °C)
- bi
common extended Debye–Hűckel parameter (bi ∼ 0.064 at 25 °C)
- Cp0
standard molar heat capacity of species at T, P (J K− 1 mol− 1)
- ΔrCpT0
standard molar heat
Other abbreviations used in calculations
Acknowledgement
The support of NANOCEM, a consortium of European cement producers and academic institutions is acknowledged. The project advisors Ellis Gartner, Lafarge Cement France, Duncan Herfort, Aalborg Cement Denmark and Karen Scrivener, EPFL Lausanne, have contributed to and encouraged our approach. Finally, the manuscript was critically reviewed by Dimitrii Kulik, PSI Switzerland, who gave valuable advise to the use of GEMS-PSI in the course of this project and John Gisby, NPL UK. Their constructive
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