International Summit on Cement Hydration Kinetics

Outcomes of the summit activity include a series of seven papers and a Roadmap for short-term research in the area of cement hydration kinetics.  The following papers:

Bullard, J. W., H. M. Jennings, R. A. Livingston, A. Nonat, G. W. Scherer, J. S. Schweitzer and K. L. Scrivener (2010), Mechanisms of Cement Hydration at Early Ages, Cem. Concr. Res., submitted.

Cheung, J., L. Roberts, A. Jeknavorian and D. Silva (2010), The Impact of Admixtures on Hydration Kinetics, Cem. Concr. Res., submitted.

Luttge, A. (2010), Experimental Methodologies for Investigating Cement Hydration Kinetics, Cem. Concr. Res. Cem. Concr. Res., in preparation.

Juenger, M., F. Winnefeld, J. Provis and J. Ideker (2010), Advances in Alternative Cementitious Binders, Cem. Concr. Res., submitted.

Scrivener, K., and B. Lothenbach (2010), The Effect of Supplemental Cementitious Materials on Hydration Kinetics, Cem. Concr. Res., in preparation.

Thomas, J. J., J. J. Biernacki, J. W. Bullard, S. Bishnoi, J. S. Dolado, G. W. Scherer and A. Luttge (2010), Modeling and Simulation of Cement Hydration and Microstructure Development, Cem. Concr. Res., submitted.

Xie, T., and J. J. Biernacki (2010), The Origin and Evolution of Cement Hydration Models, Comp. Concr., submitted.

were used as the basis upon which the Raodmap was developed.  Their extensive review of each of the six primary themes of the Summit, mechanisms, modeling, experimental techniques, alternative cements, supplemental cementitious materials and admixtures, represent the state-of-knowledge in the field.  The Roadmap, entitled, “Paving the Way for a More Sustainable Concrete Infrastructure – A Roadmap for the Development of a Comprehensive Description of Cement Hydration Dynamics,” is not as much a summary of these papers, but rather a synthesis of them into a guide for short-term research on cement hydration kinetics.

 The following excerpt from the Roadmap includes the abstract and the concluding remarks which amount to a list of research needs.  Please consider offering input prior to final publication of this document since we plan to submit it to a publisher by the end of September.

 

Paving the Way for a More Sustainable Concrete Infrastructure – A Roadmap for the Development of a Comprehensive Description of Cement Hydration Dynamics

 J. J. Biernacki¹, J. W. Bullard²

¹Tennessee Technological University, Cookeville, TN; jbiernacki@tntech.edu

²National Institute of Standards and Technology, Gaithersburg, MD; bullard@nist.gov

 Abstract

Recent advances in both experimental and computational technology are providing unprecedented insights into the nature of cement hydration.  While a comprehensive theory appears to be a decade or more off, the recent progress suggests that what were once thought to be the most elusive hurdles are now within reach.  Although the path remains uncertain, a number of simulation platforms are now available along with emerging modeling strategies that could provide multi-scale linkages for the development of engineering models and computational research tools.  Similarly, new and existing experimental strategies are yet to be fully exploited but are positioned to offer breakthroughs.  Ultimately, a more coordinated effort needs to be assembled that enables research teams to focus on the task of developing a comprehensive description of cement hydration rather than work on isolated tasks.

 This report is not intended to be a scholarly review on hydration kinetics, but rather a summary that articulates what appears to be some of the most important yet unknown aspects of cement hydration that obscure our ability to quantitatively describe the processes and assemble a predictive mathematical model.  The summary herein is a companion to a series of recent detailed reviews.

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Concluding Remarks

This “roadmap” for understanding the dynamics of cement hydration was developed to provide the research community with focal points for directing immediate and short-term research to be conducted within the next three to five year.  Among the most pressing issues at hand are those that will impact on the design of cementitious materials and systems that lead to small carbon footprints and hence improved life cycle performance.  Although significant progress has been made in the past ten years towards development of modeling platforms for cement hydration, thus far they are mostly limited to hydration of neat C₃S (C₃S in the absence of organic or inorganic admixtures or other cement or mineral phases) and even then a unique model is not yet available that describes the range of observed behaviors.  Furthermore, the body of existing experimental information is sometime contradictory or interpretable using more than one hypothesis.  In the absence of consensus and in some cases lack of detailed mechanistic information, near term development of advanced engineered materials, including modified and new cementitious systems, optimized utilization of waste and by-product materials and discovery of new admixtures, will continue at the present slow and costly pace.  The research needs identified by Summit participants are formulated here as either questions or statements.  The list of needs is not intended to answer the questions, but rather to pose them.  Nor does the list represent a proposed solution but rather are supposed elements of the solution.  Recognizing also that the topical areas discussed above are not isolated one-from-the-other, but rather that they are co-dependent, the following list is suggested in an effort to “pave the way for a more sustainable concrete infrastructure via the development of a comprehensive description of cement hydration dynamics:”

Hydration Mechanisms

1. Does a barrier layer actually form?
2. If a barrier layer forms, is it thermodynamically metastable or is it mechanically destabilized at the end of the induction period?
3. Does some form of hydroxylated surface form and does it play a role in controlling C₃S solubility and does this hypothesis replace that of the barrier layer conjecture?
4. What are the transport properties of the bulk C-S-H products formed and how do they evolve with time?
5. Does C-S-H form by a two-stage growth process and what is the bulk density of C-S-H as a function of time?
6. When and where do C-S-H nuclei form and what is the formation rate?
7. What are the form of the rate laws, e.g. what is the reaction order?
8. Along with (7), what are the elementary reactions that control the reaction rates, not only at early age, but at any age?
9. What is the actual morphology of C-S-H growth and what controls the morphology since many forms have been observed?
10. Are there signatures in early-age calorimetry measurement that indicate long-term kinetics and performance?

Modeling and Simulation

1. Continue to develop various solution phase driven models that incorporate kinetics, thermochemistry and transport phenomena.
2. Develop multiple modeling paths and strategies that corroborate findings and lead towards useful engineering tools as well as model-based research instruments including fast algorithms for PC and similar platforms.
3. Continue to extend and exploit computations resources as necessary and needed to accommodate changing needs, i.e. utilize massively parallel processing and super computer facilities as needed.
4. Consider alternative computational strategies to accelerate the development of rigorous models, i.e. fast single particle models, representative volume approaches, etc.
5. Exploit the body of knowledge on true multi-scale modeling.
6. Improve the dissemination of modeling tools to promote their use and development.
7. Incorporate more molecular-level modeling strategies, i.e. kinetic Monte Carlo, etc.
8. Develop suitable structural analogs for various anhydrous and hydrated cement phases for use in molecular modeling.
9. Develop focused experimental program driven in part by model development and designed to provide information for parameter estimation and to answer mechanistic questions.  Specific questions that must be addressed experimentally and within the construct of existing and new models to be developed are included in Section III (Mechanisms) above.

Experimental Techniques

1. Extend as necessary and apply the vertical scanning interferometry (VSI) technique in an attempt to answer at least a portion of the questions regarding mechanisms.
2. Further develop x-ray nano tomography into a quantitative technique and apply it to study the rate of cement phase reaction in both model systems and portland cements and for blended systems containing silica fume, blast furnace slag and fly ash.
3. Further explore the use of nuclear resonance reaction analysis (NRRA) as a tool for elucidating the barrier layer hypothesis.
4. At this time, broadband time-domain-reflectrometry (BTDR) dielectric spectroscopy (DS) is a rather elusive technique that needs to be made generally available to the community at large.  The datasets coming out of the single laboratory where the experiments are being conducted also need to be made generally available to the modeling community or need to be integrated with a modeling effort in an attempt to extract mechanistic kinetic information.
5. Establish an open network with researchers in the broader community, both those doing modeling and experimentation, so that they have access to datasets and instrument time on unique tools such as VSI, nano x-ray tomography, NRRA and TDDS.

Admixtures

1. Identify the elementary steps (reactions) for classic hydration acceleration and retardation, e.g. for CaCl₂ and sucrose activity respectively.
2. Isolate and identify physiochemical interactions with various cement surfaces and quantify the rate controlling processes, i.e. rate of adsorption of neat admixture chemical A.
3. Isolate and identify chemical interactions with various cementitious ionic species, i.e. rate of solution phase chelation of Ca⁺² by sucrose.
4. Design experiments explicitly to be used for kinetic model development with the objective of having quantitative outcomes.

Supplemental Cementitious Materials

1. Isolating the rate of reaction of the SCM or SCM phases is generally very difficult.  Most reactive SCM’s are amorphous rather than crystalline and some, i.e. fly ash, contain more than one reactive constituent, that is, may contain more than one reactive glassy phase each having its own reactivity.
2. Characterization of the microstructure of even the parent SCM can be difficult , for materials such as fly ash which, as stated above, may contain more than one reactive glassy phase.
3. So called “filler effects” are difficult to separate from nucleation related effects since both may have similar apparent outcomes, i.e. slightly alter the size and location of the primary calorimetry peak (Stage 3 and 4).
4. Generalized solubility models for the range of glassy phases are not readily available.

 Alternative Cements

1. While there are kinetic datasets for the various classes of cements, there is a general lack of a cohesive unified theory for the common cement forms, e.g. those that are indirectly derived from the portland family of anhydrous crystalline cements and those derived predominantly from glassy raw materials and requiring high alkali content activator solutions.
2. Although the kinetic processes share features in common with those of C₃S-based cements, it seems that side-by-side studies of these features have not been conducted.
3. There is a general lack of information regarding long-term durability for many classes of alternative cements.  While somewhat outside of the scope of this roadmap is should be acknowledge that such is an obstacle in path for more widespread development and use of such materials.
4. There are a number of economic hurdles at this time, including the use of bauxite as a raw materials and the high cost of alkali agents, which if relieved by further development or materials engineering might reduce production cost and enable the introduction of alternative cements into various markets where they are presently not economically viable.

 General Comments

1. There is a general lack of resource organization and dissemination of tools for modeling cement hydration.  A National resource for hydration data should be considered wherein a database of computer models, thermophysical properties (thermodynamic datasets and thermodynamic models), crystallographic information files (CIF), kinetic datasets, models and modeling tools and their associated source codes, etc., can be easily accessed by the research community at large.  Huge amounts of time are spent by research teams searching for, reviewing and assembling such information independently.
2. There is presently no focal point for hydration research in the US, there needs to be.  Concrete is the primary building materials for the world’s infrastructure and the US must continue to remain competitive and be a global leader in concrete materials technology.  The lack of a generalized, universal theory governing chemical transformations, microstructure development and properties of complex hydrate synthetic mineral-based materials impedes the pace of development.
3. It appears that some alternative cement systems exhibit kinetic features that are like those of the C₃S-based portland cement system, including behaviors such as an early dissolution peak, dormancy and a main hydration peak, i.e. supesulfated cements and calcium sulfoaluminate cements.  While this is well known among the community of researchers, it might be beneficial to study such systems side-by-side in an effort to resolve common or dissimilar features that could lead to a more refilled and clarified mechanistic theory of cement hydration.