Carbonation Behaviour of Low-Clinker Cements

Date This project started on 01 September 2012

Status This project is Ongoing

Authors
University of Leeds, UK
IFSTTAR, FR
Lund University, SE
ZAG, SI
CSIC, SP
Objectives

The ongoing trend towards lower clinker contents in cements is welcome from an environmental perspective, but it presents problems with regards to our understanding of material performance. In many instances the potential technological benefits of using composite cements is compromised by the refusal of contractors to recognise the need for adequate curing. Removal of formwork after a period of hours may be acceptable for CEM I systems, but with more slowly hydrating composite cement systems this may cause problems.

In such instances hydration will be far from complete and the microstructure will not be dense and impervious, but rather open, with a resultant loss of performance and allowing the ingress of aggressive species - affecting durability. Thus, there will be a complex interplay of continuing hydration, drying of the sample surface and phase carbonation (as represented schematically in figure 1).

Each of these processes will affect the pore structure and transport properties, plus the phase assemblage. Therefore, the use of models based on the properties of almost fully hydrated and non-carbonated materials for service life prediction is irrelevant. An understanding of the impact of incomplete curing upon carbonation is imperative.

Figure 1: Schematic representation of the interplay between sample hydration, drying and carbonation.

Furthermore, calcium-bearing hydrate phases (in particular CH) are the main suppliers of alkaline buffering capacity. Therefore, reducing the calcium hydrate content (as is the case with low-clinker binders) is likely to result in an increased rate and extent of carbonation. This, in turn, implies that high volume SCM mixtures are more sensitive to carbonation-induced corrosion. Usually, carbonation in OPC matrices causes microstructural changes which slow down CO2 penetration (i.e. a reduction in porosity and refinement of the microstructure).

However, in high volume SCM mixtures, an increase in porosity and a coarser microstructure can be observed, which can yield a significant increase in permeability and thereby adversely affect durability.
This programme will investigate the phase assemblages, carbonation kinetics, micro- and macrostructure and transport properties of cement systems containing less than 50% clinker.

This is too grand an aim for a single PhD student, both in terms of workload and expertise. However, by having two overlapping (but not entirely concurrent) PhD projects, individual and institutional strengths can be maximised. The overall programme may be separated into three primary aims:

  • An understanding of the phase assemblages and microstructure of low-clinker binders at early ages, i.e. the situation when formwork is likely removed during construction, followed by the coupling between simultaneous early-age hydration, carbonation and desiccation of low-clinker materials in terms of phase assemblage and microstructure.
  • Identification and generation of relevant input data for durability models, to account for the coupling between carbonation, moisture transport and hydration. This will entail an understanding of changes in pore structure and transport properties upon simultaneous carbonation, hydration and desiccation.
  • Testing of existing carbonation models and refinement where possible or appropriate.

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