Subject matter

In view of the urgent need to reduce CO₂ from cement production, more and more “blended” or “composite” cements are being developed and used.  Currently less than 25% of the cements sold world-wide are what is usually referred to as OPC (>95% Portland clinker)-  To accommodate these new blends the current “prescriptive” cement standards are getting more and more complex. For instance, the current European standards for common cements (EN 197-1 and EN 197-5) now specify 34 cement types, with more types to be added by upcoming standards such as EN 197-6 (recycled concrete fines).  In view of the diversifying range of cementitious constituents and their expanding levels of incorporations, the number of combinations as cements is bound to exponentially increase if current prescriptive approaches are maintained. Moving away from specifications based on cement composition to a performance-based approach could rigorously resolve the issue, align with policy expectations and enable a more rapid introduction of new sustainable cements.     

The aim of this committee will be to prepare the way for performance approaches to defining cements suitable for application and look at existing and new test methods of performance.

The work of this committee will be critical to preparing standards which are suitable world-wide and not just based on practice in Europe or North America.  In this context the work will be extremely important to the specification of cements in the countries where most construction will occur in the coming decades, such as Africa and making sure that the CO₂ emissions of cement can be lowered as much as possible while ensuring adequate performance.

The committee will confine its work to cements which contain calcium silicate hydrate (C-S-H) as the major hydrate, due to its well proven performance on long time scales.

Terms of reference

• The envisaged life of the committee will be 5 years, it is estimated that substantial progress on the main points can be made on this timescale.
• Great attention will be made to get adequate representation from a wide geographical range of countries, including Africa, the list below is only preliminary.
• The work will draw on the experience of the members with the use of blended cements in different geographical areas and involve extensive round robin testing.
• The work will be extremely industrially relevant, but by convening a RILEM committee with independent co-chairs we will ensure that the work is done in a neutral environment.

Detailed working programme

3 main work streams should be considered, which will be pursued in parallel.  At the end of the committee, we will consider how these different aspects can be brought together in a future standard.  In each area the performance standards currently around will be reviewed.

1. Strength Testing:

The first requirement of a cement is to deliver a certain strength.  The European system of 3 broad strength classes (32.5, 42.5, 52.5) is widely adopted in many constituencies outside Europe. However, there is already an important discussion of how to measure strength.  The EN 196 mortar bar method at constant w/c is widely regarded as preferable to the ASTM mortars method, where the water content is adjusted to give a certain slump.  Even so it is acknowledged that this method could be further improved to mirror better the performance in concrete (many companies internally use “micro- concrete” mortars with maximum aggregate size around 5 mm).  Another issue is curing temperature, testing at 20°C is not relevant in many parts of the world where the ambient temperature is always higher than this.  India, for example does cement testing at 27°C.  Higher temperatures are also favourable for the reaction of pozzolans, facilitating higher level of clinker substitution and lower CO2 emissions.  The range of temperatures considered is also important in terms of practical application in testing laboratories.  Maintaining laboratories artificially at 20°C is not practical nor desirable in many countries.

Working in a RILEM committee without commercial interest will allow these issues to be discussed and evaluated.

2. Criteria to avoid poor performance (apart from classic durability), e.g. dimensional stability:

Conventional standards contain several criteria to avoid excessive expansion or unsoundness.  Notable prescriptive limits on the content of MgO, free lime and sulfate.  It would be better to use test methods that more directly test dimensional stability by triggering the underlying mechanisms. A review of existing methods needs to be made and perhaps better tests developed. In particular, we also need to consider whether there is a lower limit to clinker at which C-S-H ceases to be the major and percolated hydrate phase.  This criteria, is crucial to ensuring that materials have the same basic performance as existing ones.

3. Durability related criteria:

In general durability concerns more the aspects of concrete formulation rather than the cement itself.  However, in some cases, such as sulfate attack, there are already specifications for cements suitable for use in such environments.  But these designations are based of complex prescriptive criteria and in general there are not performance tests to verify if new blends are suitable or not. This area of the committee’s work will be much more complex and the committee should focus on criteria which apply to cements rather than the concrete made from them.  For example, even if a sulfate resisting cement is used it may not perform well if used at a very high w/c ratio.  Therefore, the aim of the committee will be to review possible approaches and make some preliminary testing to validate different approaches. This section of the work is closely related to the on-going committee TC EBD, to avoid duplications and profit from the work already done in this committee, active work on this aspect will start later in the life of the committee.  Realistically, it may not be possible to come to robust recommendations in this area during the lifetime of this committee so this workstream will be more exploratory.