LOW CARBON CONCRETE | RULES OF THUMB
These ‘low carbon’ concrete packages are marketed as
providing embodied carbon reductions of between 30% and 70% compared to a CEM I only concrete. In a large number of below ground structures, this comparison is not valid, since the exposure conditions and heat of hydration in deep sections could require the concrete to contain more than 66% GGBS replacement and, as such, no reduction in embodied carbon could be achieved by the use of a proprietary ‘low carbon’ concrete. The major drawbacks of using high GGBS blends
occur in the construction of concrete-frame buildings and structures. Concretes containing more than 50% GGBS content will tend to have long setting times and slow strength development when cast in columns, walls and suspended slabs at normal UK ambient temperatures. A consequence of this can be extended working hours for concrete finishers, longer striking times and back-propping of suspended slabs, leading to increased costs and programme. A final anomaly is that due to the possibility of high
GGBS-content concrete not being able to achieve its required characteristic compressive strength at 28 days, concrete suppliers will often increase the binder content of the mix design to compensate for this – thereby increasing the embodied carbon content of a concrete that is being used to reduce carbon. With high cement replacement concretes, it makes
more sense to specify a compliance age of 56 or 90 days. This will enable more rational and economical mix designs to be adopted. Some major infrastructure and building projects are
now beginning to specify a limit on the total value of embodied carbon in a project, rather than a percentage reduction that can easily be achieved by comparing a concrete that contains a moderate level of CEM I replacement with a pure CEM I only concrete of the same strength grade. With CEM I being the major contributor of embodied
carbon in concrete, it makes sense to either: (i) reduce its content in concrete by as much as
possible or (ii) eliminate its use completely. The latter option may seem incredible to most
engineers but developments in geopolymer concrete and alkali-activated cementitious materials (AACM) in recent years have made it a distinct possibility. These are special concretes that do not contain any CEM but instead comprise replacement materials such as GGBS, fly ash, metakaolin and calcined clay; they are activated with the addition of alkali additives such as hydroxides and silicates. AACMs are not new. Alkali-activated slag mixes
were first developed in the late 1930s but their use did not become commercial until recently. In the
UK, the introduction of Cemfree AACM by the David Ball Group has been assisted by the publication by BSI of BS PAS 8820: Construction Materials. Alkali- Activated Cementitious Material and Concrete(1)
Opposite: . The
development of this standard was sponsored by HS2 Ltd, the David Ball Group and Hanson UK. Commercially available alkali-activated slag concretes generally have characteristic compressive strengths which are typically lower than C25/30. Geopolymer concretes are a subset of AACMs and
were developed in the 1980s by Joseph Davidovits. Geopolymer binders can use a variety of waste products including fly ash, GGBS and mining wastes. These inactive wastes, which are high in aluminosilicates, must be activated by the addition of a strong alkali solution to produce an aluminosilicate gel similar to the material produced by the chemical reactions caused by CEM I.
The placement of EFC geopolymer concrete in a piling mat by SCS JV on High Speed 2 contract S1
Above:
A 5m-thick base slab in a deep shaft necessitates the use of low heat and, consequently, low carbon concrete
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