Specifying Sustainable Concrete

Jenny Burridge provides a recap on minimising the environmental impact of concrete, focusing on the use of cement for current and future practice

We last looked at specifying sustainable concrete in Summer 2014 (CQ 248). Since then, further research has been carried out into new and novel cements and how these can be used in the UK. This article covers some of this new work, while also providing a refresher into the guidance for specifying cements in sustainable concrete now.

Specification methods
There are five methods for specifying concretes in BS 8500, their use dependent on the type of concrete product being used and its required performance (see figure 1 below).

Concrete that is to be exposed to rain, frost or chemicals will require a different specification to concrete in an internal dry environment, with a corresponding different range of allowable cement types and content. A mix that will endure for 100 years inside a building may not last as long in the sea, for example. Stronger concretes tend to be more durable, but are also higher in cementitious content, so it is worth thinking about the location and specifying accordingly.

BS 8500 gives six exposure classes for different types of environment. These are shown in figure 2. The durability required for each class is given as a function of concrete strengths and cover to the reinforcement. The stronger concretes tend to be more impermeable and therefore less vulnerable to penetration by water, chemicals, carbon dioxide (which leads to carbonation – see CQ 259, Spring 2017) or chlorides.

Reinforcement should be protected from chlorides to avoid corrosion. If chlorides are present, the tables covering XD or XS exposure classes should be followed. Protection from carbonation and chlorides is not an issue for mass concrete, as this is unreinforced.

Freeze-thaw and aggressive ground affect the concrete matrix and therefore affect both reinforced and mass concrete. The Concrete Centre’s publication, “How to Design Concrete Structures Using Eurocode 2: BS 8500 for Building Structures”, provides a summary of the tables in BS 8500, giving concrete strengths, covers and allowable cement types.

The embodied CO2 of concrete is highly dependent on the amount of Portland cement it contains. But what we refer to as cement is not only made of Portland cement (CEM I) – it can also include other cementitious  materials such as fly ash and ground granulated blast-furnace slag (GGBS).

These additions provide some useful benefits, such as durability, workability and lower heats of hydration. They are also products recovered from other industries, and are therefore low in eCO2, and their use can reduce waste to landfill. Most modern ready-mixed concretes in the UK include such material. In 2015 there was a 28% reduction in embodied CO2 from the 1990 baseline figure, due in part to the use of alternative cements as well as energy savings made in the production of cement.

Concretes that contain high levels of fly ash or GGBS have longer setting times than pure CEM I concretes, but up to 35% replacement has little effect on the setting times. In cold weather the strength gain of concrete is reduced and therefore the percentage of additions that will allow a striking time of about three days is reduced.

However, it is still possible to use higher percentages of cement replacement if admixtures are used that speed up strength gain. Figure 3 shows the relative strength gain of concretes with different proportions and types of cementitious material. The graph shows the effect of cement replacements on early strength and shows that all concretes reach the required strength at 28 days.

Cement types tend to be blended at the concrete batching plant and normally use fly ash or GGBS, not both. Fly ash tends to make the concrete darker in colour and improves its workability; GGBS tends to lighten the colour and improve its reflectance.

Responsible sourcing
The relevant standard for the responsible sourcing of construction materials is BES 6001, which can apply to all building materials and covers a range of environmental and social factors. The concrete industry adopted independent certification to BES 6001 from its launch in 2008. The latest published data shows that 90% of concrete produced in the UK is certified to BES 6001.

Multi-component cements

Finely ground limestone is a highly sustainable material with a wide array of uses. Due to its abundance and ease of processing, it has gained popularity as a filler material. Limestone powder is already used in combination with Portland cement clinker to make cement – known as Portland limestone cement (PLC) – which usually contains about 15% limestone powder.

In the UK, PLC is designed to meet performance criteria for most building applications and is permitted by application standards. While PLC is available in bulk, most is currently supplied in bags.

Recent research has demonstrated that if the grinding of limestone is optimised, PLCs can be produced to have similar performance to that of Portland cement CEM I, leading to significant cost and carbon savings without compromising concrete performance.

In the UK, fly ash and GGBS have been more popular than fi nely ground limestone, as higher levels of clinker substitution can be achieved. Possible solutions for optimising the use of limestone powder in the production of cement include either manufacturing higher-strength PLC or incorporating limestone powder in three-component CEM I-fly ash-limestone or CEM IGGBS- limestone composite cements. Such practices are now commonplace in many European countries, and such cements are covered by the European cement standard EN 197-1.

However, the use of three-component, limestone-containing cements is currently not recognised by UK application standards. An amendment to the UK concrete standard has been proposed by MPA Cement and it is expected that three-components cements will be permitted via an update to the standard during 2018. This will enable specifiers to choose from a wider range of low carbon cements, while improving resource efficiency.

Novel cements
There is continual research into new and novel cements. The aim of these new products is often to reduce the carbon impact of calcination – the process, achieved by heating the raw materials to high temperatures, that is responsible for up to 60% of the CO2 emitted from Portland cement production.

Most novel cements are hydraulic – that is, they react in a familiar way with water – but some are non-hydraulic, usually consuming CO2 to solidify and harden. The quantity of limestone being calcined is usually reduced and, in some cases, eliminated.

Cements can be made to react with CO2 if the proportion of limestone is reduced and kiln temperatures are lowered from 1,450°C to about 1,200°C. In this way, unique “calcium metasilicate” compounds are formed that react with CO2 to form a hardened paste based on calcium carbonate and silica gel.

Researchers at Solidia Technologies claim that the manufacture of metasilicates produces 30% less CO2 than Portland cement CEM I, with a potential reduction of a further 40% if CO2 is fully sequestered. Hardening is based only on carbonation, and water is used merely to mix the material and is removed later in the process.
Due to the requirement of a CO2 chamber, the technology is limited to precast applications. This product is also less alkaline than Portland cement, so it will require further investigation before it can be used with steel reinforcement.

New hydraulic cements are being developed. One is calcium hydrosilicate cement, which was invented in the 1990s at the Karlsruhe Institute for Technology in Germany. Limestone is again reduced and is heated at about 1,000°C to produce lime. Th is is then mixed with water and processed with silica under hydrothermal conditions to form partially hydrated calcium silicates.

It is claimed that this saves 50% of CO2 compared with CEM I. When fully hydrated, calcium hydrosilicate cement has a chemistry not dissimilar to Portland cement.

If limestone is partially replaced with sulfur and aluminium bearing minerals and fired at a lower temperature of around 1,200-1,300°C, various calcium sulfoaluminate compounds are formed. The Aether project, funded by LafargeHolcim, is well established and full-scale industrial trials recently found that sulfoaluminate cements can be manufactured using identical kilns to those used for Portland cement, with an estimated 25% reduction in CO2 emissions.

Some hydraulic cements can be produced without cement kilns. For example, by-product materials such as fly ash and GGBS can be activated using alkali chemicals (instead of CEM I). It is claimed that this process cuts CO2 emissions by as much as 80% compared with pure Portland cement. PAS 8820 “Alkali Activate Cementitious materials” specification was published by BSI in 2016, which means that these can be specified for some applications, such as groundbearing slabs, with client approval.

However, as some activation chemicals are synthesised using an energy-intensive process, it is important to consider the additional embodied carbon.

■ Specifiers can currently specify low embodied-carbon concretes and there are new developments that may further lower the embodied CO2.
■ Multi-component cements are widely used in other parts of Europe and will be included in the UK concrete standard in the near future.
■ Novel cements will need to be tested for all the properties we rely on for concretes (particularly reinforced concrete), such as durability, bond and shear strength – but they offer the potential to dramatically reduce CO2 emissions still further.