When the UK Environmental Agency wanted to put up a flood defence project in Woodbridge, Suffolk, it naturally used significant volumes of concrete.
Whilst this usually involves a substantial environmental impact, the agency’s engineers Jackson Civil Engineers achieved CO2 emission reductions of 67 percent by using ‘Cemfree’ – a concrete which does not contain Portland Cement and which instead consists of 95 percent ground-granulated blast-furnace slag (GGFS) and a five percent alkali activator. Concrete traditionally used on similar projects involving 50 percent GGBS and 50 percent Portland Cement emits 158 kilograms of carbon dioxide per cubic meter of concrete. By contrast, Cemfree emits just 52 kg/m3.
Using alternatives to cement is one way engineers around the world are seeking to make concrete more environmentally friendly.
For good reason, Australia’s building industry relies on concrete. As well as being versatile and affordable, it is durable and can support assets with a long-life cycle. Notwithstanding growing use of timber in mid-rise multi-storey buildings, concrete remains the primary material in high-rise construction. Its use may thus grow as urban consolidation continues.
Environmentally as well, concrete offers advantages. Being a hard-wearing product, buildings and structures made from concrete last a long time. This helps minimise waste. Concrete also has a high thermal mass and acts as an effective mechanism for storing and absorbing heat. This reduces heating and cooling energy consumption. At the end of its life, concrete is well suited to crushing and recycling into products such as road base.
Despite this, the material presents challenges. Traditional concrete consists of water, cement and an aggregate (rock, sand or gravel). The production of Portland Cement is responsible for around five percent of greenhouse gas emissions which are attributable to human activity. This is largely because the materials which form the cement (limestone and clay) need to be heated to around 1,500 degrees Celsius in order to form clinker and bond together. Concrete production also requires large volumes of fresh water and generates considerable amounts of wastewater.
That raises questions about the merits of concrete as a sustainable material.
On this score, engineering consultancy firm Cundall says concrete offers benefits and drawbacks. In a written response to questions from Sourceable, Cundall representatives David Clark, Australian Partner; Amie Shuttleworth, Global Head of Sustainability; and Lee Leston-Jones, Structural Engineering Partner, said the merits of concrete as opposed to steel vary according to individual projects.
“There is an ongoing debate between the concrete and steel industries, and depending upon the building typology and associated structural spans, loadings, etc the environmental impact can swing in favour of either steel or concrete,” Leston-Jones, Shuttleworth and Clarke said.
“As such it is essential that the relative merits of materials are assessed on a project by project basis.”
According to Leston-Jones, Shuttleworth and Clark, concrete enjoys several sustainability benefits. As well as its durability and thermal properties mentioned above, it performs well from an acoustic and fire separation perspective. This enables designers to reduce the amount of materials which are applied for acoustic and fire separation purposes.
Against this, the trio point to drawbacks.
By far and away, Leston-Jones, Shuttleworth and Clark say, the largest environment impact associated with concrete is that of cement production as well as impacts associated with the mining and dredging of virgin aggregates. On average, the trio say, each manufacture of one tonne of cement within Australia causes carbon dioxide emissions in the order of 0.82 tonnes (slightly better compared with the global average of 0.87 tonnes).
Moreover, the trio point out that even where manufacturing plants run entirely on renewable energy, this process would still emit heavy levels of carbon dioxide because of the thermal decomposition of the raw material calcium carbonate.
Concrete has other drawbacks as well, Leston-Jones, Shuttleworth and Clarke say. Because of its weight, concrete framed buildings require greater volumes of material in the foundation. Concrete is also less adaptable compared with steel. This means the ongoing flexibility associated with the later introduction of openings can be restricted.
As well, concrete buildings are not as easy to deconstruct or recycle. Accordingly, such buildings are often ‘down-cycled’ at end of life. In many cases, this produces dust emissions and alkaline water pollution. Courtesy of quality control issues associated with the presence of cement and other additives, using crushed concrete in new concrete is difficult.
Whilst this can be overcome through use of precast concrete, Leston-Jones, Shuttleworth and Clarke say that for now, many precast components are largely bespoke in nature.
Against this, the trio say other materials have advantages. Steel has excellent qualities in terms of recycling. In timber, meanwhile, the growing application of products such as CLT and glulam has seen products which are fast to erect and proven to demonstrate durability. These products involve low (or, if sequestration is considered, negative) carbon emissions along with cleaner sites and less waste as well as reduced levels of air and water pollution.
Romilly Madew, former Chief Executive Officer of the Green Building Council of Australia (GBCA) and now current Chief Executive Officer of Infrastructure Australia (IA), agrees.
Whilst acknowledging concrete’s versatility, affordability and resilience along with its favourable thermal performance, Madew says the sustainability issues associated with the manufacture of cement mean that GBCA’s Green Star certification affords recognition to projects where either use of concrete is minimised or non-cement materials such as fly-ash and blast furnace slag are used.
“While appreciating the energy efficiency savings concrete can deliver during the use of buildings, we recognise the sustainability challenges inherent in its manufacture,” Madew said recently in a written response to questions prior to leaving GBCA and taking up her role at IA.
Not surprisingly, concrete industry representatives adopt a more favourable view.
In a written response to questions, Cement Concrete & Aggregates Australia chief executive officer Ken Slattery said that, ‘few products, if any, can claim to provide the environmental, social and economic benefits that concrete can offer’. Slattery added that, ‘it’s hard to imagine a building product with better sustainability credentials than concrete’.
In assessing environmental performance, Slattery says it is important to look at the context of the material. Concrete, Slattery says, is durable, stable and resilient to environmental impacts as well as offering exceptional service life. As well, it is low maintenance, inert, recyclable and uses materials which might otherwise be treated as waste. Finally, concrete through its high thermal mass can deliver savings in energy use and carbon emissions.
Given all this, an important question involves what can be done to improve concrete sustainability.
On this score, Clarke, Shuttleworth and Jones say opportunities lie in alternatives to cement and greater adaption of recycled aggregates. Concrete’s largest environmental impact, they say, involve the embodied carbon impact associated with cement production along with the mining and dredging of virgin aggregates. Because of this, any actions which can reduce or eliminate the need for cement production and/or raw aggregate use can deliver substantial reductions in the environmental footprint associated with the material.
The trio point to several initiatives.
First, there is replacement of cement within the concrete mixture. An example can be seen through the Cemfree case described above. Locally, Australian company Wagners produces Earth Friendly Concrete (EFC). This replaces the traditional Portland cement with a geopolymer binder system made from the chemical activation of blast furnace slag and fly ash (both waste products from iron production and coal fired generation respectively). This, the company claims, reduces the carbon emissions associated with normal Portland cement by 80 to 90 percent.\
Some governments, are acting as well. In Dubai, the government mandated in 2015 that Portland Cement must not exceed 34 percent of mixtures within concrete.
On recycled aggregates, the trio points to efforts to recycle concrete at the end of its useful life as part of a broader Cement Sustainability Initiative – a global effort by 24 major cement producers with operations across more than 100 countries.
Again, governments as well as companies are acting. In the Neverlands, landfill of concrete waste is banned and all concrete is recycled except for some residual process waste. Japan has near complete recovery of concrete from construction and demolition waste. Almost all of this is used for road sub-base.
Beyond this, Clarke, Shuttleworth and Jones point to action in other areas. Concrete can absorb carbon dioxide and act as a ‘carbon sink’ by storing it in a process known as carbonisation. Several methods to maximise the potential of the material to do this can be applied. One example is to use mixes and additives to raise the level of carbon absorption associated with the material.
Madew agrees that there are opportunities in concrete alternatives and recycled materials. She says these are being increasingly used and is confident that their application will increase going forward.
All this is happening, Madew says, amid a broader shift toward sustainable building practices. In October, GBCA certified it’s 2,000th Green Star rated project – an achievement which demonstrates growing acceptance of the need for buildings which are environmentally friendly.
Slattery says the industry is making progress across many areas. Some manufacturers, he says, are using waste materials which might otherwise end up in landfill in cement kilns. This includes old tyres, spent solvents, demolition timber, used oil and other unwanted material. Better kiln technology was helping to boost cement manufacture efficiency. Advances in concrete mixes is also helping. Finally, a number of manufacturers are using recycled water in the operations of facilities and concrete batch pads.
Going forward, Slattery says there are opportunities for further improvement in these and other areas. He says the construction industry as a whole needs to move away from a ‘cradle to gate’ only mentality and toward a mentality whereby sustainability is looked at across the entire building lifecycle.
Clark, Shuttleworth and Leston Jones stress there are challenges but remain optimistic.
On challenges, they say the industry needs to develop low carbon alternatives to cement, processes for greater recovery and reuse of aggregates and solutions for concrete deconstruction.
Should this happen, concrete’s benefits along market familiarity with the material mean that the outlook for concrete is promising.
When used in construction, concrete offers benefits.
With greater recycling and more extensive use of cement alternatives, the material may improve its credentials from an environmental viewpoint.