Cement is to concrete what flour is to cake. Normally, cement and its mineral compounds make up roughly 15% of concrete (in volume), with stone, water, sand and chemical admixtures making up the rest of the recipe.
Cement, however, is not the green lobby’s favourite construction material, owing largely to the carbon dioxide (CO2) footprint of the cement manufacturing process.
On average, the amount of CO2 released during the manufactur of pure cement is between 800 kg and 900 kg for every 1 000 kg of cement produced using standard, traditional methods.
Consequently, the global cement industry is judged to be responsible for about 5% of total carbon emissions worldwide – among the highest in industry in the world, says cement producer AfriSam’s sales and marketing executive Grant Neser.
In order to shake off this reputation, the cement industry, as well as researchers active in the fields of cement and concrete production, has been working long hours to reduce cement’s carbon footprint.
Concrete now uses much less pure cement, compared with ten . . . twenty years ago, with this figure continuing to decline as research on the subject expands.
IS IT REALLY DIRTY?
Cement produces CO2 emissions because of the breakdown of limestone into lime and CO2 during the burning process. There are also CO2 emissions from burning fossil fuels in the kiln, explains Professor Mark Alexander of the University of Cape Town’s Department of Civil Engineering.
“However, as engineers, and in construction, we do not use cement in its pure form except in very specialised cases (such as grouts). We use concrete, which means the cement is greatly diluted with aggregates that, in general, have low carbon footprints.”
Comparing concrete with other major building materials worldwide, one finds that the production of 1 kg of cement emits 0.83 kg of CO2, says Alexander.
Producing 1 kg of concrete, however, emits 0.13 kg of CO2, with 1 kg of masonry products emitting 0.22 kg of CO2, wood 0.46 kg, and virgin steel 2.8 kg of CO2 emissions.
“On this basis, concrete is an environment friendly product,” counters Alexander.
However, it remains true that the lower the cement content, the lower the CO2 footprint of the concrete produced.
The cement content in concrete varies according to application. It also depends on the type of cement.
“Almost all our concretes now are blended cement concretes, meaning that the clinker content in a cubic metre of concrete can be as low as 180 to 200 kg/m3. “In cement-rich concretes, this can go up to 400 kg/m3, but this is rare,” says Alexander. (Basically, cement is produced in two steps: first, clinker is produced from raw materials, and then cement is produced from cement clinker.)
Concrete will continue to evolve under both environmental and cost pressures, he adds.
“However, there is no viable alternative to concrete as a universal building material to meet the needs of society in the foreseeable future.
“The cement industry has made great strides to reduce its CO2 emissions. Current research is looking at low-carbon or low-clinker cements where the clinker content in concrete might go down to 150 kg/m3 or less.
“Compare this with where we were just ten years ago, with clinker contents generally above about 300 kg/m3. The concrete of the 2010s is vastly different from that of even the 2000s.”
Alexander says this is the result of advances in admixture technology, increased use of blended cements, the addition of fibres, and now also the addition of nanomaterials.
However, these advances also cause some unique problems.
“What this means is that we need much greater sophistication and education in the use of this highly versatile material. In general, this is not appreciated by the structural engineering community or the specifying authorities,” notes Alexander.
CHANGE IS COMING
Through the decades, the built environment has evolved in response to the changing needs of society and, to support this evolution, cement technology has continued to advance, says Neser.
The twentieth century concept of so-called pure Portland cement – as is well known in its 50 kg bag in the retail market – is rapidly becoming obsolete with the development of technologically advanced composite cements.
“Cement manufacturers cannot afford to keep producing cements with conventional technologies that generate large quantities of CO2 emissions when we have the option of using more technologically advanced composite cements that offer additional advantages.”
Neser says AfriSam has worked to produce composite cements that harness by-products from the steel manufacturing and coal-fired power station industries, together with chemi- cal activators, to improve the characteristics and performance of traditional Portland cement.
“For the past two decades, we’ve been investing extensively into research and development focused on the production of these advanced cements, replacing the environment-unfriendly clinker and drama- tically reducing our carbon footprint.
“AfriSam has also poured considerable capital investment into upgrading our production facilities to produce advanced composite cements. As a result, we’ve been able to reduce our clinker factor from a world average of about 90% to an average of 60%, with the clinker factor as low as 35% when using Eco Building Cement, which is particularly environment friendly.”
“As announced in June 2011, Lafarge’s objective is to reduce our net carbon emissions per ton of cement by 33% [by] 2020, compared with 1990,” says Lafarge CO2 research and development programme manager Gunther Walenta.
At the end of 2013, Lafarge managed to reduce its emissions by 26%, compared with 1990 (24.7% in 2012), with 577 g of CO2 emissions per 1 kg of cement produced. This represents a 203 kg reduction in CO2 emitted per ton of cement, compared with 1990.
Lafarge has achieved this, among others, by improvements in kiln energy efficiency, as well as an acceleration in the transition towards the use of nonfossil fuels, which now account for more than 17% of the group’s fuel mix, with a 50% target set for 2020.
Product development and innovation to bring lower carbon cement solutions to market also played a role.
“We continue to reduce the carbon intensity of our cements, which is reflected by the decrease in our clinker factor from 84.6% in 1990 to 71.9% in 2013,” says Walenta.
Lafarge has also success- fully completed its second industrial trial for Aether, its new-generation clinker, formulated for lower-carbon cements.
The trial confirmed the feasibility of indus- trial-scale production of Aether cements, which offer similar characteristics to Ordinary Portland Cement (OPC), while allowing a 25% to 30% reduction in CO2 emissions.
“Work is now ongoing to industrialise the first Aether cements while identifying concrete applications,” notes Walenta.
A new partnership between Lafarge and US start-up Solidia Technologies may also assist the efforts of the global cement maker in reaching its CO2 goals.
A new low-carbon cement developed by Solidia Technologies can be made in traditional rotary kilns, used for OPC production.
Solidia Cement is made from similar raw materials to OPC – mainly limestone and silica. It is, however, produced at lower temperatures than OPC (1 200 ºC to 1 250 ºC, compared with 1 450 ºC for OPC), and needs less limestone to form the reactive phases, which generates less CO2 emissions.
Overall, this allows for a reduction of up to 70% in CO2 emissions in producing a ton of Solidia cement, compared with OPC, says Walenta.
Mixed afterwards with sand and gravel, Solidia Cement hardens through carbonation, in other words by absorbing CO2, to produce Solidia concrete.
Solidia cement and concrete will initially be targeted at the precast concrete market for nonstructural applications and some structural applications.
According to Lafarge, more than three- billion tons of cement is used each year worldwide, and the precast concrete segment represents between 10% and 20% of this, depending on the particular national market.
In addition to existing precast markets – mainly in developed countries – Lafarge believes this technology also has potential in emerging markets, where there is an opportunity to industrialise precast concrete production methods – for example, block manufacturing. This means that the potential market could, in fact, be much bigger.
Lafarge is also developing a depolluting concrete, designed to reduce nitrogen dioxide fumes in closed spaces such as tunnels.
Thermedia structural insulating concrete can help to improve energy efficiency in buildings, and Hydromedia new-generation pervious concrete can help to reduce the risk of flooding in urban areas.
In Malawi, the company has also launched DuraBric, a new cement binder specially formulated for use in soil-stabilised bricks, which can increase the life span of traditional earthen homes, which house an estimated two-billion people worldwide.
It is possible to produce zero per cent Portland cement concrete, says Murray & Roberts research manager and analytical chemist Cyril Attwell.
The construction group recently laid a concrete slab at the City Deep container terminal, where it used no Portland cement.
The concrete also required no curing with water (wetting of the concrete) afterwards to ensure it does not crack. The slab also saw less shrinkage than the concrete average of 0.045%.
“As we poured the material, it quickly hardened on the outside as it reacted to the CO2 in the air, aided by the unexpected help from the readymix truck’s exhaust pipe,” explains Attwell.
“This means the water could not escape the concrete, which meant it required no curing. Also, because the water could not escape, the volume reduction (shrinkage) was negligible.”
Attwell’s zero per cent Portland cement concrete used a pulverised fly-ash by-product from coal-fired power stations (curiously enough, heavy CO2 polluters themselves), as well as a slag by-product from the steel industry, stone, sand, a polymer and a catalyst.
The concrete slab at City Deep achieved compressive strength of 51 MPa in 28 days, and flexural strength of 4.2 MPa over 28 days.
While this should please engineers, bean counters would appreciate the fact that it costs 30% less than conventional concrete, with 25% less labour costs, especially as it requires no labour to cure the slab afterwards, says Attwell.
He adds that he applied 38 variables to produce the zero-cement concrete.
“Standard concrete technology looks at three variables, namely the water:cement ratio, particle size and particle shape. For my first project, we started by adding two variables, masking chemistry and dissolution.”
Attwell’s trajectory to the production of zero-cement concrete started in 1999, when contracts manager Anton Botha wanted concrete that could reach 12 MPa in 12 hours at –8 ºC for the winter-time construction of the Mangaung prison, in the Free State.
Twelve megapascals is equal the force exerted on a single finger when holding up two-and-a-half 50 kg bags of cement with that finger.
The problem with the Mangaung project, however, was that cement particles could not dissolve in water at temperatures less than 5 ºC, notes Attwell.
Then a junior concrete technician, he slept in the laboratory for almost a week to resolve the problem. Finally, he had an answer.
“I looked at how the human body reacts to extreme cold.”
Attwell’s search started in the Alps before the 1970s, when frostbite victims were treated by giving them brandy.
“Brandy changes the saturation levels of minerals in your blood, meaning more minerals can dissolve at a lower temperature – but only a certain amount, with the rest then staying around in clots.”
Clots, of course, spell aneurisms.
The Europeans subsequently developed a product which treated frostbite more effectively – and for which the 25-year patent had lapsed by 1998, notes Attwell.
This product led him to tetrapotassium pyrophosphate, which finally led him to creating concrete that achieved 11.8 MPa within 12 hours at –8 ºC, at a production cost of R297/m3.
“At Mangaung, we could place concrete cells on top of concrete cells before 24 hours had passed,” says Attwell.
A range of products followed, notably the Gautrain project, which Murray & Roberts constructed along with a number of partners.
Requiring 344 000 t of cement meant the product would require a rainforest of 4.5 km by 3 km, active for 40 years, to counteract the CO2 released by its production.
By substituting some of the cement with a pulverised fly-ash by-product, “we almost halved the CO2 footprint”, says Attwell.
“We call the technology we use advanced recrystalisation technology, or ARC.”
One of the newest projects in which Attwell used the ARC technology, steadily progressing towards the use of zero-cement concrete, was the construction of the Portside building in Cape Town.
For the cement portion of the concrete, Attwell used 35% cement and 65% by- product material, largely from the Saldanha Steel mill.
Attwell is positive the strength of the product equals or is greater than that of competing cement-based concretes.
However, Attwell’s specialised solutions are not for everyone, especially the do-it-yourself market, warns Alexander.
“If I understand correctly, Attwell is using non-Portland-cement-based binders or, essentially, geopolymers.
“Portland cement has higher levels of calcium silicate and the hydration reactions – the chemical reactions that impart the essential properties to the material – are different.
“In layman’s terms, Portland cement is an easy-to-use, self-setting and self-hardening binder that can be used across a wide range of applications, from the lowest and most unsophisticated end of the market to extremely sophisticated and advanced applications in normal temperature environments. This is not generally true for geopolymers.
“A further point is that geopolymers require specialised input materials, which themselves are not always in ready supply, and can be toxic.
“Further problems with these materials include nonrobustness, health and safety issues, and long-term durability.”
Alexander believes cement and concrete will continue to develop to meet the needs of society as in the past, and is unlikely to be replaced soon.
“There are huge developments taking place in materials science, with molecular engineering promising radical new materials. We may also be on the verge of exciting new biomaterials. However, whether any of these can replace the great demand for standard concrete in terms of infrastructure development, housing for human needs, industrial demand, and so forth, is, in my view, unlikely in the next several decades, if not longer.”