As one of the most widely used materials on the planet, concrete presents an ideal opportunity for the industry to minimise its environmental impact.

At the Loeriesfontein Wind Farm, the amount of activator used was reduced to 0.4%, or four litres of activator per cubic metre of concrete.
Image credit: Loeriesfontein Wind Farm

Concrete is one of the most used materials in the world, second only to water. At the same time, the cement used in concrete is one of the largest producers of carbon dioxide, contributing an estimated five to eight percent of all human-generated atmospheric carbon each year. Last year alone, 4.2 billion tonnes of cement were produced (Statista 2017). With the increasing focus on environmental sustainability and climate change, these two aspects need to somehow be reconciled. One of the areas that is helping with this is the use of alkali-activated and geopolymer concrete.

The ultimate goal of research into these areas is to obtain optimal strength from ‘green’ concrete by producing a hybrid material of superior strength with a significantly lower carbon footprint, reduced water consumption, and a lower cost. One of South Africa’s biggest proponents of low- and no-cement concrete is professional chemist Cyril Attwell from ARC Innovations. Until earlier this year, Attwell was the group concrete and research manager at Murray & Roberts, where he worked on Transnet’s City Deep Container Terminal upgrade, as well as the Loeriesfontein and Khobab wind farms and Cape Town’s Portside Tower skyscraper.

“People tend to think that concrete is simple, but this isn’t the case. Concrete is the most complex crystalline structure known to man,” Attwell explains. “There are over 500 families of crystals in normal concrete, and when you start looking at geopolymer and alkali-activated concrete, for example, this number increases to over 800. It’s extremely complex. I got into concrete research more by accident than by design, but once I got into it and started looking at the complexity of it, it captivated me. Everyone sees concrete as being a simple, highly ordered product, when in reality it is highly complex with a huge chaotic streak, and I like trying to order chaos.”

A self-described ‘bunny hugger’, Attwell has been looking at ways to reduce the carbon footprint of concrete since he first started working with it over 20 years ago. But creating green cement is easier said than done. “For every cubic metre of concrete used, the cement required releases about 0.9kg of carbon dioxide, with an average of 300–350kg of cement. Now with alkali-activated, low-cement concrete, every cubic metre used comprises 20–30% — or 200–300ℓ — of a commercial activator, usually a combination of a hydroxide with a silicate.

“The problem with this is that for every kilogram of silicate used, you produce around 0.66–0.75kg of carbon dioxide (CO2), because sodium carbonate is used to get to the sodium silicate, driving off CO2. So while this amount is lower than that produced by cement, it is not substantially so,” he explains.

“Then there is the fact that with most alkali activation, the 200–300ℓ of activator are unreactive at ambient temperatures, so you need to heat them up, increasing the CO2. And this is one of the problems I have with this whole green mentality when it comes to the silicates, especially when you’ve got 200–300ℓ of commercialised material. And with most alkali activation, the commercial activator is unreactive at ambient temperatures, so they need to be heated up, which uses more energy and creates more CO2.”

South African Projects

Attwell explains that he was involved in the Gautrain project. “Murray & Roberts brought me in to design the concrete and do the production on the Gautrain. On that project, we were actually able to reduce the amount of cement used from the original estimates by 144 000 tonnes, from the original 354 000 tonnes estimate to 210 000, and we replaced that 144 000 tonnes with 140 000 tonnes of ash from the Lethabo Power Station. Generally, people don’t use fly ash for precast, especially not for such a big project, but by applying advanced re-crystallisation (ARC) technology, we were able to utilise a 32% substitution for the precast, and we didn’t use any steam at all.”

ARC is the process of optimising the crystallography of a hydration, polymer, or hybrid material to increase the strength and durability of concrete while reducing the environmental and economic costs.Strengths of up to 52MPa at 28 days have been achieved on certain sites using ARC technology and zero cement, with the optimisation of the crystal lattice through mineral synergy increasing the strengths beyond what is considered normal.

“Now if you look at the environmental impact of cement, the 344 000 tonnes in the original estimate would have required about 4.5km by 3.5km of rainforest to be alive for 40 years to counteract the CO2 produced to make it. By applying ARC technology and extension to reduce the amount of cement used in the concrete, we were able to reduce that to 2.5km by 3.5km — a basic saving of 2km by 3.5km of rainforest that would need to be alive for 40 years to counteract the CO2. And this without impacting the strength.

“In fact, during the project we were experiencing temperatures of down to -6°C overnight, and usually cement stops dissolving at 5°C. However, because of the ARC technology we were able to withstand the temperatures going down to -6°C and still strip the next morning and move those 50 tonne precast elements at 16 hours because of the strength we achieved. We didn’t have to apply steam, which would have chased up the carbon footprint even more.”

After the Gautrain project, Attwell and his team moved on to the Portside skyscraper — Cape Town’s tallest building, and also its greenest. “The original specification for the project allowed for 30% substitution of cement, but we ended up substituting up to 85% with slag waste,” he explains. And from there, to 102 Rivonia in Johannesburg. “For 102 Rivonia, we used 60% substitution, but this time with Ulula ash rather than slag waste. And that was up to 80MPa concrete as well, achieved at ambient temperature with no steam curing or anything like that to increase the carbon footprint.”

Then came the groundbreaker: Transnet’s City Deep Container Terminal upgrade: the first commercial application of 100% cement replacement in South Africa.

Professional chemist Cyril Attwell from ARC Innovations is one of South Africa’s biggest proponents of low- and no-cement concrete.
Image credit: Robyn Grimsley

Transnet’s City Deep Container Terminal

In September 2013, Murray & Roberts was involved with Transnet’s City Deep Container Terminal upgrade. “What made the project unique,” explains Attwell, “was that there was zero cement used in the concrete, and 40ℓ (4%) of commercial activator — only 20ℓ of silicate — was used instead of the standard 200–300ℓ (20–30%), resulting in an approximate 92.7% reduction in the carbon footprint compared to a standard 45MPa concrete.” And this was without compromising the strength.

“We achieved a 50MPa concrete, which went up to 75MPa after one year,” says Attwell. “We’re actually monitoring it on a yearly basis — coring it, checking the strength and durability, and making sure that it is getting stronger and not deteriorating, because there’s very little history on this material.”

The specifications for the project called for a maximum substitution of 30% with ash. Murray & Roberts proposed a reverse: 30% cement and 70% ash, the highest substitution ever done in South Africa at that time. Using the data from the extensive testing the concrete had undergone — five to 10 times the amount of testing standard concrete undergoes, including some tests that normal concrete never sees because of the need for a deeper understanding of the material — the company sold the idea to Transnet. It was at this point that Attwell proposed to Murray & Roberts’ then-MD Anton Botha that they actually go for 100% substitution.

“Anton said he was happy for us to propose the idea to Transnet, but that I would have to be the one to sell it to them. So, we set up a meeting with the technical project manager, and he and I arrived there a bit earlier than Anton and the other guys. Before the others had even walked in the door, I had negotiated to 100%, and based on the success we’d had with the 70% substitution, within two minutes, they had accepted. Although I think the fact that it would be the first site in South Africa to do 100% cement replacement was probably also a factor. So we went ahead, and on 13 September 2013, we cast the first slab of 15m3 of concrete — the first commercial application of 100% cement replacement in South Africa.”

Three years later, in October 2016, the concrete was cored and tested. Not only had it maintained the 75MPa strength, but the durability had actually increased compared to the tests done a year prior.

The Way Forward

After the success of City Deep, the next big project for Attwell and his team was the Loeriesfontein and Khobab wind farms. While the cement substitution at the wind farms was also high — between 89% and 95% substitution with slag waste — the real improvement came in the decrease in the amount of commercial activator used. Whereas the City Deep project had seen the amount of activator used reduced to 4%, at Loeriesfontein this was dropped even further to 0.4%, or four litres of an activator per cubic metre of concrete. And this activator was a waste material from a different industry.

“While the City Deep project was a massive accomplishment, the problem was that the activators we used there — the hydroxide and silicates — are very aggressive; they are not nice materials to work with. To counteract this, we decided to use a waste product from another industry as the activator. So we used 10kg, or four litres, of a neutral salt rather than one with a high alkalinity, which meant you could actually hold it in your hand without it doing any damage, making it much easier for people to work with.

On the Gautrain project, Murray & Roberts was able to replace 144 000 tonnes
of cement with 140 000 tonnes of fly ash from the Lethabo Power Station.
Image credit: Cornerstone

And then of course there is the fact that this is a waste product rather than a commercial product, which lowers your cost factor significantly as well. “Through cement replacement, we were able to reduce the price at City Deep by about 30%, and at Loeriesfontein by about 35%. Now I am looking at ways of increasing the amount of waste used in concrete while decreasing cement — which is the most expensive part of concrete — with the aim of reducing the price by at least 50%,” says Attwell. “In future, the aim is to reduce the price by as much as 90–95%.”

Through his new company, ARC Innovations, Attwell is working on ways to reduce the commercial content of concrete — including aggregate — to zero. He shows off a sample of his latest batch, saying, “This sample here has no cement in it, and it is still using alkali, but the twist is it only contains 4% commercial product. What we are doing is combining a liquid waste with solid waste, with approximately 100kg of commercial material per cubic metre of concrete to give you this product. So, we are not using any water at all, only liquid waste, and for a water-scarce country, this is very important.

“So here we are using just under 5% of commercial product, and I believe the future is going to be using 100% solid waste and 100% liquid waste, combining it to make a concrete or an epoxy or a resin at costs that are ludicrously low, and with all the strength, durability, and other properties you need from the material. This sample here costs approximately R86 per cubic metre, compared to concrete of a similar strength that is probably in the region of R700 to R800 a cubic metre. In the future, the cost of your concrete is going to be primarily made up of your transportation and handling costs for the materials you use. So you will be reducing your costs by around 90–95%, and reducing your greenhouse gases by 90% at a minimum. And this is for the second most used material in the world after water, so the implications are staggering.

“This is what is on the cards for the future.”

Source: Mining Online