Using more sludge and less slag in their recipe, researchers were able to create a material that's stronger than even today's enhanced cements and highly resistant to corrosion by acid. It could solve a serious problem facing sewer pipes worldwide.
When manufacturers produce cement for concrete, they can add a material called ground granulated blast furnace slag (GGBS) to their mixture. GGBS is a by-product of iron produced by blast furnaces and consists of iron ore, coke, and limestone that's been dried and powdered. Among other things, adding GGBS to a cement mixture makes it stronger and enhances its permeability to certain substances.
Still, engineers at the University of South Australia (UniSA) thought they could do even better, so they turned to sludge, which is basically the material that settles out of water when it goes through purification at a treatment plant. In this case, they focused on alum-based water treatment sludge, which is formed when aluminium sulfate binds with solids and organic material during the water-treatment process.
When they replaced 20-40% of the slag with dehydrated sludge in a cement mixture, they found that the resultant material had over a 50% higher compressive strength than cements using only GGBS as an additive. Equally importantly, the mixture was highly resistant to sulphur-oxidizing bacteria as well as acids.
This is significant because traditional concrete sewer pipes made from cement are highly susceptible to degradation by acid and action by microbes. According to the researchers, this leads to bills close to $70 billion per year shouldered by the Australian taxpayers.
Environmental Win
In addition to creating less waste caused by broken and corroded sewer pipes, using the new cement in concrete mixtures reduces its environmental impact because it makes use of waste produced by the water treatment industry and cuts down on greenhouse gas emissions.
“Sludge is usually disposed of in landfill sites, which not only reduces available land for other uses, but also harms the environment, creating CO2 emissions from transporting the waste,” said study co-author and UniSA civil engineering PhD candidate, Weiwei Duan.
Considering that some estimates say that there are enough sewer pipes in use globally to circle the equator multiple times, if the new material finds its way into sewage systems in the coming years, it could have a dramatic impact on pollution.
“This has the potential to extend the service life of sewage pipes, reduce maintenance costs, and promote the reuse of water treatment byproducts, thus contributing to the circular economy," concludes AniSA professor Yan Zhuge, who was the principle supervisor and lead researcher on the project. “The construction industry is one of the world’s biggest greenhouse gas emitters, so if we can cut down on the need for cement, we will be helping to lower carbon emissions.”
The researchers say that more study needs to be done to evaluate the long-term real-world use of their new material in actual sewer lines and to figure out just how scalable the cement is. If those hurdles are crossed, the new mixture could join other enhanced and greener cements such as a circular cement that uses old building waste; a cement that uses seawater, electricity, and carbon dioxide to replace the sand in the mixture; and a cement that's produced with 98% fewer CO2 emissions than traditional methods.
The concrete-creation process in which cement is a key ingredient is estimated to be responsible for about 8% of global CO2 emissions per year, so anything that can help clean it up could go a long way to improving the environment.
The new cement has been reported in the Journal of Building Engineering.
Source: University of South Australia
