Nanotubes in cement

Hari Harikumar, CHASM Advanced Materials, outlines the benefits of using carbon nanotubes in cement.

Global Cement (GC): What are carbon nanotubes and how are they made?

Hari Harikumar (HH): Carbon nanotubes (CNTs) are long cylindrical networks of carbon atoms connected in hexagonal patterns. The smallest are single-wall carbon nanotubes (SW-CNTs), which have just one wall and are about 1nm across. Multi-wall carbon nanotubes (MW-CNTs) have several layers, like a Russian doll. These are typically 10 - 20nm in diameter.

Both types are made by decomposing a carbon containing gas (at 600 - 800°C) in the presence of a metal catalyst embedded on a metal oxide catalyst support particle. This provides the source of carbon atoms for the crystallisation growth of nanotubes of pure carbon, which also occurs at the surface of the catalyst. CHASM Advanced Materials makes SW-CNTs in fluidised beds using carbon monoxide feed gas, while their MW-CNTs are made in rotary kilns using ethylene feed gas. The length and diameter of the CNTs produced can be tuned in both processes.

GC: What are their properties?

HH: As they are typically 1 - 10µm in length (against diameters of 1 - 20nm), CNTs have extremely high aspect ratios, which is ideal for providing mechanical reinforcement or for creating electrically conductive networks. They also have massive surface area per unit mass. Whereas a cement particle might have a surface area of 50m²/kg, a carbon nanotube will have an area greater than 100m²/g, more than 1000 times greater. It is this property that will make CNTs, specifically MW-CNTs, really interesting for cement makers.

GC: Why is that?

HH: Thanks to their high surface area, adding just 1g of MW-CNTs to 1kg of cement doubles the available surface area for CSH dentrite formation in the initial stages of cement setting. This aids greatly in early strength development. In tests conducted at the University of Texas Arlington Center for Advanced Construction Materials, the addition of 0.1% of CHASM Advanced Materials' NTeC™-C 'hybrid' MW-CNT led to 41% higher 3 day strength in a laboratory testing. 28 day strength also rose by around 20%. The high aspect ratio also caused stiffness, as defined by the Young's Modulus, to rise by 86%, while flexural strength doubled.

While these figures reduce somewhat when we translate from the laboratory to the real world, the use of MW-CNTs means that we can get a bigger 'bang for our buck' from the same amount of clinker. We can add more supplementary cementitious materials (SCMs), some of which lead to lower early strength, and still achieve the same performance. Higher flexural strength would allow for reductions in the thickness of concrete road surfaces and higher tensile strength will permit the use of less steel in high-rise buildings. All of these contribute to a massive potential to reduce the mass of cement - and thus clinker and CO₂ - used in construction. We estimate that in our home market of the US, we could reduce the amount of CO₂ produced by the cement sector by 30Mt/yr, just by dosing MW-CNTs at 0.1% by weight.

MW-CNTs may also present 'smart concrete' applications. For example, CHASM Advanced Materials is also looking at how the conductive properties of CNTs might be harnessed to measure stress in structures like bridges, as well as to assess building safety after earthquakes.

GC: What are the cost considerations for cement manufacturers?

HH: Until recently MW-CNTs cost around US$200 - 300/kg, but thanks to modification in processes and scale to support high volume applications like plastics, this is falling by a factor of 5 - 10. We at CHASM are targeting even lower cost by fine-tuning the nanotubes for cement.

The benefit to cement producers primarily comes from the potential reduction of the volume of clinker used, the most energy-intensive and expensive component in cement. If we take a baseline of OPC produced at a cost of US$50/t, switching to a 50:50 mix of OPC and a locally available SCM at US$5/t, with 0.1% of MW-CNTs would reduce the cost of production by around US$15/t. Of course, the difference will be less for plants that already use SCMs, but it is still very worthwhile. In cases where the cost of production remains the same, or even rises, there are opportunities to use the product's sustainability credentials and performance benefits to justify a moderate increase in price. In markets where CO₂ emissions attract taxes or other costs, the advantages of using MW-CNTs will be enhanced even further.

GC: How close are these to market?

HH: CHASM Advanced Materials is unique in the market as both a producer of CNTs and a developer of novel applications. This has enabled us to develop a production process for cement-optimal MW-CNTs. From a 100kg/yr laboratory-scale set-up in 2020, we were able to expand to a 50t/yr pilot plant in 2022. The next step is a 1500t/yr reactor in 2024 or 2025, which would be twice as large as the biggest MW-CNT reactor in operation today. We will be able to supply cement producers with advanced

MW-CNTs. The rotary kiln process we use is nearly infinitely scaleable. There is no huge leap needed to build reactors 10 or even 100 times larger.

GC: How will cement producers use MW-CNTs?

HH: One unhelpful aspect of MW-CNTs is that they tend to aggregate together, which reduces the surface area and reduces their efficacy. To prevent this, CHASM has developed two dispersion systems. The first encapsulates them in water-soluble pellets. This has been tailored for ready-mix producers, just like any other additive. The second approach is to directly bond MW-CNTs to cement particles to produce what we call 'activated cement.' We envisage this being an add-on at the end of the process at a cement plant, much like the blending steps we have today. It would also be possible to combine these two approaches for even greater performance.

GC: What are the disadvantages of using CNTs?

HH: There are two main objections regarding CNTs in cement. Firstly, non-optimised MW-CNTs do not work well in cement applications. For example, longer MW-CNTs are best for strength. Great... but if you use longer MW-CNTs, the concrete is so thick that it becomes unworkable. This has led some potential users to discount all MW-CNTs out of hand.

Unfortunately, CHASM, having developed bespoke solutions, spends a lot of time and effort battling the perception that MW-CNTs 'don't work.' Hence CHASM’s strategy to design and manufacture 'Fit For Purpose' nanotubes will overcome these adoption barriers.

Another problem is that their high aspect ratio presents risks to respiratory health, if they are inhaled. However, by keeping MW-CNTs in a suspension, encapsulated within a water-soluble polymer, or even bound to a cement particle, we can trap the individual CNTs and ensure they can't be inhaled, either during manufacture or use. These methods to ensure safety during its lifecycle are already well established in mature markets, like nanotube reinforced plastics for automobile applications, and will be adopted in the construction industry as well.

GC: What advantages do MW-CNTs have over graphene (sheets of carbon atoms)?

HH: Graphene is a fantastic material that has opportunities in the cement sector. However, due to various barriers, there are yet to established techno-commercial solutions for the cement sector. Infact, we at CHASM have started to leverage our experience from the nanotube journey to understand graphene better. We are even exploring new nano-composites for the benefit of the construction sector.

GC: What are the biggest barriers to the adoption of MW-CNTs in the cement sector?

HH: While there are undoubtedly many advantages to using MW-CNTs in cement and concrete, we find that the sector can be somewhat hesitant when it comes to new technologies. However, this is changing more and more, particularly when it comes to low-CO₂ approaches. We hope that, by marketing to both cement producers and concrete producers, we can minimise the difficulties of introducing MW-CNTs to the market.

GC: What would you like this sector to look like in 10-20 years' time?

HH: MW-CNTs are very lightweight materials, so transporting them over long distances is akin to moving air. Therefore, the future will need large MW-CNTs production units colocated next to cement plants and / or within major cities that use a lot of ready-mix concrete. We can envisage 20 - 50 such plants in strategic locations, providing nanotubes for our partners in the cement and concrete sectors, in locations all around the world.

GC: Thank you Hari for a very interesting discussion today.

HH: You are very welcome indeed!