Just When I Thought I Was Done With Cement Replacements, A Look At Geopolymers – CleanTechnica

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One of the many people who have suggested I look at something in the space of cement and concrete decarbonization suggested red mud. All I knew was that it had something to do with aluminum, and hence I thought it would be another metal slag supplement to cement. You know what they say about assumptions.

Let’s step back from red mud to the class of cement replacement solutions, geopolymers. That’s from geo for ground and polymer, a large molecule made up of many smaller repeating units called monomers. These monomers are bonded together in a chain-like structure. Polymers can be natural, like DNA and proteins, or synthetic, like plastics and nylon. They are used in a wide variety of products due to their versatility and strength. For example, the plastic used in water bottles and the rubber in tires are both made from polymers. Because of the length of the chains, they can be really good at holding things together,

Geopolymer precursors are made from industrial waste, including coal fly ash, blast furnace slag, and red mud from aluminum manufacturing, mixed with caustic alkaline substances such as sodium hydroxide combined with sodium silicate to activate them. The process of using them is different than for Portland cement, which is typically mixed with sand, gravel, supplementary cementitious materials, and water at concrete batching plants and trucked to job sites, at least in densely populated areas. Remote construction sites, whether buildings or infrastructure projects, solve this by building concrete batching facilities or by mixing small batches by hand.

The activators only require salt, sand, water, and energy to make, so aren’t geographically constrained, but are fairly expensive. However, there’s an interesting side note on sodium hydroxide. Its manufacturing includes the chlor-alkali process, which electrolyzes salty water. One of the waste products of that process is hydrogen. While using hydrogen as an energy carrier makes no sense, it is required as an industrial feedstock, especially for ammonia fertilizers. Places with large geopolymer manufacturing facilities, if designed intelligently from the beginning, could also be producing products that require hydrogen. The scale of the hydrogen demand might be a concern, as existing chlor-alkali plants I’ve looked at produce far less hydrogen per day than current scaled ammonia plants require.

Batching plants and trucking through urban traffic are made possible by the lengthy pot time — the length of time it will stay liquid in the truck — and geopolymers cure much more quickly and hence have a shorter pot time. As a result, geopolymer concrete tends to be made on job sites closer to where it will be poured. The caustic substances are more challenging to work with on many construction sites, which inhibits their use somewhat.

They are like epoxy cements in that regard, a subject I tackled recently, as epoxies have fumes that have to be carefully ventilated, but require more sealed-off spaces to cure properly. They share other characteristics with epoxy cements, including being polymers, the need for activators, shorter curing times, and more resistance to harsh chemical environments. Geopolymers have different heat creation during setting characteristics than Portland cement, much less intense but longer lasting. Both epoxy and geopolymer concrete have problems with regulations and building codes because they aren’t Portland cement, which is written into so many, specifically for anything structural that holds up humans, whether it be buildings or bridges.

However, 20% to 25% of concrete is used for preformed concrete components manufactured in factories, where it’s much easier to control for the workplace safety concerns of the caustic activators, the work flow, inventories, and where regulations about Portland cement tend not to apply. This is a big chunk of the global concrete market, about a billion of 4.1 billion tons. And remote and offshore infrastructure has a special place in this cement mixer of technologies and use cases, often avoiding municipal building codes.

An interesting bit about geopolymers is slipforming, a continuous pouring process used for constructing high-rise buildings and other tall structures like towers, silos, and bridge piers. Concrete is continuously poured into a moving formwork that gradually rises as the concrete sets. Rebar reinforcements are added to the top of the structure as the form rises. This allows for uninterrupted construction of vertical structures. Portland cement cures slowly, with most formulations taking up to 28 days to reach 70% strength, and then continuing to strengthen for a long time after that. Fast setting Portland cement only takes seven days to reach high strength levels, a fundamental limiting factor on slipforming speed.

The central core of the Burj Khalifa in Dubai and the CN Tower in Toronto were both slipformed, as notable examples. Slipforming is already used extensively in offshore wind farms, where subsurface spars are poured on shore, transported to the site, and lowered with heavy-lift cranes onto the prepared foundation on the seabed. Communication towers are another application, of less relevance to an interesting intersectional use case I looked at recently.

Geopolymers, by contrast, cure much faster than Portland cement, in minutes or hours depending on the formulation. While it gives off heat, adding more heat can cure it faster. Further, running an electrical current through geopolymer concrete as it is setting, or electrocuring, can both hasten curing and improve the results. It does this by increasing the mobility of ions within the geopolymer mixture, leading to a more efficient dissolution of alumina and silica. This accelerates the polymerization process, which is critical for the development of strength in the geopolymer matrix.

What this means for slipforming is a radical increase in the speed of the process. The limiting factor is more the labor of adding the reinforcing steel bars than the curing of the geopolymer. Where time is of the essence, for example, pouring a hundred wind turbine spars for a GW-scale offshore wind farm, this can be very helpful.

One other thing to draw out is that supplementary cementitious materials aren’t used with geopolymer concretes. The geopolymer substances replace both the Portland cement and the supplementary materials. Only two-thirds of the water is required as well, as the chemical process is different. This means that while Portland cement is perhaps 10% of concrete, the geopolymer precursor is 25% to 30%. A ton of Portland cement is replaced by perhaps three tons of geopolymer precursor, and a couple of tons of  supplementary materials are replaced by ton or so of activating hydroxides and silicates, in other words. The ratios become important as we consider cost workup comparisons, and are necessarily rough.

And so, back to red mud. It’s the waste from refining bauxite into alumina, or aluminum oxide, the precursor to smelted aluminum. According to the International Aluminium Institute, for every ton of alumina produced, between 1 to 1.5 tons of red mud are generated. Given that producing one ton of aluminum requires approximately two tons of alumina, this means that for every ton of aluminum, between 2 to 3 tons of red mud are generated. In the storage ponds or dry stacking fields, it’s highly caustic, which makes it an environmental problem.

In 2023, about 70 million tons of aluminum were manufactured. That suggests a waste stream of 140 to 210 million tons of red mud. While red mud geopolymers are not significantly commercialized, there’s enough data to suggest that every ton of red mud can make a 0.5 to 0.7 tons of geopolymer precursor. Given the three tons of geopolymer precursor to one ton of Portland cement, that means that a six to nine tons of red mud are required to displace a ton of Portland cement, assuming my reading of a lot of sources is correct. Due to the alkalinity, less of the hydroxide caustic solution is required than for other geopolymers.

This suggests 16 to 35 million tons of Portland cement can be displaced with the annual new aluminum output (not recycled aluminum). This isn’t a big number compared to the 4.1 billion tons of Portland cement used annually. There is, however, a lot of red mud sitting in storage ponds and dry stacking fields globally. Some of it is used to remediate acidic soils and water treatment, but far smaller than the amount sitting around. In some places it’s still dumped offshore as a result, although no longer in North America or Europe where that is banned.

The timing of those bans are interesting, by the way. The offshore dumping of red mud was prohibited in the United States under the Marine Protection, Research, and Sanctuaries Act (MPRSA), commonly known as the Ocean Dumping Act. This act was enacted in 1972 and regulates the disposal of materials into ocean waters to prevent marine pollution. The Act specifically prohibits the dumping of hazardous materials, which includes red mud due to its high alkalinity and potential environmental impact.

In Canada, the regulation of ocean dumping is governed by the Canadian Environmental Protection Act (CEPA), which was enacted in 1999. CEPA strictly controls the disposal of waste at sea and prohibits the dumping of hazardous industrial wastes such as red mud.

In Europe, the ban on offshore dumping of red mud was implemented later, with stricter environmental regulations coming into effect around 2016. This delay compared to North America can be attributed to the complexities of coordinating policies across multiple countries and the significant economic considerations associated with transitioning to more sustainable waste management practices.

The estimated cost of producing a ton of red mud geopolymer is approximately $140 to $250, which includes $10 to $20 for red mud, $50 to $100 for red mud processing costs, $50 to $70 for sodium hydroxide, and $30 to $60 for sodium silicate. Remember that we need three tons of precursor and a ton of activators to displace a ton of Portland cement and supplementary materials in concrete, however. At 10% Portland cement in concrete, a ton has about about $10 to $15 of Portland cement, but would require perhaps $50 of red mud precursor. Similarly, the activators are a lower percentage of the mix, but are much more expensive per ton than the supplementary cementitious materials used with Portland cement, so cost more.

What’s the payoff then? In theory, lower greenhouse gas emissions, but it doesn’t seem to stack up. A ton of Portland cement has a carbon debt of about a ton of carbon dioxide. A ton of concrete is about 10% to 15% Portland cement, so the cement provides 100 to 150 kg of carbon dioxide debt. The replacement with the larger amount of red mud geopolymer precursor and activators is around 120 kg of carbon dioxide, per my napkin workup. This is in the same order of magnitude.

Red mud geopolymer’s cheap resource cost is offset by more of it being required and the higher cost of activators. The higher overall cost is balanced by the advantages of rapid slipforming and higher chemical resistance. The chemistry and costs of this have been well understood, with geopolymers as a class being first developed in the 1970s by Joseph Davidovits, a French materials scientist. Research on red mud geopolymers specifically started over 20 years ago.

But now red mud is piling up onshore. What was a cheaply disposed of waste is now an expensive waste management problem. That means that bauxite refineries are desperate to find off-takers for the caustic material and will pay higher tipping fees, making the economics work out better. I wasn’t able to find any commercial rates, but that virtually no red mud geopolymer is being used today indicates that the costs have outweighed the benefits. As I found in the SCM assessment, the cheapest materials that can be found in sufficient masses the nearest to the cement and concrete demand centers are the ones that have been used.

Remember that geopolymers can be produced from other industrial wastes. Coal fly ash geopolymers are one of the most common, as we generate 600 to 800 million tons of it annually, an order of magnitude more than red mud. As the dive into supplementary cementitious materials showed, however, the biggest use of fly ash is as an SCM to directly displace Portland cement and lower the overall cost of concrete. Turning it into higher cost geopolymers with limited carbon dioxide saving advantages likely isn’t going to pencil out well. It’s quite likely to just be shoveled into SCMs until it’s gone, at which point more expensive SCMs like calcined clay will be required. There’s an awful lot of it in huge tips simply because we’ve been burning coal for electricity since 1882. It’s going to take a while to finish it off.

Ditto blast furnace slag, about 300 million tons a year. That’s used widely as an SCM where blast furnaces used to exist such as the USA’s Rust Belt and the many industrialized parts of China. It’s much more widely used mostly as is for asphalt aggregate, railroad ballast and shoreline erosion control. Still, massive tonnages of it. The legacy slag still in landfills might be as much as a billion tons. It’s going to take a while to chew through the old stuff as we green steel.

There are specific places where geopolymer concretes are used, but they are in very limited tonnages compared to Portland cement concrete. The high cost factor means that the fast-setting capabilities must be strongly advantageous and the setting is factories or infrastructure sites, not urban building lots due to the high tonnages of caustic chemicals.

The last thing to say about geopolymers is that like SCMs, they are going to be used in bulk where they exist in bulk. Fly ash and blast furnace slag dominate geopolymers because the volumes are big and they are in many places, specially coal generation plants.

Bauxite refineries are spread across various regions globally, but their distribution is somewhat uneven due to the location of bauxite deposits, economic factors, and industrial infrastructure. In the Asia-Pacific region, Australia stands out as one of the largest producers of bauxite and alumina, with major refineries in Western Australia and Queensland. India’s significant bauxite refining capacity is primarily located in Odisha and Andhra Pradesh. In South America, Brazil is a major player with key refineries in states like Pará and Maranhão.

The distribution of bauxite refineries is influenced by various factors. Resource availability plays a primary role, as proximity to bauxite mines reduces transportation costs and logistical challenges. Economic factors, such as a strong industrial base and supportive economic policies, also contribute to the development and maintenance of refineries. Adequate infrastructure, including transportation networks, energy supply, and port facilities, is crucial for the establishment of refineries. Investment and development are other significant factors, with regions attracting substantial investment in mining and refining technology hosting more refineries. Consequently, countries like Australia, China, Brazil, and India have high concentrations of bauxite refineries, while African countries, despite having large bauxite reserves, have less refining capacity due to underdeveloped infrastructure and economic constraints.

And so to the intersectional opportunity. One of the biggest bauxite refineries in the world happens to be in Ireland in the Shannon estuary, effectively right in the Shannon Foynes port. It’s the Augninish facility, owned by Russian firm RUSAL, which at least claims that sanctions have not impacted it. Its red mud ponds are capable of processing about a million tons of the stuff per year. Until 2016, that was fine because they were just dumping it in the ocean, apparently. Now they can’t. Their plan to expand the ponds to support eight million tons of red mud, several year’s output at their normal pace, were stymied by local opposition which didn’t want a massive environmental disaster on their hands.

At an average of million tons a year since 2016, they have probably nine million tons of the stuff lying around, presumably in dry stacked fields and the ponds. That would produce enough red mud precursor to displace three million tons of Portland cement while creating about 30 million tons of rapid curing, easily slipformed structures in the port.

Structures like offshore floating wind turbine platforms and even masts. Slipformed concrete wind turbine masts were a thing until the 2008 economic downturn, and quite probably will be again as it’s vastly easier to ship tons of geopolymer precursor, caustic chemicals, and the like to wind farm sites than it is to get huge diameter steel wind turbine masts under bridges and around corners. Floating moored wind turbine bases are typically big hollow spars, often submerged a few meters at neutral buoyancy so waves don’t smash into them and make the wind turbine pitch and yaw like a mad thing.

A little math and research suggests a 16 MW wind turbine floating base and mast combined requires about 4,000 tons of steel. While geopolymer cement is great stuff, it’s not as strong for the same volume as steel. It is, however, about a third as light as steel for the same volume. Assuming twice as much geopolymer concrete is required (a guesstimate), that suggests around 3,000 tons of geopolymer concrete would be required. That would require about 1,200 tons of red mud geopolymer precursor.

The current supply of eight million tons of red mud could build most of the mass of about 6,500 offshore floating 16 MW wind turbines. That’s a bit over 100 GW of wind generation.

Why is this relevant? Well, Shannon Foynes is the targeted port for building Ireland’s offshore floating wind energy in the massive resource area they have to the west, where winds whipping across the Atlantic are an incredible resource. The 2030 Irish target that everyone agrees is not going to be met is 5 GW, but proposals for up to 70 GW of offshore wind generating roughly 300 TWh annually to feed Europe’s big appetite for energy are part of the late Eddie O’Connor’s vision for Ireland as an energy superpower. A very large percentage of the raw material to build floating offshore wind is already in the port where floating offshore wind is going to be built. Zero shipping costs, while at the same time Augninish is getting desperate enough to pay a reasonable amount per ton to anyone willing to take if off their hands.

The roughly million tons of red mud waste could build perhaps 800 16 MW wind turbines a year to be towed out into the Atlantic, hooked up to the emerging HVDC mesh network and power countries as far away as Romania, if a true European supergrid emerges. That’s a green industrial project on a grand scale.