Before Gabe Falzone and I sat down for coffee to talk about concrete, we met in a building with concrete foundations and concrete walls. After exiting the building, we stepped down a set of concrete stairs that led us to a flat concrete slab walkway in the heart of a college campus, the concrete stretching in every direction, only ever ending in another concrete building or another set of concrete stairs, or, as it ended for us, a concrete block that housed a small coffee shop.
Of course, I had patrolled these same buildings and walkways for the better part of four years and never taken notice of their composition. Like modern technology, concrete has become so ubiquitous, so important, that it perversely disappears altogether. The second most consumed substance on our planet after water , concrete defines our industrialized world, and yet before coffee with Falzone, I couldn’t tell you what concrete was other than spitting some vague, hazardous guess of “grey stone.” I certainly couldn’t differentiate between ‘concrete’ and ‘cement.’ “Nobody knows this stuff,” Falzone reassured me. “Even people who work with it still get it wrong.”
Cement is the primary building ingredient for concrete. It’s the dry powder that, when mixed with water and other rocks, gravels, or sands of the maker’s choice, hardens into concrete — a strong, durable, perfect building material. Essentially, cement is the binder, the glue, that enables the formation of concrete. “Most people say, ‘Oh, the cement’s not dry,’ but it’s not actually drying...It’s hydrating, which means it’s reacting with water so that water gets bound into the final phases,” Falzone explained. “It’s curing.” To make cement, engineers mine assorted rocks and minerals out of the ground and heat them up in a big kiln. The resulting concrete can vary wildly depending on what kinds of rocks and minerals are included. “There’s a lot more design into everything than you think,” Falzone said.
Research on modern cement production has proliferated in the past several decades in step with our growing industrialized world, but also because such research has become crucial. As with most every other aspect of industrialization, the rapid expansion of cement usage over the last century has come at a cost. Cement production accounts for 5% of the world’s total carbon dioxide (CO2) emissions . The rule of thumb, confirmed by Falzone, is that one ton of cement begets approximately one ton of CO2 . And production is only expected to increase, particularly as modernization and growth continue in India and China; China alone accounts for nearly half of the world’s global cement production .
Falzone is a graduate student at UCLA working in the Laboratory for the Chemistry of Construction Materials run by Dr. Gaurav Sant. They’re working on ways to improve cement production, testing new techniques and chemical mixtures to make more durable, more sustainable concrete. Speaking with him about his research revealed the rich and distant world of cements and concretes, a realm of seemingly endless variety, where assorted rocks, minerals, and chemicals are mixed and matched into unique cement blends, resulting in concretes that can meet the demands of specific makers and climates.
Most of the world’s cement is Portland cement (originally found on the Isle of Portland in Dorset, England in the 18th century), which is made primarily from limestone and therefore contains mostly calcium oxide, or lime. But it also contains substantial amounts of silicate, aluminate, iron, magnesium, and sulfate, all of which can be tuned as desired. The extent to which the whole enterprise resembles a chef experimenting in the kitchen is remarkable. Magnesium phosphate cements are used for repairs because they cure quickly and have very high early strength. Calcium aluminate cements are particularly resistant to abrasion and acid attack. The addition of silica fumes produced from silicon smelting or natural volcanic ash can yield much higher strength by increasing silicate content. Iron slag, the by-product of iron-making in blast furnaces, can help reduce radiation damage.
The most recent success coming out of Falzone’s lab is set to make future concrete better suited for an Earth in the throes of climate change. This March they announced the first successful results of CO2NCRETE, a concrete analogue made from a cement substitute forged from recycled CO2 . If deployed at scale, this technology could constitute a major step towards keeping global cement emissions in check.
The emitted CO2 in the cement production cycle comes from two sources: calcination and kiln fires. Calcination is the release of CO2 from heated limestone. “When you’re making the cement, you dig up limestone from the ground, CaCO3, and what we want for cement is just lime, the calcium oxide, CaO — this is the chief chemical constituent of cement,” Falzone told me. “So when you burn the limestone in the kiln, it decarbonates, and that is what produces CO2 directly.” Indirectly emitted CO2 comes from powering the high-energy kilns that bake the collected chunks of earth and stone.
CO2NCRETE addresses these emissions by flipping them on their side: using already emitted CO2 to harden the lime into concrete, rather than leaving that gas in the atmosphere and curing the concrete in other ways.
The process behind CO2NCRETE is, in some ways literally, as old as dirt. “The ideas and pieces go back forever. Lime mortars have been around since 6000 BC, people have used these same kind of processes since then,” Falzone said. Before the advent of Portland cement, which sets extremely quickly (as fast as 15 minutes after it mixes with water), the cement of ancient times took months, sometimes years, to set. In ancient Greece and Crete, limestone was gathered from quarries and then heated, baked into lime, and then mixed with water to produce what’s called slaked lime. Once the excess water evaporated, this dry slaked lime cement would sit, exposed to air, and slowly react with the carbon dioxide of the atmosphere, carbonating into hard calcium carbonate — effectively concrete. Referred to as the “lime cycle,” it produces a stronger or more durable version of the starting limestone, with all the pores filled in by the lime's reaction with carbon dioxide. Falzone and the lab are employing that same process, but doing so more efficiently: “To be fully carbonated takes years, so our thing is to do that quickly and under a lot of control.”
In CO2NCRETE, that carbonation comes from flue gas from power plants. Flue gas is trapped in a chamber that contains a special membrane, able to separate CO2 in a controlled way such that it can be re-used. That CO2 is then combined with the slaked lime in a pressurized tank, where the pores are filled as it reacts. Before the CO2 is added, the slaked lime is mixed with water to become a slurry paste that can be shaped and printed by a 3D printer. Then, as the CO2 penetrates and reacts, the paste sets and the concrete substitute is born. “The idea is to follow something similar to pre-cast concrete where they make pieces, let them cure, and then ship them and put them in place.”
“The real issue is where you get the CO2 from,” Falzone said. “Other people are carbonating concrete now too. But one, they can’t hold very much CO2 in their concrete — very low uptake percentages. And two, they’re just buying CO2 from someone so you’re limited by what you can pay for.” The picture above is a proof of concept; it will be a while before production can be scaled such that it can be used in actual construction, but the idea is there.
Revitalizing ancient techniques recalls their advantages; in concrete technology's infancy, back before modern industrial principles of optimization and speed took root, it was more durable than ever. The ancient Romans added to their lime cement mixtures volcanic sand, ash, and rocks from a volcanic region known as Pozzouli, dubbing those siliceous materials as pozzolans. Pozzolans required long setting times in water but they significantly increased the concrete’s strength and durability, allowing structures to last for thousands of years, far superior to modern concrete. The dome of the Pantheon, the Baths of Caracalla, and the Colosseum of Rome are all made entirely from this Roman concrete — without the reinforcing steel frame common today — and yet they still stand, after nearly two thousand years. Modern concrete, on the other hand, sets faster but is only meant to last for a little over a hundred years .
Researchers in Falzone’s lab aren’t the only ones using recycled CO2. Companies are popping up all over the world looking to capture and store CO2 into something useful like concrete: Calera from California plans on using flue gas to make a similarly white, chalk-like cement; Carbon Sciences in Santa Barbara hopes to use flue gas rich in magnesium and calcium leftover from mining operations; Carbon Sense Solutions from Halifax, Nova Scotia seeks to expose flue gas to lime on large scales .
As climate change becomes more imposing, these cements may become the norm. Walking back from coffee through the concrete walkway and concrete buildings, I thought about whether the view might change, if the foundations of our industrial world might shift, built more from these white concretes realized from cements of recycled CO2 — the greenhouse gas once viewed as a menace turned, instead, into a resource.
Staff Writer, Signal to Noise Magazine
PhD Student, Earth, Planetary, and Space Sciences
1. Cement Technology Roadmap 2009. http://www.wbcsd.org/web/projects/Cement/Cement_TechnologyRoadmap_Update.pdf
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