Posted on November 11, 2021
In a full frontal assault on atmospheric carbon, it is important to develop all the potential weapons in the arsenal.
In the concrete industry, carbonation is a key ally in the quest. CO2 in the concrete pore fluid reacts with calcium hydroxide and calcium silicate hydrate to form calcite (CaCO3). The process strengthens the concrete while absorbing atmospheric CO2. Carbonation is an ongoing process in exposed concrete. Over time, it pulls a significant amount of CO2 from the air.
However, given the current pace of climate warming, it is not possible to wait decades for carbonation. Nor is it adequate to fully offset the industry’s emissions. Carbonation is still an important carbon reduction strategy, however. To maximize carbonation, concrete should remain exposed whenever possible.
Titanium dioxide (TiO2) and magnesium oxide (MgO) are also helpful in the effort. Both make cement more eco-friendly, although in different ways. Let’s take a closer look at each.
Titanium Dioxide: Advantages
Titanium is the ninth most common element in the Earth's crust. It naturally reacts with oxygen to form titanium oxides found in ores, sands and soils. Pure titanium dioxide is a fine, white powder that’s been widely used for more than a century. It is a pigment in paint, printing inks and ceramics. In sunscreen, titanium dioxide (TiO2) filters the sun’s UV rays. It is even used as a color additive in toothpaste.
Absorption of pollutants
Purdue University researchers looked at adding nanoscale titanium dioxide to concrete mixes. Even small quantities almost doubled concrete’s absorption of greenhouse gases. Check out the details of the study in the journal Construction and Building Materials. This YouTube video is another resource. Fortunately, the addition of TiO2 does not compromise compliance with building codes.
Increased reflectance
TiO2 also increases the reflectance of a wide range of building materials. Commercial roofing and white concrete are two examples. TiO3 reduces heat absorption which may reduce cooling costs. It is also a valuable tool in the hands of creative designers and architects.
Chula Vista, CA, commissioned its Cool Pavements Study to evaluate the impact of white concrete. It notes that fresh gray Portland cement has an albedo of 0.35. White concrete has an albedo of 0.80. Both are far better than fresh asphalt’s albedo of 0.05. Although TiO2 concrete’s edge lessens over time, it's self-cleaning properties do help.
Self-cleaning
Akira Fujishima discovered the photocatalytic properties of TiO2 in the late 1960s. A photocatalyst uses solar radiation to accelerate a chemical reaction without being consumed. Titanium dioxide concrete oxidizes pollutants like nitrogen oxide and sulfur oxide. It neutralizes or breaks down some of the substances that darken concrete over time. As a result, TiO2 concrete retains its superior reflectance over time.
The photocatalytic effect also breaks down volatile organic compounds (VOCs). Researchers at the University of Pittsburgh looked at TiO2s unique ability to deal with VOCs. They found that TiO2 concrete removes 60 mg of VOCs per square meter of exposed concrete surface per day. It also removes other pollutants, including soot, particulates, mold, algae, bacteria and allergens.
Barriers to adoption
Sulfate attack occurs when sulfate ions react with calcium hydroxide and calcium aluminate. Gypsum and ettringite form in the concrete. The chemical process may cause concrete to crack, lose strength or even disintegrate. In a 2018 study, researchers concluded that “TiO2 has robust effects on the rate of sulfate attack on OPC.” The study found that TiO2 reduces the size of coarse pores. This increases crystallization pressures as ettringite and gypsum form. The use of slag-blended mixes can counter this effect.
Magnesium Oxide: Advantages
Magnesium oxide is yet another entry in the quest to reduce the clinker required to produce Portland cement. Producers make MgO cement using either magnesium carbonates or magnesium silicates. Its impact on the concrete industry is very different from that of titanium dioxide. Magnesium oxide may become a key ingredient in a “green” concrete that is white in reality. Smithsonian explores the idea in depth.
Authors of one study see MgO cement as “an environmentally friendly, sustainable, and efficient new material.” MgO concrete doesn’t require the same high temperatures to produce. This reduces energy consumption and emissions. The relatively rapid hydration of active magnesium oxide is another plus. Magnesium hydroxide reacts with carbon dioxide to form magnesium carbonates that increase strength. In certain applications, this may reduce or even eliminate the need for reinforcement.
MgO-based additives also perform well as expansion agents. These agents compensate for early-stage concrete shrinkage. They can out-perform traditional ettringite additives in limiting late age thermal shrinkage.
Barriers to adoption
There are challenges that limit the adoption of MgO cement. In a new study, researchers sound a cautionary note. They say that “care should be taken before MgO-based cements (are) heralded as environmental saviors for the construction industry.” More research is needed before magnesium-based cements become alternative binding sources at scale.
At present, MgO cement is more expensive than ordinary Portland cement (OPC). This is due to the “relatively high cost of reactive sources.” Costs do vary by region, however. Finally, internal pH prevents its use in regular steel-reinforced concrete. Non-steel reinforcements may be an alternative.
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