Construction and demolition debris generates 550 million metric tons of waste each year in the United States—more than double the weight of the nation’s annual municipal waste. Of that, concrete waste from demolished buildings, damaged infrastructure, renovation work, disaster cleanup, and concrete deemed noncompliant for new construction accounts for greater than 300 million metric tons per year of waste, enough to fill 50,000 Olympic-size swimming pools.
Approximately 76 percent of this waste is crushed and separated into its main components—cement and aggregate (sand, gravel, and stone)—and recycled into hydrated cement paste for construction plus aggregate to make new concrete. However, given the low cost of aggregate and the potentially inferior quality of downcycled aggregate, the rest—132 metric tons per year—is landfilled rather than reused.
A Better Case for Recycling
Jiaqi Li joined the Laboratory in 2021 as a Lawrence Fellow with a background in cement structure and properties. “One of my goals at Lawrence Livermore was to reduce the environmental impact of concrete waste while developing bio-based cement materials to meet the nation’s growing need for construction materials in new buildings and infrastructure repair,” says Li, who is now an assistant professor at the University of Michigan. Li had published research on making a bio-based supplementary cementitious material (SCM) from silica containing diatoms, a type of microalgae. Conventional SCMs react with cement hydration products to improve the strength and durability of concrete. However, SCM sources, such as coal fly ash from coal combustion, are less available as industry standards change. A compatible and scalable SCM source could address a projected U.S. shortage of 20 metric tons per year and a global shortage of more than 600 metric tons per year. Other researchers had produced such SCMs from diatoms at bench scale, but Li, along with a cement industry partner, wanted to scale up the process. All he needed was a high volume of diatoms to meet his goal.
Laboratory researchers Xavier Mayali and Rhona Stuart offered the expertise to grow diatoms through their work advancing microalgae for biofuel production. Diatoms, the microalgae species Li sought to grow, can produce lipids (oils) through photosynthesis. The lipids stored inside their cells can be extracted and converted into biodiesel, bioplastics, polymers, coatings, and platform chemicals used to derive synthetic materials. Microalgae represent a promising pathway to biofuels and other bioproducts because they flourish in water unsuitable for growing crops and reduce the demand on U.S. farmland. However, growing diatoms at an industrial scale is costly because the microalgae require a substantial amount of silicon to build frustules—silicon dioxide (silica) shells—as part of their growth cycle. The current practice to provide this critical nutrient is to add expensive purified silicon to microalgae growth media.
Working together, Li and Stuart, in collaboration with Livermore researcher Ty Samo, arrived at a potential process to support both their research goals. “Researchers have long observed that diatoms grow better in glass flasks versus plastic because diatoms take up silica, a primary component of glass,” says Stuart. “Recycled cement contains silica, but no one had considered whether diatom strains would grow on recycled cement.” Building on this observation, Stuart and Li speculated that diatom production could scale up with the addition of waste cement extracted from discarded concrete. The diatoms could then produce enough silica structures to generate a bio-based SCM and simultaneously generate lipid-rich bi omass at sufficient scale to be converted into fuels and other commercial products.
Speeding Diatom Growth
Two projects were set in motion to test how cement-derived minerals could boost diatom growth. Samo led a Laboratory Directed Research and Development (LDRD) project focused on the basic science of how silica, other minerals, and carbonates influence diatom physiology. Samo and his team selected the diatom Phaeodactylum tricornutum, one of the only diatoms known to not require silicon for growth, to define optimal growth conditions and observe possible carbon fixing outcomes. This diatom species also has a long, well-documented testing history the team could reference for their fundamental science research.
By the end of the LDRD project, Samo’s team had identified mechanisms to explain the positive effect of cement on diatom growth. First, cement releases a mixture of metals, such as iron, aluminum, silicon, and zinc, after soaking in algae growth media. Some of the metals were assimilated by the tested diatom P. tricornutum and proved beneficial for the diatoms’ photosynthesis, respiration, and potential frustule production. Cement also increased the alkalinity of the growth environment to help drive diatom growth.
In a separate project Stuart, Li, and their team partnered with microalgal company MicroBio Engineering (MBE) for work sponsored by the Department of Energy’s (DOE) Office of Critical Minerals and Energy Innovation. This project explored the industrial feasibility of using cement waste to speed diatom growth for the benefit of two supply chains: biofuel and SCM production. In the final stage of the project, the team planned to demonstrate that the biosilica-based product provided comparable or improved strength and durability in concrete, relative to traditional SCMs.
Given the applied science and industry focus of the project, in contrast with the basic science focus of the LDRD effort, Stuart and Li’s team sought diatoms that required silicon for growth and that would scale rapidly using recycled cement. “We wanted diatoms that required silicon so we could understand the conditions in which the diatoms’ growth was limited and where they thrived,” explains Stuart. The researchers cultured 10 diatom strains provided by academic and industry research partners using recycled cement as the sole silicon source. They measured each strain’s growth in terms of its maximum chlorophyll fluorescence, expressed in relative fluorescence units (RFU) as a proxy for biomass yield. All 10 strains had higher RFU with added cement compared to no added silicon, indicating they can use all the silicon from the added cement. Based on these results, two strains were selected: Thalassiosira pseudonana, the top saltwater performer, and Cyclotella meneghiniana RWW13, the top freshwater performer.
Scaling Up
Stuart and Li’s teams moved forward to grow the two selected diatom species for biosilica production on recycled cement in flasks. The diatoms were tested on two cement concentrations— 0.01 percent cement and 0.1 percent cement—and with two controls—no added silicon and a non-cement silicon source, sodium metasilicate nonahydrate. A model cement was used for testing rather than cement from reclaimed concrete to avoid batch effects. Higher concentrations of cement proved toxic for diatoms, likely because the larger pieces of material blocked sunlight. The mixture also became too alkaline for the diatoms.
As expected, the diatom species were starved with no added silicon, providing a control value. Both diatom species grew and added silicon to their biomass as anticipated, and at similar levels whether with the non-cement silicon source and 0.01 percent cement source. Addition of 0.1 percent cement yielded more biosilica per cell—nearly quadruple the amount of biosilica produced in the biomass for both diatom species. “These results indicated that the diatoms were loading biosilica onto their cells, which we confirmed with scanning electron microscopy and energy dispersive x-ray spectroscopy,” says Stuart.
When the team moved to higher volume (10-liter), outdoor test mesocosms (a controlled testing environment simulating natural conditions) with aged cement, both diatoms grew and generated biosilica at levels similar to those from the bench-scale tests. Positive results held in the team’s industrial-scale, 1,000-liter trials in diatom raceways (channels for large-scale, outdoor cultivation of diatoms). MBE conducted these trials, which included designing efficient and nondestructive systems to deliver the silicon from the cement to the diatoms in the raceways.
Combining scanning electron microscopy for imaging with energy dispersive spectroscopy for elemental analysis, Stuart and Li found a higher silicon-to-carbon ratio for the diatoms with cement added, indicating that the diatoms could generate thicker or more silica-rich frustules compared to diatoms in the non-cement control. To explain why the higher concentration cement additions yielded significantly more biosilica production, Stuart says, “Dissolved silica is taken up by the diatoms all at once, and then it’s gone. The higher concentration of cement represented almost an order of magnitude more silica, but with a slower release. Prior to this project, we didn’t know how much silica these diatoms could extract. Now, we understand they take on as much silica as they can, a condition we refer to as ‘luxury uptake.’”
As the team closes out work on this project, it will produce and test cement blended with biosilica from diatom biomass and measure compressive strength, the key element for cement applications in construction. The team is also conducting a technoeconomic analysis and lifecycle assessment to model the viability of large-scale bioproduction systems, including options to co-locate diatom growth facilities with existing cement plants.
The industry-scale tests in this project support ongoing research by both Li and Stuart as they pursue their respective work in SCMs and bioproducts. “Given the promising outcomes to date from our work, we’re proposing follow-on funding to further optimize cement enhancement using different diatoms with distinct biosilica properties,” says Stuart. The beneficial relationship between a frequently discarded demolition waste stream and materials needed for the future offers further applications as well. Adds Stuart, “Since our study demonstrated that diatoms dissolve cement to extract silica, we can also explore whether they can dissolve other mineral types to extract critical minerals, essential materials for the electronic and aerospace manufacturing industries.”
—Suzanne Storar
For further information contact Rhona Stuart (925) 422-3493 (stuart25 [at] llnl.gov (stuart25[at]llnl[dot]gov)).
