NASA mixed cement aboard the space station; it hardened with strange stripes and highly porous structures that may affect its strength

Sometime in 2021 during an average day at the International Space Station, astronauts did something they had never done before: mixing cement. Not to repair some hole or build a shelf. This was not about construction in space; NASA wanted answers for a much more important question: what if mankind wanted to live on the Moon or Mars for real? If they did, they might have to learn how to build shelters. And cement looks like a promising material for that. Thus, astronauts mixed powdered cement material with water inside a sealed pouch and waited to observe the solidification process in the absence of gravity.

The result was far more than unusual, in addition to its scientific significance. According to NASA's own research recap, astronauts aboard the space station physically mixed tricalcium aluminate and gypsum, two of the basic elements present in regular cement, as part of an investigation known as MICS, short for Microgravity Investigation of Cement Solidification. It was an experiment designed and later analyzed on the ground by a research team led from Penn State University. However, the simplicity of the goal does not imply the simplicity of the process and results.

The cement came out looking like nothing on Earth


The hardening of cement takes place through a procedure called hydration, which is essentially a chemical interaction between the cement powder and water, forming a hard crystalline structure. On Earth, gravity helps heavy particles settle while air bubbles rise and escape. But in space, there was no such restraining force, resulting in strange striations or stripe-like patterns and a high degree of porosity in the samples.

The porosity essentially refers to a high number of air pockets formed in the reaction due to the absence of gravity. Porosity is generally considered a drawback in construction materials. This is because trapped air pockets are understood to weaken a structure. And apparently it matters here because a construction material for Mars would also need to offer astronauts solid protection against harmful cosmic radiation.

The research behind the headline

This was not a one-time experiment. In a 2021 article titled “Hydration of tricalcium aluminate and gypsum pastes on the International Space Station,” published in the journal Construction and Building Materials, led by Aleksandra Radlińska, a researcher at the Department of Civil Engineering at Penn State University, together with her colleagues, noted that even in just a few seconds after the start of hydration, the microstructure of samples in microgravity conditions differed from the microstructure of their analogs created on Earth, in particular, thick striations were formed during dissolution of gypsum. In turn, controlled samples on Earth were hydrated more, forming a more conventional multilayer microstructure.

Moreover, this research was a development of previous studies. As early as 2019, the same research group in a separate study titled “Microgravity Effect on Microstructural Development of Tri-calcium Silicate (C3S) Paste,” published in the journal Frontiers in Materials, showed that cement paste that hydrated in space conditions has more porosity and unusual crystal shapes, compared to cement created under normal gravity due to the absence of buoyancy effect. Besides the mentioned increase in porosity, this space cement had an advantage: it was more homogeneous, unlike Earth cement, with a layered structure due to gravity-induced sedimentation of particles. Uniform density is good; trapped air is bad. So it seems outer space is a give-and-take proposition.

Why this actually matters

This is where the problem comes in, especially considering NASA's broader ambitions beyond the Moon: the Artemis program is currently working toward its next crewed lunar landing no earlier than 2028, and NASA has described that lunar work explicitly as a proving ground for the harder, longer-term goal of sending astronauts to Mars sometime in the 2030s; Artemis itself is a Moon program, not a Mars mission. If humans wanted to create any kind of infrastructure off Earth, sending metal and concrete would be impractical and costly. Ideally, construction of infrastructure should be based on local resources, which would include mixing lunar or martian soil with minimum amounts of water. But if the cement itself exhibits irregularities depending on the gravity level, there is a long way of research that must be done before starting construction of any kind for shelters on Mars.

This is the truth, even a slightly deflating one: at the time of NASA's reports, the scientists had not yet tested the strength of the hardened cement material in space. They noted the porosity, which, based on how porosity behaves in ordinary construction materials, would be expected to weaken the structure; but nobody has actually measured the strength of this space-hardened cement yet, so whether it ends up weaker in practice is still an open question, not a settled result. Structural concrete, pioneered by Roman engineers, has supported human building projects for roughly two thousand years. Simpler lime-based mortars go back even further, closer to five thousand years, but weren't used as a primary structural material the way Roman and modern concrete are. It is now being subjected to a stress test in a place where it may not be ideal to test. Indeed, NASA's cement experiment is a fascinating initial effort but, at the same time, serves as a reminder of the fact that building in space is far from being an easy task solved by sci-fi films. There are still chemistry problems to solve before blueprints for a lunar base can move forward.