Should we ever get human settlers on Mars, they'll likely have to construct their buildings at least partially out of Martian soil. That said, it would be nice if they didn't have to bring along a bunch of kilns, in order to fire that soil into bricks. Well, according to a NASA-funded study conducted at the University of California San Diego, Martian bricks can actually be made without any heat whatsoever.
Led by Prof. Yu Qiao, the UCSD team was initially trying to make bricks by mixing simulated Martian soil with binding polymers. What the scientists discovered, however, was that the polymers weren't even necessary if the simulant was subjected to a high enough pressure.
In order to make that happen, the material was first placed in a flexible rubber tube. That tube was then compacted at a pressure equivalent to "someone dropping a 10-lb [4.5-kg] hammer from a height of one meter [3.3 ft]." This resulted in round soil pallets, which were subsequently cut into bricks.
When tested, those bricks were found to be stronger than steel-reinforced concrete.
It is believed that the secret to the bricks' success lies in the tiny iron oxide particles which coat the larger basalt particles that make up the simulant. When pressed together, the clean, flat facets of the iron particles bind to one another, thus also binding the basalt particles together.
Of course, it's entirely possible that the Martian settlers won't be laying down bricks, but will instead be 3D-printing their structures using the Red Planet's soil. The UCSD technique may still be applicable, however, as the astronauts could deposit a layer of soil with their 3D printer, compress it to a brick-like consistency, then deposit another layer on top of that one and compress it, and so on.
A paper on the research was recently published in the journal Scientific Reports.