As people exhale and perspire, they release moisture into the air. Therefore, if a number of people are present in a room with little or no ventilation, all of that airborne moisture can cause the room to feel uncomfortably stuffy and humid.

To keep that from happening – particularly in hot climates – electric ventilation systems are often used throughout the day, continuously moving the moist air out of the room. Needless to say, this arrangement uses a lot of electricity.

In an effort to address that problem, Prof. Guillaume Habert and colleagues at Switzerland's ETH Zurich university have developed a hygroscopic material that passively absorbs moisture from the air throughout the day. That moisture is released back into the air when the room cools at night, at which point the ventilation system only has to run briefly in order to get the moist air out.

The material consists mainly of finely ground marble, obtained as an otherwise-unwanted waste product from quarries. In a binder jet 3D-printing process, a print head moves through a bed of that powder, depositing a liquid geopolymer made up of a mineral known as metakaolin and an alkaline solution.

That geopolymer instantly sets as it's deposited, binding the powder to which it was applied. By repeating this process over and over, three-dimensional objects can be built up in successive layers.