From Debbie, one of our readers.
“I believe Enrico Dini’s 3D printed geopolymer formula is:
dolomitic limestone sand (aggregate)
mixed with magnesium oxide powder (catalyst),
sprayed with a magnesium chloride solution (binder)
This is sprayed in many, many successive layers until the whole house is completely built up. I pieced together the information from his interviews and the patent for his machine. This is actually a geopolymer that was invented in 1867 called Sorel cement. He says: “The binder transforms any kind of sand into a marble-like material (i.e. a mineral with microcrystalline characteristics) and with a resistance and traction much superior to Portland Cement, so much so that there is no need to use iron to reinforce the structure. This artificial marble is indistinguishable from real marble and chemically it is one hundred percent environmentally friendly.”
He must also have a little phosphate in the mix because Sorel cements had a problem holding up in wet conditions:
Sorel Cement
Enrico Dini’s 3D Printed Structures
Geopolymer House Blog 