An update to this article has been posted here.
Normal concrete is cement + aggregates + water = concrete. On the moon, this present a problem. For all intents and purposes, water is non-existent. Portland cement is carbon intensive like water, carbon is almost non-existent (5-280 ppm in the regolith).
Dr Houssam Toutanji, a civil engineer at the University of Alabama-Huntsville has developed a new process for making concrete on the Moon. Dr. Toutanji proposes plain regolith be used as the aggregate and sulfur baked out of the regolith be used as the binding agent. The sulfur needs to be in a liquid or semi-liquid state, so it needs to heated to between 130 and 140 °C. To strip the sulphur out of the regolith will require a solar oven capable of heating the regolith to very high temperatures to extract the sulfur, which is only present in regolith around levels of 400-1300 ppm.
This new lunar concrete cures in hours, versus 7 to 28 days for normal concrete. NASA's Marshall Spaceflight Center tested the new process using a lunar simulant. Mixing 35 grams of pure sulfur to every 100 grams of simulant into 5 cm, the blocks were cured and then subjected to thermal stresses before their compressive strength was meaured. Plain lunar concrete withstood 17 MPa. Silica, which is also present on the Moon, can be added for strength and that boosted the number 20 MPa.
As Milton Friedman said, there is no free lunch. In order to get enough sulfur for the process, tons and tons of regolith will have to be processed. Now if a full blown bootstrapping operation is going on, this is no problem, just one more step. But if not, that is alot of expense to go through for just concrete. It's another reason to make sure when we get up to the Moon we bootstrap to keep costs down.
Another NASA researcher, Peter Chen, came up with using epoxy as a binding agent. However, epoxy cannot be made on the Moon and must be shipped up from Earth. With current launch prices hovering around $10,000 per pound, it seems a long shot.