I developed a cement made from iron bacteria, then made pots from it to test its ability to set and not dissolve in water. Iron bacteria lives by oxidizing dissolved iron 2 oxide in the water table, converting it to insoluble iron 3 oxide, which precipitates out of solution forming an orange slime. This material can be concentrated to a paste that can be shaped into objects. After drying out completely, the material becomes insoluble in water just as cement does. I don't know for sure why it does this but I suspect that while it's wet, there's still some dissolved iron 2 oxide in the material. When air gets to the iron 2 oxide and converts it to iron 3 oxide it precipitates out. It then becomes a solid, cementing itself to the other iron 3 already present. This is probably why iron bacteria sometimes forms solid lumps known as bog ore. Bog ore was once primarily used for smelting into iron but was also occasionally used as a building material.