When mixed with water how does gypsum act as a binder in creating concrete?
When I first started in consulting engineering on of my first jobs was inspecting the leveling of condominium ground floor slab on grade floors that has sunk about 300 mm (1') from where they were supposed to be. Turns out the ground underneath had settled a lot more and there was a huge void under the slabs in these units. It was fixed using two products.
One they called foamy concrete which today I think I hear it referred to as cellular concrete and the other they called self leveling concrete. Both were very similar in nature. All the sand and aggregate that you would normally see in a concrete mix were removed and essentially replaced with gypsum powder. Incredibly fine particle size compared to sand. It makes the concrete in both case essentially behave like water.
To support the slab, they drilled two holes in the slab in opposite corners. They shoved the hose nozzle in one and and started to pump foamy concrete into the void until it started to come out the opposite hole. Let it sit and settle for a bit and repeat the process a couple of more times until it no longer settle back into the hole. What makes the foamy concrete foamy is they actually blow air into the concrete mixture which causes the concrete liquid mix to turn into foam. Its relatively weak compared to structural concrete, probably around 1 MPa. But considering bearing capacities tend to be measured in kPa, it is plenty strong to support a residential slab on grade.
In order to level the concrete floor back up to its original position, they used "self leveling" concrete. This was basically the foamy concrete without the air in it. The just poured that out of the hose creating a giant puddle on top of the slab. The concrete really seemed to have the consistency of chocolate milk. They just kept pouring it until the top of the puddle was where the slab should have been.
The gypsum powder is allowed allowed the mixture to essentially behave like water, and reduced the cost of just using cement and water.
So to sum up, using gypsum can allow you to change the viscosity of the concrete essentially turning it into water. It also allows you to use less cement compared to just mixing cement and water.