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.
I think portland cement is the primary "binder" agent in the concrete mixture, and gypsum acts as an additive and serving an important function when need.
Role of gypsum in cement:
"Gypsum is a mineral and is hydrated calcium sulfate in chemical form. Gypsum plays a very important role in controlling the rate of hardening of the cement, so it is generally termed as retarding agent of cement. It is mainly used for regulating the setting time of cement.
When and how gypsum is added to cement:
In the manufacturing process of cement, clinkers are formed. These cement clinkers are cooled down and added with a small amount of gypsum. The mixture then sent to the final grinding process. For ordinary Portland cement, it remains between 3 to 4% and in the case of Quick setting cement, it can be reduced up to 2.5%.
Purpose of gypsum in cement:
The main purpose of adding gypsum in the cement is to slow down the hydration process of cement once it is mixed with water. The process involved in the hydration of cement is that, when the water is added into cement, it starts reacting with the C3A and hardens. The time taken in this process is very little, which doesn't allow time for transporting, mixing, and placing. When gypsum is added into the cement and water is added to it, a reaction with C3A particles takes place to form ettringite. This ettringite is initially formed as very fine-grained crystals, which form a coating on the surface of the C3A particles. These crystals are too small to bridge the gaps between the particles of cement. The cement mix, therefore, remains plastic and workable. The time allowed for mixing, transporting, and placing plays an important role in the strength, composition, and workability of concrete.
Effects of gypsum in cement:
- Gypsum prevents the Flash Setting of cement during manufacturing.
- It retards the setting time of cement.
- Allows a longer working time for mixing, transporting, and placing.
- When water is mixed with cement aluminates and sulfates get to react and evolve some heat but gypsum acts as a coolant and brings down the heat of hydration.
- Gypsum concrete possesses considerably greater strength and hardness when compared to non-gypsum concrete.
- Water required in gypsum-based cement for the hydration process is less."