Cement Flashcards
What are the main components of cement?
Limestone, gypsum, clay. The minerals are calcium, silicate, alumina, sulphate and iron.
CASSI
What does tricalcium silicate do? What are its letters?
C3S. It brings early strength due to its hydration reaction, it forms a lot of CH which provides early strength but isn’t durable for the long run. Gives early strength because it reacts fast.
What does C2S do? What is it called?
Dicalcium silicate. It provides ultimate strength because through hydration, it produces less CH and more CSH which reacts slower but is stronger and more durable in the long run.
What cement components are responsible for flux?
C3A (tricalcium aluminate) and C4AF (tetracalcium aluminoferrite)
Describe what happens with C3A hydration reactions.
During its exothermic hydration reaction, large amounts of heat are generated which will cause a fast set (too much crystal growth without enough plastic time). It’s reaction with sulphate (provided by the sulphates) will produce ettringite which when in contact with monosulphoaluminate, slows down the reaction.
What is flux?
The reduction of clinkering temperature. It reduces the temp needed for minerals to from cement.
What is the chemical composition of gypsum and why do we need it?
CS(with a bar)H2. With the gypsum, there isn’t the intermediate phase of ettringite formation which we need for the concrete set to slow down. Ettringite is insoluble and it’s formation during early hydration slows down water contact with remaining C3A particles.
What is C4AF and why do we need it?
Tetracalcium aluminoferrite. It doesn’t do much in terms of strength. It’s used for aesthetic purposes, gives more grayness and it’s used for flux purposes.
Describe the microstructure of CSH
Dense and amorphous (fuzzy). Interstitial pores are too dense and therefore it’s responsible for impermeability.
Describe the microstructure of CH
Thin flat hexagonal plates. High surface area to volume ratio. More reactive and causes leaching during efflorescence.
What do ettringites look like?
They are long and thin, spiky and crystalline. Usually grow unidirectionally. They are needles.
What are the phases of hydration mechanics? In order
Dormant period, initial set, curing
What is the dormant period?
The mixture is still flowable and workable. The mixture hasn’t yet set therefore if you add more water, it will change the W/C ratio. More water = lower strength.
What is the initial set?
A matrix begins to form. The water in the cement paste is used for the hydration reactions occurring. More water = more strength. Initial set occurs when crystals cannot move anymore and are not suspended.
What is curing?
Additional water does not change w/c ratio and it contributes simply to hydration. Hydration stops below relative humidity of 85%. Ideal curing takes 28 days.
