HYDRATION OF CEMENT 2.4.1 The Hydration Reaction 2.4.2 Volume products of hydration

Progress of cement hydration

Hydration of PFA … PFA reacts with the lime produced by the hydration of cement to give more hydration products - reducing the voids and lowering the permeability. The nature of the pozzolanic reaction should be explained. When Portland cement reacts with water hydration products form which give the material is strength. However, a by-product is also created - lime. This lime can react with a pozzolana, such as PFA / fly ash, to form more hydration products. This reaction is rather slow at normal temperatures and little contribution to strength/durability is given by 28 days after the concrete is cast. In the fullness of time there is a gradual build up of hydration products from the pozzolanic reaction that blocks the pores in the concrete. This makes it highly impermeable giving PFA / fly ash concrete is excellent durability properties. These reactions deplete the lime in the concrete. It has been argued that the reduction in lime may lead to any reinforcing being unprotected in an alkaline environment. In real concrete is has been shown to be impossible to deplete all the lime plus the low permeability of the matrix is such that oxygen cannot permeate to cause an corrosion.

Cement Hydration Gel pores 0.005 nm Capillary pores 10 nm

The need for Gypsum If pure C3A is mixed with water the reaction leads to uncontrolled setting. To prevent this Gypsum (Calcium Sulphate CaSO4.2H2O) is added to the cement clinker. This reacts with the C3A to form insoluble calcium sulphoaluminate, but eventually tricalcium aluminate hydrate is formed.

Heat of Hydration

Definitions (conventions for cement and concrete) Bulk volume: The overall volume defined by the outer dimensions of the sample Net volume: The volume of solids (excluding pores) Density: The mass divided by the bulk volume Specific Gravity: The mass divided by the net volume Porosity: Volume of Pores Bulk Volume = Bulk Volume - Net Volume Units: 1m3 = 1000 litres (l)

Effect of w/c on permeability

Pore size distribution

2.4 HYDRATION OF CEMENT 2.4.1 The Hydration Reaction 2.4.2 Volume products of hydration

Section 2.4 Question 1 Volumes in litres NOTE THAT THE TOTAL VOLUME IS CONSTANT When mixed Part Hydrated More Hydrated Cement 0.063 Cement 0.127 Cement 0.317 Hydrated cement 0.35 Hydrated cement 0.465 Water 0.4 Pores 0.24 Pores 0.19