Question

Which one of the following statements is not correct with respect to the properties of cement?
1. Highly reactive Pozzolanas enhance the early age strength of the composite cement.
2. Pozzolanic activity refines pore structure which decreases electrolytic resistance of concrete.
3. The expansion due to alkali-silica reaction can be controlled by replacement of as high as 60% of OPC with high-calcium Pozzolana.
4. Such high amounts of replacement of cement resulting in higher accelerated carbonation depths compared to the pure use of OPC only.

Answer 1

Correct Answer - Option 1 : Highly reactive Pozzolanas enhance the early age strength of the composite cement.

Pozzolana
Definition	It is a natural or artificial material containing silica in a reactive form. Initially pozzolana does not have cementitious property, but when it reacts with calcium hydroxide(generated by hydrating cement) it acts as a cementitious material.
Properties	Even if the highly reactive pozzolanas are used the early strength obtained is less, as the is ca(oh)2 is needed, which forms later from the hydration of c3s and c2s. Pozzolana has good resistance against sulfate attack and chloride ion penetration, hence it can be used in the aggressive water conditions. When the concrete allows penetration of aggressive media such as chloride ion and sulfate etc, the electrolytic reaction starts which damages the steel reinforcement and thus the entire structure. The pozzolana when refined and grounded properly with Portland clinkers, fills the pores and restricts the aggressive media. The pozzolana reacts with available alkali in the concrete, thus makes alkali less available to react with silica minerals contained in aggregate.