Question

why concrete is weaker than cement paste?explain the affects of water /cement ratio and supplemantary cementitious materials on transition zone

Answer 1

Concrete is weaker than Cement paste, is not entirely a true statement, however for the sake of this question we will consider it to be true.

The strength of concrete is lesser than the strength of cement paste because of the behavior of the transition zone. The transition zone is the point of contact of the cement paste and the aggregate surface. Concrete is a heterogeneous mixture of cement paste and aggregates and the zone of interaction of the cement paste with the aggregate surface is the weakest link in concrete. Angular aggregates give better strength than rounded aggregates as the angular aggregates provide better interlocking between the particles and also the cement paste is able to better bind at the rough surface rather than a smooth surface. In the microstructure of concrete when the cement paste is not able to entirely bind to the aggregate surface it forms the location of weakness. The Cement paste in itself forms a homogeneous compound and the aggregate itself also has a monolithic composition, hence the surface of contact i.e surface of aggregate forms the link. In the case of an ideal transition zone, the strength of concrete will be much higher, however, due to various reasons, like excess water in concrete can lead to voids/ water pockets at the transition zone and hence weakening the overall strength of concrete as the paste will not bind to the aggregate at this location.

The ideal quantity of water required in a concrete mix is only the water required by the cement to react and form the cement paste. This is about a water-cement ratio of 0.13. However additional water is required in concrete to compensate for losses, absorption by aggregates, workability of concrete, and also for the since entire water can not be ideally provided to cement in the mix. Considering the above reasons a w/c ratio of about .3 to .4 is generally used for good quality concrete( when admixtures are not used). Excess water in concrete can form water pools on the aggregate surface ( visible in microstructure) weakening the transition zone. A weakened transition zone will result in low strength concrete.  

Supplementary cementitious materials like silica fumes, slag, etc, help in refining the microstructure of the concrete. Due to their extremely small and refined microstructure, the supplementary cementitious materials help in strengthing the transition zone by better binding the cement paste to the aggregate surface. Most of the supplementary cementitious materials have extremely small particle size and hence they also increase the surface area of contact and enhanced packing between the particles. The enhanced packing leads to better interlocking and hence higher strength of concrete.