Question

Closed pores and open connecting pores and shrinkage relation in concrete

Answer 1

A typical pore size distribution for hardened cement encompasses a large range, extending from about 10 £gm to as small as 0.5 nm or less in diameter (see Table 7.1). The larger pores, ranging from 10 £gm to 10 nm, are the residual unfilled spaces between cement grains, earlier defined as capillary pores. The finest pores, ranging from approximately 10 nm to 0.5nm, are called gel pores sincethey constitute the internal porosity of the C-S-H gel phase. While this is certainly a useful distinction, it shouldbe kept in mind that the sizes of capillary and gel pores overlap, and the spectrum of pore sizes in a cementpaste is continuous. Internal features with dimensions of 0.5 nm or smaller are formed by the interlayer spaces of C-S-H gel. Water located in these features is not in the liquid, so these are not true pores as defined for cement paste. Voids greater than 10 m often exist in concrete, either from the unintentional entrapment of air during the mixing procedure, or from intentional air-entrainment, which purposefully disperses air voids of approximately 50 m in diameter throughout the paste to relieve pressures induced from the freezing of water in pores.