Intrinsic Instability of Hardened Portland Cement Pastes and its Signifi- cance

A critical evaluation of shrinkage of saturated cement paste due to thermal cycling, increased ion diffusion through saturated cement paste due to thermal cycling, Powers water permeability data of dried and resaturated cement paste, and dying shrinkage data of Pickett and others reveals an intrinsic instability of Portland cement pastes. The instability is such that even a very mild treatment of the paste causes a redistribution of pore space such that pores are narrowed in some area and widened in other areas. It is to be noted that mild drying to 94% relative humidity is sufficient to initiate this instability. These changes occur even in saturated specimens and are irreversible. These observations are inconsistent with extensive chemical bonding in hardened cement paste or the strength of chemical bonding, in cement paste, is so low that a very mild treatment can destroy them. Relevance of this redistribution to water absorption isotherms, mercury intrusion porosimetry, etc has been discussed. These techniques provide information on the treated samples and not on the microstructure of virgin wet cement pastes and as such they can not be used to rationalize the properties of cement pastes or concrete. In actual use of a concrete structure, the effect of this intrinsic instability accumulates with each cycle of drying and wetting, heating and cooling etc. After a period visible, macroscopic cracks appear even in a well cured concrete structure. These macroscopic cracks are seldom healed especially within the structure where concrete is unsaturated. The accumulated effect makes the structure vulnerable to attack of all types of degrading processes like frost attack, sulphate attack, carbonation etc. These cracks may not affect the compressive load bearing capacity of a concrete structure.