Hygral stress in hardened cement paste

A specially constructed stress cell, permitting variation of degree of restraint, was used to measure the hygral stress produced in thin-walled hardened cement paste cylinders due to water absorption. The effects of porosity, relaxation and relative humidity on the hygral stress were investigated using a Portland and a Portland composite cement. It was found that capillary suction transports water to the gel pores causing an initial rapid stress development. This is followed by a gradual increase over ca. 3 days governed by the redistribution kinetics of water molecules in the cement gel. The hygral stress developed is proportional to the volume fraction of cement gel. The cement gel itself produces a uniaxial stress of ca. 8 MPa for degrees of restraint above ca. 80%. About 70% of the stress is caused by changes in surface energy at the gel particle/pore water interface; the remainder is due to the disjoining pressure. A change in surface energy of 0.17 J/m² was estimated based on measurements of specific surface and porosity. The development of hygral stress is also controlled by stress relaxation. This appears to be enhanced by the effect of the disjoining pressure which weakens bonds between gel particles to create a more mobile structure under stress.