Drying shrinkage mechanisms in Portland cement paste

The shrinkage mechanisms of portland cement paste were in-vestigated from shrinkage, weight loss, and pore structure measurements using nitrogen sorption and mercury intrusion porosimetry (MIP). Thin samples (2.3 mm) of well-hydrated (165 d) pastes of 0.4 and 0.6 water-to-cement (W/C) ratios were dried directly from saturated surface dry state to 75%, SO%, 11%, and 0 % relative humidity (rh). From equilibrium shrinkage vs calculated increase in surface free energy curves two active stress mechanisms were identified. The Gibbs-Bangham (surface free energy) effect is the major stress mech-anism, which is active in the entire rh range investigated, whereas the capillary stress effect is active above 25 % rh. From elastic modulus calculations it can be concluded that true Gibbs-Bangham shrinkage accounts for only 33 % of total first drying shrinkage. Thus nearly 67 % of first drying shrinkage may be due to a decrease in interlayer spacing caused by Gibbs-Bangham and capillary induced stresses. Further, nitro-gen measures the true external surface area, and total external pore volume can be obtained from combined measurements using nitrogen sorption and MIP. I