Drying Shrinkage Mechanisms in Portland Cement Paste

The shrinkage mechanisms of portland cement paste were investigated 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%, 50%, 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 mechanism, 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, nitrogen measures the true external surface area, and total external pore volume can be obtained from combined measurements using nitrogen sorption and MIP.