Origin of Drying Shrinkage of Hardened Cement Paste: Hydration Pressure

摘要

Driving force of drying shrinkage of hardened cement paste has been attributed to RTln(h)/v with external relative humidity h and water molar volume v either in theories of capillary tension and disjoining pressure. However these theories failed to explain considerable hysteresis observed in length-change isotherms. In this study, sorption isotherm and length-change isotherm of cement pastes were determined with different water to cement ratios and cement types, and an internal pressure of shrinkage was calculated using measured strain and elastic modulus of the skeleton. Based on the concept of disjoining pressure and the model of adsorbed water and hardened cement paste matrix, equation w+K(V/V)=0 as derived,where the w: is volumetric water content (m~3/m~3) at the reference temperature, : incremental disjoining pressure from the reference (N/mm~2), K : bulk modulus of hcp (N/mm~2), and V/V : volume change strain of hcp. From this equation and experimental results, disjoining pressure (e) (= hydration pressure) as a function of statistical thickness e was obtained as (e)=4500exp(-2e/0.95). Obtained disjoining pressure decays exponentially showing the similar tendency with the hydration forces observed in many other hydrophilic substances. In other words, this internal pressure is a kind of disjoining pressure originated from hydration force which is built up within adsorbed water films as a result of interactions between hydrophilic solid surface and water molecules. The proposed model can give a rational explanation for the relationship amoung water content, surface area of hardened cement paste, bulk modulus, and drying shrinkage of hardened cement paste.