Downscaling Based Identification of Nonaging Power-Law Creep of Cement Hydrates

Creep of cementitious materials results from the viscoelastic behavior of the reaction products of cement and water, called hydrates. In the present paper, a single isochoric creep function characterizing well-saturated portland cement hydrates is identified through downscaling of 500 different nonaging creep functions derived from three-minute-long tests on differently young cement pastes with three different initial water-to-cement mass ratios. A two-scale micromechanics representation of cement paste is used for downscaling. At a scale of 700 microns, spherical clinker inclusions are embedded in a continuous hydrate foam matrix. The latter is resolved, at the smaller scale of 20 microns, as a highly disordered arrangement of isotropically oriented hydrate needles, which are interacting with spherical water and air pores. Homogenization of viscoelastic properties is based on the correspondence principle, involving transformation of the time-dependent multiscale problem to the Laplace-Carson space, followed by quasi-elastic upscaling and numerical back-transformation. With water, air, and clinker behaving elastically according to well-accepted published data, the hydrates indeed show one single power-law-type creep behavior with a creep exponent being surprisingly close to those found for the different cement pastes tested. The general validity of the identified hydrate creep properties is further corroborated by using them for predicting the creep performance of a 30-year-old cement paste in a creep test lasting 30days: the respective model predictions agree very well with results from creep experiments published in the open literature.