Poroplastic properties of calcium-leached cement-based materials: Fid-Bau Portal

Poroplastic properties of calcium-leached cement-based materials

Heukamp, F.H. / Ulm, F.J. / Germaine, J.T.

Asymptotic calcium leaching of cement-based materials produces a new material composed of C-S-H with low C/S ratio on the order of 1 and a high porosity generated by the dissolution of Portlandite (CH) crystals, which creates a new pore-size family in the micrometer range. This paper investigates the role of these two phenomena in the multiaxial inelastic and hardening deformation behavior, in compression, of calcium-leached cement pastes and mortars. From triaxial tests and SEM microscopy, it is shown that the low C/S C-S-H matrix is highly plastically deformable, which is consistent with the high degree of polymerization and the effect of the C/S ratio on the intrinsic cohesion of C-S-H. The validity of the effective stress concept is experimentally proven for calcium-leached cement paste and mortar and provides evidence that the low C/S C-S-H solid phase of the cement paste is a pure cohesive incompressible material. In turn, the large pores created by the CH dissolution provides expansion space for the incompressible solid during compressive loading. Once this porosity is filled, the volume deformability is exhausted, and the material dilates to failure. In a similar way, the early tendency of mortars to dilate is found to be a consequence of a competition between plastic material behavior of the matrix (plastic hardening) and porosity-controlled structural deformation (geometrical hardening) triggered by frictional dilation mechanisms in the Interfacial Transition Zone (ITZ).