Deformation Mechanism of Cement Paste and Hydrates under High Stress

Abstract

Hardened cement paste (HCP) and compacts made of hydrates were analysed to understand the deformation mechanism of hardened concrete. A normal creep test and a creep test with stepwise increasing load were performed on HCP, and the results indicated that the deformation of HCP is governed by dislocation creep. The stepwise creep test was performed on the compacts of calcium hydroxide, synthesized ettringite, and calcium silicate hydrate (C-S-H), and crushed HCP at a confining pressure of 50 MPa. The hydrates, except for C-S-H, demonstrated similar behaviour, and the slope of the strain rate-differential stress curve was approximately three. According to a common classification, this slope indicates the deformation governed by dislocation creep. The synthesized C-S-H showed a higher strain rate compared to the other hydrates, and the slope in this case was negative or approximately zero. Therefore, we inferred that the deformation mechanism of C-S-H is different from those of the other hydrates and that C-S-H is not dominant when HCP deforms under high confining pressure. However, C-S-H occupies the largest volume in HCP. We justified this contradiction by assuming that C-S-H is squeezed out rapidly in the deformation; the structure then formed is composed of hydrates other than C-S-H, and the deformation of this structure is governed by dislocation creep.

Deformation mechanism of hardened cement paste under high stress and application of flow law

“…If the response of C-S-H under high stress is similar to metal glass and if dislocation creep occurs in HCP, crystalline hydrates such as calcium hydrates or ettringite may govern the deformation under high stress. As these crystalline hydrates are not dominant in volume, they likely form a skeleton to resist the applied load, and the deformation of this structure may be governed by the dislocation creep [43]. The calcium hydroxide crystal is large, and its aspect ratio is high.…”