Deformations in Pastes During Capillary Imbibition and Their Implications

Abstract

In capillary imbibition (or absorption) experiments performed on cementitious materials, a non-linear evolution of water uptake with the square root of time is frequently reported. This anomaly is derived from analytical methods that assume that the pore structure remains invariable during water ingress. A more comprehensive understanding of the transport phenomena should consider calcium silicate hydrate (C-S-H) swelling during water ingress. Such swelling may cause changes in the pore structure and interferes with water uptake. To analyse that process, the external deformations occurring in cement paste samples during capillary imbibition tests were measured. Pastes were prepared using Portland cement (CEM I 52.5 N) and water/binder = 0.4 and 0.6. After 7 days of curing, strain gauges were glued on the samples to register the external strain. Differences in the deformation results show that the restriction of the system plays an important role. Results indicate that the anomalous water ingress during capillary imbibition coincides with the measured external deformations. In this sense, calculations based on the model of Thomas and Jennings showed that pores shrink. Such pore changes are synchronous with the capillary rise. Hence, an improved description of the delaying process in the transport mechanism is here presented considering the effects of the changing pore structure. The extent of this physical effect is related to the porosity and amount of non-deforming and deforming phases (C-S-H) of each system.