Investigation of the dimensional stability in DSP cement paste.

摘要

The dimensional change of hardened cement paste due to external factors such as humidity and temperature change has significant impact on the design and performance of concrete structures. This work deals with shrinkage and thermal expansion of cement pastes with low w/c ratio and densified with silica fume, i.e., DSP paste (Densified with Small Particles).; In the first part of this work, investigation of the autogenous shrinkage and drying shrinkage was performed. Autogenous shrinkage was found to originate from self-desiccation as a result of cement hydration and pozzolanic reactions in the hydrating pastes, and it increases with lowering w/c ratio and increasing silica fume amount. On the other hand, drying shrinkage of hardened paste with low w/c ratio was substantially reduced by the addition of silica fume (∼50%), while water loss was unaffected. Drying shrinkage-weight loss relationships in DSP pastes was interpreted based on the knowledge of the drying shrinkage of conventional pastes and model Vycor glass system, and was found to originate from small mesopore and micropore drying. The concept of total shrinkage in DSP paste, which is a combination of autogenous shrinkage and drying shrinkage, was proposed, and evidence was presented to show the influence of autogenous shrinkage during hydration on the drying shrinkage of resulted hardened paste.; In the second half of this work, thermal expansion property was systematically investigated for cement pastes with a wide range of microstructure features. It was found that saturated paste behaves anomalouly in response to temperature change, i.e., the thermal expansion is followed by a time-dependent delayed contraction at isothermal condition. This characteristic was found to be related to the pore structure and the elastic properties of the paste. A permeability-based mechanism was proposed to quantitatively explain the observed thermal deformation, and was used successfully to model the delayed contraction and calculate the permeability coefficient of mature, dense cement paste in the order 10 −16 m/s.