The role of C-S-H microstructure and calcium hydroxide on creep and shrinkage of hardened portland cement paste.

The thesis provides new research related to the mechanisms of creep and shrinkage of hardened Portland cement paste. The emphasis is put on the role of calcium silicate hydrate (C-S-H) microstructure and calcium hydroxide the most important phases in hydrated Portland cement paste. Specially designed specimens were used for creep and shrinkage measurements. These were well hydrated and very thin in order to avoid the occurrence of moisture gradients during the drying process. Length change of specimens following organic solvent exchange was used as a microstructural probe. Length change data of compacted CH powder, pre-conditioned from 11% to 85% relative humidity, are presented in order to support inferences on the role of CH in cement paste. Similar measurements were conducted on Portland cement and tri-calcium silicate pastes. It is suggested that the length change on removal of water may be related to surface energy changes not always specific to the C-S-H phase alone. Mass change measurements versus time during exchange were carried out to support the interpretation of the length change behavior. The re-saturation of Portland cement paste systems with synthetic pore solution after several drying pre-treatments was adopted to facilitate electrical measurements in order to investigate the shrinkage, creep, and creep recovery behavior. The state of the water in each pre-treated specimen was characterized using the DTGA methods. The coupling of a.c. impedance spectroscopy (ACIS) and creep and shrinkage measurements established ACIS as an effective method for evaluating pore structure modification as well as the nature of changes to the pore network of cement paste during creep and shrinkage experiments. Cement paste specimens from which all evaporable water was completely removed were found to creep a significant amount contrary to the hypothesis that they should not creep at all. The compacted calcium hydroxide specimens were found to creep with a creep coefficient less than one. This suggested that calcium hydroxide makes a contribution to the overall deformation of cement paste systems