In this study, the effect of expansive additives on autogenous shrinkage and delayed expansion of ultra-high strength mortar was explored. The specimens made for the study were composed of ultra-high strength mortar, which was mixed with ettringite-lime composite type expansive additive. Two series of experiments were conducted with the specimens. The experimental results confirmed that the autogenous shrinkage of specimens was effectively decreased by increasing the proportion of the expansive additive. On the other hand, for the specimens, which had 7% expansive additive, were cured for 7 days at a constant temperature of 20℃, and then cured for a long time in either an underwater, moist (Relative humidity: 100%) or dry air (Relative humidity: 60%) environment, excessively large expansion strain occurred. Specifically, typical turtle shell-like swelling expansion cracks were confirmed in the specimens that underwent long-term curing in an underwater and moist environment. According to the result of hydration analysis, the formation of expansive substances, calcium hydroxide and ettringite contribute to the occurrence of delayed expansion.
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 justi-fied 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.
This paper reports the influence of nano-TiO2 on hydration reactions and drying shrinkage behavior during the first drying process of hardened cement pastes. Cement pastes containing 3 wt% nano-TiO2 (by external addition) in cement and reference samples were prepared with water to cement ratios (W/C) of 0.40 and 0.55. The results of experiments on samples hydrated for 6 months show that in the mature state of hydration, the addition of TiO2 retards the reaction of belite and slightly reduces the precipitation of portlandite in both W/C ratios. Regarding the formation of C-S-H, the addition of TiO2 increases the mean chain silicate length in C-S-H in the saturated state, and nano-TiO2 is present in the vicinity of low density (LD) C-S-H during the hydration process. The drying shrinkage of the cement paste with nano-TiO2 in the first drying process is larger than the reference sample when W/C = 0.55, but the opposite occurs when W/C = 0.40. This is explained by the increased precipitation of low density C-S-H due to the addition of nano-TiO2 when W/C = 0.55; however, when W/C = 0.40 the amount of LD C-S-H in the sample with nano-TiO2 does not increase due to their being less space for precipitation in the sample.