In this study, the influence of early hydration characteristics of ultra rapid hardening cement with different amounts of additives on hardening shrinkage crack of the paste at the temperature of 5°C was investigated by the observation of the texture of hydrates as well as characterization of early hydration and test of crack occurrence, using FE-SEM, conduction calorimetry, DSC, XRD and so on.
The relationship between amounts and morphology of hydrates, and hardening shrinkage was also discussed. The results obtained are as follows :
1) Judging from the observation of occurrence of crack, the influence of H3B03 in ultra rapid hardening cement on crack is far greater than that of Na2SO4, CaSO4. Ultra rapid hardening cement paste added more than 0.5% of H3B03 has no crack due to hardening shrinkage.
2) The measured hardening shrinkage agrees well with the degree of crack by the test of crack occurrence.
The degree of crack becomes larger with increase of hardening shrinkage determined by rubber method.
3) Ultra rapid hardening cement with larger amount of H3BO3 produces larger amount of ettringite in early hydration stage. The form of this ettringite is cubiform shape with aspect ratio of nearly 1.0.
4) Hardening shrinkage of ultra rapid hardening cement paste doesn't correspond to the degree of hydration directly.The amount of ettringite and its morphology are considered to be much influential to hardening shrinkage.
