Abstract: Under low water to binder ratios (W/B), the hydration products' characteristics and micro-structure of hardened cement paste are studied by XRD and SEM methods. The test results show that, the blended cement cannot hydrate completely under low W/B and no water-curing conditions, and the early hydration rate is accelerated. There are large amounts of AFt formed in the expansive cement, which have two contrary effects. One is to enhance the early structure strength, and the other is to cause the paste structure crack.