In this study, nano calcium silicate hydrate was used as an early strength agent to promote the compressive strength of concrete at 1 day. The strength development and the microstructure of standard concrete (SC), autoclave-free ultra-early strength concrete (ESC) and autoclaved concrete (AC) were comparatively studied. The development of hydration products, morphology and pore-structure with ages were investigated via XRD, TG, microhardness, SEM and NMR tests to reveal the mechanism of early strength of ESC. The results showed that the compressive strength of ESC at day 1 achieved 60% of the designed strength, as strong as 45.6 MPa, and only 3% less than that of SC after 90 days. While the compressive strength of AC was significant increased over 90% of ultimate at 1 day, then slightly raised after that. The hydration products did not changed between ESC and SC, but the content of C-S-H gel, Ca(OH)2 and non-evaporated water of ESC was higher in the same specific age. New hydration products such as hydrogarnet and tobermorite were found in AC under autoclave conditions. The microhardness of the paste and ITZ of ESC were also higher than those of SC. The porosity of ESC at 1 day was larger than that of SC, which was contributed by gel pores (1-10 nm). However, AC with higher ratio of large pores than ESC and SC exhibited the largest porosity. The results proved that nano calcium silicate hydrate as an early strength agent significantly increased the early strength of concrete under autoclave-free conditions. Nano calcium silicate hydrate particles supplied additional nucleus in pores and ITZ, accelerated the formation of C-S-H gel, hardened hydration products, and improved the porosity structure. However, with autoclave curing, the hydration products in AC formed with larger size and higher crystallization, which benefited for early strength. However, the large porosity with large size pores might cause damage.
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