Strength properties and hydration of ultra-high performance concrete incorporating calcined clay and limestone with steam curing regimes

UHPC blended with low-energy mineral admixture and cement supplementary material is beneficial to the environmental protection and energy conservation. This paper investigated the hydration, micro-structure and strength development of LC³ based UHPC matrix with steam curing at 55 and 90 ℃ for 1 d and 2 d. The compressive and flexure strengths of UHPC mixtures incorporating limestone and various replacement levels of metakaolin were measured. Hydration, pore structure and micro-morphology of LC³ based UHPC mixture were investigated by various modern analysis and testing technologies. The test results indicate that the 2 d or 3 d compressive strengths for 90 ℃ steam cured UHPC mixture containing 15 % MK are about 60 % greater than that of standard cured mixture. While its flexure strengths for steam cured UHPC matrix blended with LC³ are generally smaller than that of standard cured mixture. It is noted that the degrading effect of flexure strength can be relieved when the UHPC mixtures are steam cured at 55 ℃ for 1 d. For the hydration of UHPC matrix, steam curing temperature is a key factor for stimulating the MK pozzolanic reaction, which contributes to a 30 % and 50 % higher MK reaction degree than that of standard mixture at 28 d when the steam curing temperature are 55 and 90 ℃, respectively. In addition, the significant MK pozzolanic reaction does not obviously restrict the cement hydration of steam cured LC³ based UHPC matrix. The pore size distributions of LC³ based UHPC matrix with steam curing regime are similar with that of standard cured mixture at 28 d, among which the ratios of pores with sizes smaller than 20 nm are over 70 %. Therefore, satisfactory mechanical strengths and micro-structures of LC³ based UHPC matrix can be obtained with steam curing regimes.