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Iron tailings have caused serious potential environmental risks with the massive accumulation. This study developed pervious concrete using novel spherical tailings aggregates (STA) synthesized from tailings sand, and explored the mechanism of the effect of STA on macro- and mesoscopic properties. Pervious concrete with natural coarse aggregate and STA replacement up to 100% were prepared. Compressive strength, splitting tensile strength, water permeability and porosity were tested. The thickness and distribution of cement paste at the top, middle and bottom of the specimens were analyzed. The uniformity of the pore distribution was quantified. The economic and environmental benefits are evaluated. The results showed that no more than 50% STA had little effect on the mechanical properties and permeability of the pervious concrete, and high contents of STA led to a decrease in mechanical properties and a significant increase in permeability. The porosity increases with the STA content and linear relations with the mechanical properties and permeability are observed. STA reduced the percentage of paste with thickness below 0.6 mm and increased that above 0.9 mm, and increased the average paste thickness. The aggregate particle interference phenomenon has been proven both theoretically and practically to cause non-uniform distribution of paste and pores, which affects macroscopic properties and water permeability. This study improves the fundamental understanding of the effect of aggregates on the properties of pervious concrete and contributes to the alleviation of environmental stress. [ABSTRACT FROM AUTHOR]

This study applied a type of fly ash microsphere (FAM) collected directly from a high-temperature furnace using ceramic dust tubes to high-strength concrete, and with silica fume as a control admixture, investigated the effects of FAM on the hydration and hardening processes of the cementitious materials and the macroscopic properties of the high-strength concrete. Two cement replacement levels (8% and 15%) and two water-to-binder (W/B) ratios (0.35 and 0.25) were utilized. The results show that FAM has a relatively high level of early activity; SEM images indicate that a significant portion of the FAM reacted at early ages in the cement-FAM hardened paste. Though the early activity of FAM is lower than that of silica fume, the cement-FAM complex binder has similar hydration properties with the cement-silica fume complex binder. At 90 d, FAM consumed less Ca(OH) 2 than silica fume and a significant amount of unreacted FAM remained in the hardened pastes. The contributions of FAM to the pore structure of the hardened pastes are lower than those of silica fume at early ages; however, FAM can significantly improve pore structure at late ages, similar to silica fume. In addition, FAM can improve the flowability, late-age strength, and permeability to chloride ions of concrete, while decreasing early-age autogenous shrinkage. [ABSTRACT FROM AUTHOR]