Effects Of Porous Superfine Powder On The Performance Of UHPC

Compared with ordinary concrete, Ultra High Performance Concrete?UHPC? is a new cement-based composite materials,which have higher strength, better toughness and durability. UHPC have some features, such as low water-cement ratio?0.14 to 0.20?, the high amount of cementitious materials and mineral admixtures. During the setting and hardening process, UHPC will be subjected to a greater shrinkage and deformation, increasing the risk of early-age cracking. Studies have shown that rice husk ash?RHA? partly replacing silica fume?SF? in UHPC could reduce the autogenous shrinkage of concrete. Rice husk ash is a porous material, and ha s high specific surface area, and contains higher amount of active SiO₂. It was found that acid leaching residue of boron mud?ARM? are similar to characteristics of the rice husk ash. In this article, porous superfine powder is define as the ultrafine powder with a porous structure, high specific surface area, the active SiO₂. The influence of acid leaching residue of boron mud partly replacing silica fume on properties of UHPC, such as fluidity, setting time, strength,autogenous shrinkage and drying shrinkage were studied in this paper. Meanwhil e, differential thermal analysis, mercury intrusion porosimetry, scanning electron microscope were used for analyzing the microstructure of UHPC. Based on the study of this article, we can conclude those following results:?1?Acid leaching residue of boron mud are a porous material, and have high specific surface area, and contain higher amount of active Si O 2, which are similar to rice husk ash.?2?When the amount of silica fume repacled by acid leaching residue of boron mud were increasing, the fluidity increased at first and decreased subsequently. The effect of acid leaching residue of boron mud on slump flow of UHPC was consistent with that of rice husk ash; At the same time, the flow loss of UHPC containing acid leaching residue of boron mud was lower t han plain group, but the flow loss of UHPC with rice husk ash increased.?3?Acid leaching residue of boron mud could extend the setting time of UHPC. When the rice husk ash replaced silica fume, initial setting time of UHPC had no obvious change, and final setting time became short.?4?Compared with plain group, the hydration degree of UHPC with rice husk ash was higher, leading to more C-S-H products, and microscopic structure became more dense, and strength increased. Compared with plain group, UHPC with the acid leaching residue of boron mud contained less amount of C-S-H gel in the early age, flexural and compressive strength of which also decreased. However, in the latter peiod, the amount of C-S-H gel in UHPC increased, flexural and compressive strength increasing.?5?With the increased amount of boron mud acid leaching residue, autogenous shrinkage and drying shrinkage of UHPC decreased gruadually. The rice husk ash had the similar effect on the shrinkage of UHPC as acid leaching residue of boron mud. When silica fume were replaced by porous superfine powder, total porosity of UHPC reduced, and the proportion of small holes in pore structure of concrete increased, which led to more refined structure of UHPC.