Study On Concrete With Ground Ferronickel Slag

With the development of the nickel industry,a large amount of industrial solid wastes are produced due to its ultra-high slag discharge rate.The comprehensive utilization rateof ferronickel slag(FNS)in China is still low.Developing the FNS concrete is an effective method for the utilization of FNS.However,the composition of FNS is complex due to different mineral sources and smelting method.Although many researchers have focused on the utilization of FNS in concrete in recent years,the researches are still lack of systematicness.On the basis of summarizing the characteristics of different FNS,systematic investigations are conducted in this thesis to understand the effects of lateritic FNS,as a mineral admixture,on the workability,mechanical properties and long-term durability of concrete.Moreover,special attention is paid to the leaching issue of chromium from FNS concrete.The main results are as follows:(1)The FNS contains a great amount of vitreous and the water demand is lower than cement.Thus,a reasonable content of FNS can improve the workability of fresh concrete.But the cohesion of fresh concrete becomes worse and bleeding increases when the content of FNS is too high.Due to the low activity of FNS,the setting time of fresh concrete is delayed.(2)The mechanical properties and drying shrinkage of FNS concrete with the same water-binder ratio are studied.The compressive strength of FNS concrete increases firstly and then decreases as the FNS replacement goes higher.When the FNS replacement is over 20%,the strength of FNS concrete develops slower than plain concrete and this difference can be compensated by 90 days’ curing.Moreover,the FNS weakens the splitting tensile strength and improves the elastic modulus of concrete.The FNS can be partly activated by steam curing,but it cannot cover the loss of mechanical properties due to the decrease in cement content.FNS concrete with a FNS content less than 20% has a smaller drying shrinkage than pure cement concrete.(3)The long-term performance of FNS concrete is studied based on the mix proportions with the same strength level(28d).Due to the bleeding problem caused by FNS,micro defects formed by the bleeding channel may appear inside the concrete,which reduces the resistance to water penetration of concrete.The FNS replacement results in the lower CH content and alkalinity of concrete,which leads to faster carbonation.The permeability of chloride of FNS concrete increases slightly with the increase of FNS replacement.The high temperature during the steam curing weakens the capability of binding chloride of concrete.But the FNS can mitigate this negative effect.Moreover,the refined pore structure of FNS concrete also slows down the processe of sulfate attack.The performance of steam-cured concrete is better than that of plain concrete during sulfate attack,which benefits from the lower CH content after steam curing in FNS concrete.(4)The utilization of FNS in concrete is safe.Even though the total chromium content in FNS is up to 1%,the proportion of soluble chromium is ultra-low.During the hydration process,the free hexavalent chromium ion is absorbed and solidified by hydration products.Furthermore,the high alkalinity environment of concrete also contributes to the improvement of chromium stability in FNS.During the leaching processe in salt solutions,the leaching amount of hexavalent chromium in the FNS-cement paste is rather high in the sodium sulfate solution compared to the pure cement paste,while it is greatly reduced in the magnesium sulfate solution.