Preparation And Investigation On The Chromium Slag Green Concrete

Chromium slag is a common waste slag in metallurgical industry.Chromium slag is toxic to the human body when it contains hexavalent chromium.The accumulation of chromium slag can lead to air pollution,surface water pollution,groundwater pollution and ecological degradation and other issues if it is lack of harmless or the harm reduction.In order to promote the large-scale comprehensive utilization of chromium slag,the research is focus on application of chromium slag as building materials in cement and concrete on the basis of evaluation of harmless.This can be effective utilization of valuable physical and mechanical performance of chromium slag and also can save precious natural resources.In this paper,combined with physical,chemical and structural characteristics of three kinds of chromium slag,the physical and mechanical performance of concrete prepared with microchromium slag(MCS)as admixture,water-cooled chromium slag(WCS)and silicon chromium slag(SCS)as as fine aggregate,high carbon chromium slag(HCS)as coarse aggregate are studied,and the effect of heavy metal(chromium)immobilized in the cement and concrete is tested and evaluated.Emphasis are given on the analysis that the effect of different content of MCS for the physical properties,hydration process,type of hydration products and mechanical properties of hydrated cement paste.This paper explores the difference of mechanical properties,penetration resistance of chloride ions,freeze-thaw resistance,carbonation resistance and microstructure between chromium slag concrete and ordinary concrete,in allusion to the performance problems of chromium slag concrete,silica fume or natural river sand are compounded to improve mechanical properties and durability of chromium slag concrete.The changes of interfacial transition zone(ITZ)are measured by X-ray diffraction(XRD),scanning electron microscope(SEM),microhardness and so on.Experimental results show that:(1)With the content of MCS increasing,the water demand for normal consistency of cement shows a trend of slow shorten,the initial and final setting times of cement is slightly prolonged and the autoclave expansion of samples increase.It can be obtained that when content of MCS increased to 50 wt %,the autoclave expansion is 0.22% and still less than 0.5% of expansion rate limit according to the relevant standard.Incorporation of MCS can reduce the total hydration heat and the early compressive strength of hardened cement paste,but the rate of later compressive strength development is higher than ordinary cement.The Cr concentration in leachate was 0.015 mg/L when the content of MCS reached 40 wt % at leaching cycle 0-7 d.It is far below 0.05 mg/L of the allowable concentration given by World Health Organization(WHO)for Cr in drinking water.(2)Compounded with silica fume can improve the microstructure,durability,mechanical properties and ITZ's microhardness of MCS concrete.With the content of MCS increasing,penetration resistance of chloride ions and freeze-thaw resistance of MCS concrete improve gradually,compounded with 5wt% silica fume is the best on performance of the carbonation resistance,compounded with 10wt% silica fume is the best on improvement of the 7 d and 28 d compressive strength.(3)The SCS and the WCS have a certain pozzolanic activity and used as fine aggregate can effectively raise strength of ITZ.The later compressive strength of mortar made by SCS and WCS develop rapidly,the 28 d strength of mortar are 56.2 MPa and 51.8 MPa separately,the strength of SCS mortar is even higher than natural river sand mortar.The Cr concentration of SCS mortar and WCS mortar in leachate are 0.042 mg/L and 0.033 mg/L at leaching cycle 0-7 d.(4)The SCS and the WCS used as fine aggregate can effectively improve mechanical properties,penetration resistance of chloride ions,freeze-thaw resistance,carbonation resistance of concrete.Compounded with a certain amount of natural river sand can further raise the mechanical properties of the SCS concrete and the WCS concrete,the optimal content of SCS and WCS used as fine aggregate in concrete is 60-80wt%.(5)The compressive strength of the concrete prepared by HCS as coarse aggregate is equivalent to that of ordinary concrete at 28 d.The surface of HCS is porous,the porosity of HCS is higher than common crushed stone and this provides permeability channel for external medium into internal concrete,it leads that the penetration resistance of chloride ions and freeze-thaw resistance of HCS concrete are slightly worse compared with the ordinary crushed stone concrete.(6)According to the design scheme of a variety of chromium slag concrete in this paper,the early compressive strength of concrete is lower than ordinary concrete,but higher the later compressive strength is.The concrete prepared by a variety of chromium slag can effectively improve performance of penetration resistance of chloride ions.The Cr concentration of concrete in leachate is below the allowable concentration given by World Health Organization(WHO)for Cr in drinking water at leaching cycle 0-28 d.This shows that the feasibility of comprehensive utilization of a variety of chromium slag in concrete from the angle of environment and health.