A Primary Study On Molybdenum Tailing’s Application In Concrete

Tailings produced in the process of beneficiation in the mining industry are the main industrial waste in China.Ecological problems such as air,soil and groundwater pollution are caused by the accumulation of large amounts of tailings,and human health is potentially endangered.Molybdenum tailings（MT）are a composite mineral raw material which is the residual waste of molybdenum ore after the concentrate is moved,its chemical composition is similar to that of cement and fly ash,mainly composed of Si O₂,Al₂O₃ and Ca O.In order to minimize the land occupation as well as environmental pollution problems caused by the accumulation of tailings and promote the building materials utilization of MT,an effective method was developed for the application of MT in concrete production.In this thesis,MT is taken as the research object,the main control factors are MT replacement rate and curing age.Based on the two replacing methods of fly ash and cement paste,a total of 15 mix proportions of concrete were designed.The workability test of fresh concrete was carried out,and the strength growth trend of hardened concrete at early age was analyzed.Combined with the evolution law of microstructure,the change mechanism of strength is explained,and the relationship between micro morphology as well as strength is revealed.The results show that:（1）The effects of different replacement rates and methods on the fresh properties of concrete were studied.The results show that as the MT replacement rate increases,the flowability and filling ability of fresh concrete decreases,and the air content basically increases.When 25%fly ash is replaced by MT,the passing ability of self-compacting concrete（SCC）is improved and the working performance is perfect.However,when the MT replacement is more than 50%,the bad phenomena such as excessive viscosity and aggregate separation was appeared in SCC.In order to ensure the workability of fresh concrete,it is suggested to increase the dosage of water reducer in mix proportion,and MT replacement rate should not exceed 50%.（2）Based on the replacing method of fly ash to prepare MT ordinary concrete.The results show that the total internal water content of early-age concrete increases as the increase of MT replacement rate,and the internal water is mainly composed of physically-bound water.The cubic compressive strength（1）_cu)of concrete at each curing age increases linearly as the increase of internal water consumption rate.1d and 3d water consumption rate is accelerated due to the incorporation of MT,and a higher degree of hydration,the1)_cu of MTF are greater than CG0.After 7d,the1)_cu increases first and then decreases as the increase of MT replacement rate.At 28 d,the1)_cu of MTF25 is higher than that of CG0,which is 39.0MPa.The micro morphology image shows that the powder particles are evenly distributed and the filling effect is obvious.The optimum replacement rate of MT is 25%.（3）Based on the replacing method of fly ash to prepare MT SCC.The results show that the early drying shrinkage rate of SCC increases as the increase of MT replacement rate,and the rapid growth stage of dry shrinkage is within 24 hours.The1)_cu of MTS increases first and then decreases as the increase of MT replacement rate,and the early（1,3,7d）1)_cuis greater than that of CG0.The1)_cu of MTS25 increases fastest,and its 28d1)_cu and splitting tensile strength（1）_sts)are the maximum,which are 38.5 and 2.67MPa.The results of NMR analysis showed that the total porosity of concrete decreases by 25%MT replacement rate,and the changes of pore distribution were as follows:gel pores（<10nm）and medium pores（10-100nm）increases,while capillary pores（100-1000nm）and macropores（>1000nm）decreases,both the1)_cu and1)_sts are inversely proportional to the porosity.The micro morphology image shows that the development of pore structure is poor when the MT replacement rate exceeds 25%.（4）Based on the replacing method of cement paste to prepare MT ordinary concrete.The results show that the 28d1)_cu and1)_sts of MTC decrease by only 3.8%and 1.8%to 46.1and 2.80 MPa when the MT replacement rate is 2.5%,which are equivalent to the strength of CG0 and can be applied to practical engineering.As a large amount of cement paste is replaced by MT,the porosity of concrete increases,especially the number of capillary pores（100-1000 nm）and macropores（>1000 nm）in MTC7.5 and MTC10.The micro morphology image shows that the number of pores and cracks in the cement stone increases significantly,and the structural compactness is poor,which leads to the low strength.It is suggested that the MT replacement rate not to exceed 2.5%.