| Oxidation of sulphide in sulphide-containing tailings produces acid mine drainage,which will lead to serious acid and heavy metal pollution,thus resulting in soil acidification and poisoning.Using cementitious binder to stabilization/solidification sulfide-containing tailings to prepare cemented paste backfill(CPB)(stabilization/ solidification)is an effective measure to solve the problem of sulfide-containing tailings.In this study,NaOH and active MgO are used as the activator to actived slag,and then used as the binder(NaOH-activated slag(NAS)and MgO-activated slag(MAS))to prepare cemented paste backfill(NA-HG-CPB and MAS-HG-CPB)containing sulfide-containing tailings.In addition,OPC-CPB is prepared from Ordinary Portland cement(OPC)for control test.The effect of calcined desulfurization gypsum(?-hemihydrate gypsum,HG)gradually replacing NAS or MAS on the basic properties of the solidified body is studied by single factor variable method in laboratory.The mineral composition and microstructure of the solidified body are studied by microanalysis(XRD,SEM,MIP)and leaching solution test.The solidification/ stabilization mechanism is also discussed.The main conclusions are as follows:(1)The main hydration products of NAS-HG-CPB are crystal dihydrate gypsum and ettringite,hydrated calcium silicate aluminate gel(C-A-S-H)and hydrated sodium silicate aluminate gel(N-A-S-H).The 28 d UCS of NAS-HG30-CPB(NAS is replaced by 30% HG in binder)increases by 42% compared with that of NAS-HG0-CPB(no HG in binder),as the binder content is 14%.The reason is that as the proper HG substituted NAS,HG formed ettringite completely,which accelerated the hydration process of slag in alkaline solution.And the microstructure became denser with the addition of hydration products.Both NAS-HG0-CPB and NAS-HG30-CPB have stable long-term compressive strength(180 d)without strength loss,which is attributed to the dense microstructure.When excess HG replaced NAS(?50%),the 28 d UCS of NAS-HG50-CPB and NAS-HG70-CPB decrease,which is due to the reduction of amorphous hydration products would lead to loose microstructure.In addition,due to the loose microstructure,the sulphide in the tailings is oxidized and decomposed into sulfate and generates secondary gypsum with expansion property,the solidified body is not stable.(2)The main hydration products of MAS-HG-CPB are crystal dihydrate gypsum,ettringite and hydrotalcite-like,amorphous calcium silicate hydrate gels and magnesium silicate hydrate gels(C-S-H/ M-S-H).Compared with MAS-HG0-CPB,14 d and 28 d UCS of MAS-HG30-CPB increase by 460% and 91%,respectively.The reason is that the formation of ettringite accelerate the hydration process of slag in alkaline solution,increase the hydration products leads to the denser microstructures,as well as the interaction of the appropriate crystals and amorphous hydrated product.Compared with MAS-HG0-CPB,the 28 d UCS of MAS-HG50-CPB and MAS-HG70-CPB decrease by 9% and 73%,respectively,which is mainly due to the reduction of amorphous hydration products leading to the loose microstructure.However,whether HG is mixed or not,MAS-HG-CPB has stable long-term compressive strength,mainly because after long-term curing,the p H value of the leachate of MAS-HG-CPB is very low,and the chemical reaction is slow or stopped(including secondary gypsum and ettringite).(3)The stabilization/ solidification process of both NAS-HG-CPB and MAS-HG-CPB can be divided into four stages: plasticity stage,setting and curing stage,strengthening stage and stabilizing stage.(4)Compared with OPC14-CPB,the 28 d UCS of NAS-HG30-CPB and MAS-HG30-CPB are increased by 58% and 39%,respectively,both NAS-HG30-CPB and MAS-HG30-CPB have stable long-term UCS(the long-term strength of OPC14-CPB decreased).Therefore,alkali activated slag cement mixed with appropriate amount of HG can replace ordinary Portland cement to prepare cemented paste backfill containing sulfide tailings,which is more economically and more environmentally friendly. |
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