Low Carbon Ultra High Performance Concrete(UHPC)

Ultra-high performance Concrete（UHPC）is a new generation of cement-based material,which is characterized by high strength（≥150MPa）,excellent durability and toughness.It has huge prospects to apply in bridge structure,super high-rise building,ocean engineering structure and other special engineering.However,due to the high cement usage（700-1000 kg/m3）,UHPC has high carbon emission and high material cost.Therefore,to reduce the dosage of expensive raw materials and find substitution of raw materials with scarce sources（such as high-quality natural river sand）,a variety of solid wastes are added to achieve ecological development of UHPC.However,there are some problems while the solid wastes are added to UHPC,such as low utilization rate of solid waste resources,complex pretreatment,dense particle packing damaged by solid wastes with changing particles size,and unclear interaction mechanism between UHPC paste and toxic elements introduced by solid waste.To solve the problem aforementioned,firstly,this paper uses the concept of full scale to design UHPC（tailings used as raw materials）,and the solid waste does not need to be screened.Based on the Modified Andreasen and Andersen Model,the low-carbon UHPC with excellent properties is designed.Then,the influence of raw materials’particle size distribution on the optimal range of distribution modulus（q）,a key parameter in MAA model,is explored.The performance of UHPC is controlled by dynamic q value,and the solid wastes with a wide range of particle size can be introduced to better design the low-carbon UHPC.Finally,the effects of three typical soluble heavy metal ions on the properties and hydration process of UHPC is studied,because numbers of heavy metal ions are discovered in common industrial solid wastes.The main conclusions and achievements of the research are follows:（1）In view of the current low utilization rate and complex preprocessing of solid waste,the low carbon footprint UHPC with gold tailings based on the idea of full scale recycling is designed.Due to the fact that the particle size distribution of the recycled gold tailings is normally wild,from micrometer to millimeter,it could be utilized to replace cement and aggregates in UHPC simultaneously,which can get rid of the sieving process and improve the recycling efficiency.The fresh and hardened properties of the newly developed UHPC are investigated,including flowability,compressive strength,hydration kinetics,durability and microstructure.In addition,environmental evaluation proves that the designed UHPC with gold tailings is a green building material with advanced properties and sample process.（2）The particle size gap between different solid wastes is obvious,and the influence of such particle size difference on the packing structure is seldom considered in the design process.Therefore,the influence of raw materials’particle size on optimal range of distribution modulus（q）is investigated.In this chapter,the deviation between experimental mixtures and different target curves（with different q）are concluded based on Weighted Residual Sum of Squares（WRSS）.The decrease of q can increase the wet packing density and reduce the water film thickness（WFT）,which enables performance control of UHPC.When the distribution modulus（q）is controlled in the range of 0.17-0.19,a sustainable UHPC with advanced properties and microstructure can be produced.This implies that the normally selected distribution modulus（q）value–0.23 can not always guarantee the maximum packing density and best characteristics for UHPC matrix.（3）Solid wastes often contain a large number of toxic elements,and seldom research focus on the performance variation caused by heavy metals in UHPC.Therefore,on the basis of previous experiments,three specific heavy metal elements in gold tailings are selected to discuss the effect on the properties and hydration process in the soluble state,including Mn²⁺,Pb²⁺and Cr O₄^2-.The obtained experimental results show that UHPCs performance depends on the character of precipitation,which is the product of metal ions in alkali environmental.Mn²⁺ions accelerate the hydration process,while the precipitation of Pb²⁺and Cr O₄^2-cover the surface of cement particles,inhibiting the hydration process.The results above are confirmed in FT-IR,XPS,and solid state NMR,indicating that the addition of Pb and Cr inhibits the polymerization degree of Si-O tetrahedron.The introduction of a small amount of heavy metals（about 1g metal ions/kg raw material）has a significant effect on performance.Through the research,the limitation caused by solid wastes’particle size range on efficient utilization is solved.The idea of full scale and dynamic adjustment of distribution modulus----q,contributing to the design of low carbon UHPC with dense particle packing.Meanwhile,the effects of three kinds of soluble heavy metal ions on the properties and hydration process of UHPC are studied,which contributes to the utilization of metal contained wastes in UHPC.