Study On Mix-ratio Design And Mechanical Properties Of High-toughness Desert Sand Engineered Cementitious Composites(DS-ECC)

In the high-performance building materials fields,it is one of directions to prepare engineered cementitious composites(ECC)based on local materials.Typically,high-toughness desert sand cementitious composites(DS-ECC)made by replacing river sand with desert sand comes into our focus.It can relieve the pressure caused by insufficient river sand and has characteristics of environmentally friendly,economy and high performance.However,traditional DS-ECC is more empirical and it's properties are greatly affected by desert sand type,which leads to poor universality to prepare DS-ECC with desert sand.It is of great significance to propose DS-ECC's mix-ratio design method which is suitable for different desert sand through optimizing the empirical method.Based on above all,many studies about DS-ECC,mainly made with Ningxia's Mu Su/Tengger desert sand and local low-cost PVA fiber,were carried out.First,based on uniaxial tensile/compression tests and theoretical analysis to study the mix-ratio design method of DS-ECC and propose optimal basic mix-ratio,which leads to good fluidity and ultra-high early performance.Then,uniaxial tension/compression tests,three/four-point bending tests and microstructure analysis of 28d and 56d DS-ECC were carried out.Finally,verify the feasibility of the mix-ratio design method by comparing the performances of DS-ECC,ordinary river sand ECC and desert sand cementitious matrix(DS-C).Results are as followings.(1)The mix-ratio design method based on slurry coating theory is feasible,which realizes the universality of preparing DS-ECC with desert sand.DS-ECC can be made by Mu Su/Tengger desert sand with the particle size under 0.315mm,slurry coating thickness of 0.09mm,W/B of 0.35,S/B of 0.46 and fly ash content of 50%.It has characteristics of stable workability,obvious strain hardening,high ductility and high toughness.Ultimate tensile strengths and strains of 7d DS-ECC are about 10-13MPa and 5-8%,respectively.Ultimate tensile/compression strengths of 28d/56d DS-ECC are about 10-14MPa and 30-50MPa,and corresponding strains are up to 3-6%and 0.9-1.6%,respectively.Meanwhile,it's ultimate fracture/bending strengths are about 4-8MPa and 21-30MPa,respectively.High-strength,high-ductility and high-toughness DS-ECC can be made by reducing W/B,and ultra-high ductility DS-ECC with enough strengths can be made by increasing fly ash content.(2)At the age of 28d?56d,uniaxial tensile properties of DS-ECC are better than that of ordinary river ECC,such as the ultimate tensile strength/strain and the tensile fracture energy.DS-ECC's ultimate compression strength is slightly lower than that of ordinary river ECC,while it's ultimate compression strain and compression toughness are better than the latter.Aging slightly decreases DS-ECC's ultimate tensile strengths,but increases ultimate compression strengths.However,aging has a negligible effect on the the ultimate tensile/compression ductility.Besides,DS-ECC's ultimate fracture strength is comparable to that of ordinary river ECC,while it's ultimate bending strength is superior to the latter.Both fracture and bending ductility of DS-ECC are better than the latter.Especially,Tengger DS-ECC has the ultra-high bending ductility.All bending toughness indexes,equivalent residual strength indexes and bending toughness ratio of DS-ECC meet the requirement of high-toughness cementitious material.Aging increases ultimate fracture strengths and ultimate bending strengths of DS-ECC,while reduces it's fracture toughness and bending toughness at different degree.(3)In terms of the microstructure and the hydration degree of DS-ECC,results reveal that DS-ECC's matrix is denser than that of ordinary river ECC,which leads to improved interface properties between desert sand/fiber and matrix.Deboned and pulled fibers are clearly observed under loading,which promotes the fiber bridging effect and the reinforced and toughened matrix.DS-ECC's hydration degree is comparable to that of ordinary river ECC at the same age.However,28d DS-ECC's hydration degree is slightly low and diffraction peak strengths of both CH and C-S-H are weak.