Effects And Mechanisms Of Manufactured Sand On Rheological Propeties Of Cement Mortar

With the rapid development of infrastructure,river sand(RS)is in a great shortage for market demand.Using manufactured sand(MS)as a substitute of RS in civil engineering construction has become an industry trend.However,the shape and gradation of MS differs significantly from that of RS,which result in variety of rheological properties of concrete.Rheological property is of high importance for characterizing concrete workability.It is noteworthy that the mortar rheological property determines the property of concretes with the same gravel.Thus,the influences of shape and gradation of MS on mortar rheological property and the relevant mechanisms were studied in this thesis,which can provide guidance for MS popularization and solve the problem of RS shortage.In this thesis,a novel digital image processing(DIP)method was proposed,with which different sand shapes were measured precisely.The influences of sand shapes on mortar rheological property was studied using the designed experimental methods.Based on Fuller gradation model,MS with different gradations was used to study the effect of MS gradation on fluidity and rheological parameters of mortar.Finally,Chateau model and excess paste theory were applied to clarify the mechanisms of sand shape and gradation influencing the fluidity and rheological property of mortar.Main conclusions are as follows:(1)Aspect ratios(including length/width and width/thickness)and Roundness were identified as sand shape indexes for DIP method,and the thickness of sand was measured directly by DIP method.Evaluation tests show that the results of Aspect ratios and Roundness measured by DIP method and with a Vernier caliper are equal.The relative error between average of shape indexes measured by DIP and actual values is lower than 1%.Results show that the Aspect ratios of MS are 22% bigger than that of RS,and the Roundness of MS is 10%lower than RS,which indicates that MS is elongated,flaky and more angular than RS.(2)The effects of sand shape and sand volume fraction on rheological property of mortar were studied.Results show that the irregular sand increases yield stress and plastic viscosity of mortar,and the increment effect becomes more significant for high sand volume fraction mortar.Specifically,as for 15% sand volume fraction,the yield stress and plastic viscosity of MS mortar are 5% and 20% respectively higher than RS mortar,while as for 35% sand volume fraction,which are 24% and 154% respectively higher.(3)Based on Fuller gradation model,the effects of Tabol index n on fluidity and rheological property were studied.If n<0.4,the mortar fluidity increases with the increase of n until 245 mm.Yield stress decreases with the increment of n until 29 Pa.And,plastic viscosity decreases with the increment of n until 6.3Pa·s.However,if n>0.4,segregation leads to a complicated situation.(4)Chateau model can successfully describe the relationship between sand shape and yield stress,plastic viscosity.Results indicate that the decrement effect of packing density caused by bigger Aspect ratios and the increment effect of intrinsic viscosity caused by lower Roundness both increased the yield stress and plastic viscosity calculated with Chateau model,and the effects become more significant for high sand volume fraction mortar.As for 15% sand volume fraction mortar,the yield stress of MS calculated with Chateau model is 1.09 times as RS,and the plastic viscosity is 1.25 times as RS.As for 40% sand volume fraction mortar,the yield stress of MS calculated by Chateau model is 1.87 times as RS,and the plastic viscosity is3.82 times as RS.(5)Excess paste theory reveals the mechanisms of MS gradation influencing fluidity and rheological properties of mortar.Moreover,the thickness of excess paste(TEP)calculated by CPM model and excess paste theory is the key parameter that changes fluidity and rheological properties.If TEP<0.60 mm,the mean distance between particles increasing with TEP causes reduction of the particles collision resistance.Consequently,mortar follow resistance decreases and fluidity increased,yield stress and plastic viscosity reduces with the increase of TEP.If TEP>0.71 mm,segregation occurs during fluidity and rheological property tests,MS particles settlement increases the shear stress of testing region,makes a “parallel increment” of flow curves,which leads yield stress increasing with TEP and plastic viscosity maintaining constant.In this situation,the flow circle of mortar fluidity test is constituted of cement paste with less sand particles,and the fluidity keeps constant.