Properties And Mechanisms Of Cement-Based Composites With New Types Of Nano-fillers

The innovation and modification of cementitious composites have attracted much attention in recent years.The objective is to improve the mechanical property and durability of cementitious composites,thus making the sustainable development of building materials.Existing research results show that there are obvious small size effect and surface effect of nano-materials,which show great potential in enhancement of strength and modification of the functional property of cementitious composites.In this thesis,nano-BN,nano-cellulose fibers（NCFs）,nano-MgO（NM）,nano-Al₂O₃（NA）and nano-ZnO（NZ）are added into reactive powder concrete（RPC）.The mechanical properties of composites with different types of nano-fillers are studied,and their underlying mechanisms are investigated.Moreover,the rheological properties of cement paste with NCFs are studied.The main contents and results are as follows:1 The effects of different BN sizes on the flexural and compressive strength of RPC are studied.Based on the results,120nm BN as the best enhancing nano-particle was added into RPC with different filler dosages and under different curing methods.It is illustrated that the compressive strength of RPC with 0.5%120nm BN increased by 15.7%and 12.6%compared with that without BN.BN is widely distributed in the cement matrix and achieves nucleation effect.The early hydration of cement is promoted and the quantity of C-S-H internal product increases.The network of C-S-H is more abundant and CH crystal is minimized.Thus the matrix is denser.2 The rheological behaviors of NCF solution and cement paste with different NCF dosages were studied by fitting with rheological model.The rheological parameters of cement paste with different NCF dosages are analyzed.The mechanical properties of RPC with two types of NCFs are studied.The results show that NCF thickens the solution especially when the dosage is above 2.5%.The NCF cement paste exhibits shear thinning behavior.When the NCF dosage increases,the yield stress and plastic viscosity of cement paste rise significantly and gradually being faster.The flexural strength of RPC with NCF at 3d increases compared with RPC without NCF.However,the compressive and flexural strength of RPC with NCF at28d reduces.The reason of the strength decrease is that the fillers didn’t disperse well,so the matrix cannot be well-distributed.3 The effects of NA/NM/NZ on mechanical properties of RPC with different filler dosages and their underlying mechanism are studied.Results show that NA enhances the early strength of RPC by pozzolanic reaction at early stage.However,the agglomeration of NA with high dosages will reduce the strength of RPC at 28d.Due to expansion and filling effect,NM can improve the flexural strength of RPC at 3d,but its effect the strength at 28d is little.NZ reduce the strength of RPC due to its retardation effect.