Study On Key Preparation Method And Muti-scale Fiber Reinforcement Of Ultralight Foamed Cement-based Materials

The ultralight foamed cement-based material is a novel energy saving and environmental building material,which is outstanding in the lightweight,high specific strength,economics,good heat preservation,and insulation.Meanwhile,the ultralight foamed cement-based materials are widely employed in construction,transportation,decoration,pro-environment and energy conservation,and it has important research and economic values in these areas.Whereas,the rheological properties of chemical foaming cement slurry are rather complex,and that it has lead a key problem to meet the different requirements of pore structure in the different application environment.This has become an unsolved issue of preparation both in foamed cement-based materials and other inorganic porous materials.Besides,numbers of closed pores in the ultralight foamed cement-based material have inevitably decreased the thermal conductivity as well as mechanical performance of materials.Thus,the reinforcement of fibers in ultralight foamed cement-based materials used to be a breakthrough direction to solve this issue.However,works focused on this issue are still insufficient.In this paper,the foamed cement-based materials have been prepared according to the chemical foaming method,and the raw materials are the ordinary Portland cement,accelerating agent and foam stabilizer.Moreover,a multi-scale hybrid fiber reinforced foamed cement-based material is prepared by the short polypropylene fiber and calcium carbonate whiskers.The preparation method,pore size adjustment method and design of mix proportion of the foam cement are investigated by the rheological properties of the slurry test,pore structure analysis,and foam temperature test,respectively.The reinforcement and toughening mechanism of the multi-scale fibers are also studied by the uniaxial compressive experiment and three-point bending test of the notched beam.The main research contents and conclusions can be stated as follows:(1)The ultralight foamed ordinary Portland cement-based materials were prepared by chemical foaming method at room temperature and pressure.Meanwhile the ordinary Portland cement,hydrogen peroxide,sprayed concrete liquid accelerator and calcium stearate foam stabilizer were the only raw materials for preparation.The rheological property and foaming stability of the cement slurry was investigated by the experiments which include the foaming process,slurry rheological performance,hardening of cement and foam mechanism.The equilibrium relationship between the yield stress and the gas diffusion ability of the cement slurry was the key to the prevention of the bubbles coalescence.And the equilibrium relationship between the pore radium and properties of the cement slurry were put forward.The cement slurry rheological performance and foaming process were effectively adjusted by the accelerator and foam stabilizer agents,and the adjustment of pore size and material density were also achived though this relationship.Afterward,the mixture ratio design method of the ultralight foamed cement-based material was proposed based on this equilibrium relationship.And the foamed cement materials with pore radium ranged from 2 to 3mm and density of 0.2g/cm³,0.3g/cm³ were prepared according to this method,and the validity of this method has been confirmed.(2)The foamed cement-based materials were prepared with mono PP fiber,mono Ca CO₃ whisker,and hybrid fibers,respectively.The strength,stress-strain curves,compressive failure,and energy absorbing ability of the ultralight foamed cement-based materials were discussed based on the uniaxial compression experimental results.The plateau stress,energy absorption efficiency,densification strain energy and densification strain were induced to investigate the energy absorbing ability of the muti-scale fibers reinforced ultralight foamed cement-based materials.The results showed that the stress-strain curves of fiber reinforced ultralight foamed cement-based material consisted of three stages which were the elastic-plastic stage,stable platform stage and densification stage.Furthermore,the energy absorbing ability of ultralight foamed cement-based material was further increased by the incorporation of hybrid fibers,which was attributed to the synergetic enhancement of PP fiber and Ca CO₃whisker.The elastic modulus,ultimate linear strength,peak strength,plateau stress,and densification strain energy of ultralight foamed cement-based material was increased to 252%,133%,212%,104%,and 70%compared to the plain sample,respectively.The damage evolution equation was established by the parameter of damage variables and uniaxial compression stress-strain curves.Meanwhile,the quasi-static compression damage constitutive model of the fiber reinforced ultralight foamed cement-based materials was also established by the Weibull function and the specific three-stage under compression,and the quasi-static compression damage constitutive model was established.The constitutive model was a continuous function which can reflect the whole process of the compressive curve.It was also verified by the calculation of compression and energy absorbing capacity.(3)The flexural and fracture behaviors of mono PP fiber,mono Ca CO₃ whisker,and hybrid fibers reinforced ultralight foamed cement-based materials were respectively investigated by the notched beam three-point bending method.Furthermore,the fracture toughness of fiber reinforced ultralight foamed cement-based materials was calculated and analyzed.The critical fracture propagation length,cohesion toughness,fracture energy,the critical value of J integral and crack mouth opening displacement were also calculated.The results showed that the contents of PP fiber and Ca CO₃whisker has significant influences on the anti-crack performance of the ultralight foamed cement-based materials.With the increasing of the PP fiber content,the initial toughness of the ultralight foamed cement-based material was increased first and decreased at a high fiber content.When the PP fiber content was at0.5wt%,the initial toughness was increased up to 30%.The failure toughness of the ultralight foamed cement-based material was increased with the increase of the PP fiber content,and when the PP fiber content was at 1.2wt%,the failure toughness of the ultralight foamed cement-based material was increased up to 100%.Besides,with the increasing of the Ca CO₃whisker content,the initial toughness of the ultralight foamed cement-based material was increased first and decreased at a high fiber content,and when the Ca CO₃whisker content was at 1wt%,the initial toughness and failure toughness of the ultralight foamed cement-based material were increased up to25%and 76%,respectively.After incorporating the hybrid fiber,at the optimum hybrid fiber content,the initial toughness and failure toughness of the ultralight foamed cement-based material was increased up to 25%and 76%,respectively.The results indicated that the fracture toughness of hybrid fiber reinforced ultralight foamed cement-based materials was improved remarkably.Either the initiation fracture toughness and instability fracture toughness of hybrid fiber reinforced ultralight foamed cement-based materials were higher than the incorporation of mono PP fiber or the mono Ca CO₃ whisker.(4)The enhancing effects and hybrid effects of the hybrid fiber reinforcement in the ultralight foamed cement-based materials were calculated to investigate the toughening mechanism of hybrid fiber in the ultralight foamed cement-based materials.The optimum mix ratio of the fiber was also proposed.The result showed that the hybrid fiber reinforced specimens have exhibited the advantages of multi-scale fibers by incorporating PP fiber and Ca CO₃ whisker,which presented the effects of crack grading resistance,strengthen and toughening in the foam cement.The outstanding performances of multi-scale hybrid fiber in cement-based materials were attributed to the crack resistance during different fracture process,and that can provide a toughening effect of 1+1>2.The positive hybrid effects of initiation fracture toughness,instability fracture toughness and J_c of hybrid fiber reinforced ultralight foamed cement-based materials were all observed.Finally,the hybrid fiber combination of 0.8wt%PP fiber and 7wt%Ca CO₃ whisker was confirmed as the optimum mix proportion of hybrid fiber reinforced ultralight foamed cement-based materials.