| Ecological ultra-high performance cement-based composites(ECO-UHPFRCC) can simultaneously gain high performance and economy by combined use of several mineral admixtures, superiorities of each kind of mineral admixture can be made use of. On the basis of earlier studies, this paper focus on using rice husk ash to replace part of silica fume in order to improve the mechanical properties of ECO-UHPFRCC. Rice husk ash of high reaction activity obtained by the study of combustion and grinding process. The influence of incorprated rice husk ash on UHPFRCC’s flow properties, mechanical properties, microstructure and hydration were studied, to reveal the effect of rice husk ash on the performance of ECO-UHPFRCC. Main experiments and results are as follows:Performance of rice husk ash were test by several analysis methods:using X ray diffraction technique to analysis the phase composition, SEM analysis of rice husk ash structure, conductivity test and thermal analysis test were carried out to analysis reactivity of rice husk ash. The results show that the ash obtained by combustion of rice husk for 1h at 650℃ can be mainly composed of amorphous SiO2. Rice husk ash is sheet honeycomb structure, lamellar structure is accumulated by tens to hundreds of nanometers SiO2 particles. The activity index of rice husk ash is higher than that of silica fume at early age. With the reaction going on, the 28-days activity index of the mix approachs to the same.Slurry fluidity test and rheological analysis of low w/b cement paste with different rice husk ash dosage show that silica fume reduces the viscosity and the yield stress of the slurry within a dosage of not more than 10%. While rice husk ash dosage increases, fluidity of the paste decreases and the corresponding viscosity and yield stress of the slurry increases. Silica fume and rice husk ash at each content of 10% paste shows the lowest yield stress and the viscosity in condition of 20% combined mixture dosage, from the complementary perspective should be selected rice husk ash and silica fume. Combining use of silica fume and rice husk ash would be better. Mortar fluidity decreases with the increasing of sand binder ratio; flowability decreases slowly with the content of steel fiber steel fiber dosage of 0% to 2%. The flow rate decreased rapidly, when the steel fiber content is more than 2%. The influence of silica fume and rice husk ash admixture content, steel fiber volume ratio, sand cement ratio on flexural and compressive strength of UHPFRCC have been studied. The results show that the maximum compressive strength of UHPFRCC was 199 MPa by steam curing at 85℃ for 3 days. In this recipe, water binder ratio was 1.65, silica fume dosage was 10%, rice husk ash was 10%,fly ash dosage was 30%, steel fiber volume fraction was 3%, and sand binger ratio was 1.0. Combined using of rice husk ash and silica fume can improve the strength of ultra high performance cementitious composites.Results of fracture ductility test show that:silica fume and rice husk ash each content of 10% improve the fracture toughness and fracture energy of UHPFRCC, either the dosage of silica fume or rice husk ash increases the fracture toughness, fracture energy decreases. The increases of fiber volume fraction can significantly enhance the fracture ductility of the matrix. While the fiber volume fraction is larger than 2%, the enhancement effect is not obvious.Scanning electron microscope, mercury intrsion test showed that, the UHPFRCC incorporated with rice husk ash and silica fume has more gel pore while the cell pore porosity decreases. No obvious interfacial transition zone of UHPFRCC, aggregate and the mortar, steel fiber and the matrix were found. Cement hydration heat experiment and quantitative Rietveld analysis results showed that rice husk ash can accelerate early hydration with a short dormant stage, combining using of rice husk ash and silica fume, the cement hydration degree increases more obviously. |
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