| With the improvement of strength and durability requirements of concrete materials for structural engineering,more and more concrete materials was made with low water-to-binder ratios and mineral admixtures to meet the various aspects requirements for practical engineering.Compared to the materials with high water-binder ratios,the characteristics of the strength development rules,the hydration degree and the pore structure of the low water-to-binder ratio cementitious composites have changed in different degrees.This paper systematically studied the influence mechanisms of water-to-binder ratio,mineral admixtures,and curing temperature on the material properties of above-mentioned by tests,such as macro-mechanical performance tests,chemical bond water experiments,hydrochloric acid dissolution experiments,mercury intrusion experiments,and capillary water absorption experiments.Meanwhile the influencing factors and change mechanism of the mechanical properties,pore structure and hydration degree of cementitious composites were also studied.(1)In this paper,the change rules of the mechanical properties of the cementitious composites were studied by mechanical properties experiments.Under the low water-binder ratios condition,the strength of materials was not monotonously increased with the decrease of the w/b ratio.The critical water-binder ratio of the slurry was defined;and the increase of the curing temperature would significantly reduce the critical water-binder ratio of the cementitious composites.Moreover,different mineral admixtures also showed significant differences in the change of the critical water-binder ratio;fly ash cenosphere would reduce the critical water-binder ratio of slurries under different curing regimes,while silica fume was exactly the opposite.(2)The results of chemical bound water experiments showed that the water-binder ratio was still the most important factor that restricted the hydration degree.In the slurry above 0.15 water-binder ratio,the hydration degree of cementitious materials was basically linear with the water-binder ratio.The addition of fly ash cenosphere and silica fume would cause the hydration degree of the cementitious composites to decrease.The curing temperature could increase the hydration degree of the cement paste,while reduce the total hydration degree of the cementitious composites.The contribution of the bound water of the fly ash cenosphere in the cementitious composites was separated by the hydrochloric acid dissolution method.The results showed that the incorporation of the fly ash cenosphere actually reduced the hydration degree of the cement.(3)The results of mercury intrusion experiment and capillary water absorption experiment showed that reducing the water-binder ratio of the slurry,incorporating mineral admixtures,and increasing the curing temperature could finely refine the pore structure of the hardened slurry and reduce the porosity.Comparison of pore changes in different sizes,it showed that the change of transition hole was the most significant.When the water-binder ratio is low to 0.13,some of the transition pores coarsening phenomenon might occur,which would lead to reduced strength and permeability of the hardened slurry,particularly in the high-temperature curing.(4)Finally,by analyzing the strength,hydration degree and pore structure of the materials under different influencing factors through lateral comparison,it was found that there was an intrinsically positive correlation between the material strength and the pore structure.To a certain extent,the refinement degree of pore structure influences the mechanical properties of the hardened paste with low water-to-binder ratio,while the hydration degree of the cementitious material is only a supporting role.Therefore,when configuring high-strength or high-performance concrete,more attention should be paid to the dense packing of materials and pore structure refinement,which is more conducive to improving the mechanical properties and durability of materials. |
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