Influence Of Pore Structure On Cement-Based Materials Frost Resistance Property

Freeze-thawing damage caused by cold environment is a kind of erosion form distributed widely, the salt lake environment, marine environment and the use of de-icing salt will accelerate cement-based materials damage rate, the rate is several times, even dozen times of pure water freez-thawing environment. Cement-based materials are inhomogeneous multiphase multi-scale composite materials. To reveal the association between the structure and macroscopic durability of these materials, to study the multi-dimensional/multi-scalar characteristics of their pore structures, microstructures, variations in pore structures, and evolution of micro-cracks during salt attack and freeze-thawing are of great importance.According to the different requirements of bridge engineering structure for the performance of concrete, concrete strength grades are C35 and C50. The main indicators are:fresh concrete workability, mechanical properties of concrete, chloride ion permeability, etc. Results show that increasement of air content of concrete will cause the strength reducing, every 1% increase in air content, the concrete strength decreases by 3%-5%. The ratio of bending-compressive strength increases as the air-entraining is higher, and chloride ion penetration resistance is improved; mineral admixtures can also reduce the chloride ion penetration. Research results show that the aggregate in cement-based material can not only play the role of aggregate frame effect, also affect the spatial distribution of pososity. The air-entraning effect of cement paste is lower than concrete, and the mortar has the best air-entraning capability.In this paper, using the straight-line wire method, computed x-ray tomography method (X-CT), mercury intrusion method (MIP) and the analysis of gas adsorption method etc. analyze cement-based materials pore structure. Proposing a multi-scalar pore structure characterization method consisting parameters of porosity, pore size distribution, pore morphology and realizing the real show on the spatial distribution of pores. By studying the different air content of cement-based materials microstructure, multi-scale pore size distribution, pore morphology and the spatial arrangement characteristics and salt frost resistance properties, to establish the relationship between the characteristics of pore structure and macro performance. Straight wire method and X-CT method can measure pore diameter range of μm-mm, which includes in most of the air pores; mercury intrusion method and the analysis of gas adsorption method can measure pores diameter of 3nm-400 u m and 0.4nm-100nm respectively. The result shows that air-entraining agent introducing pores in the range of 50 μ m-350 μ m, which affects most to the cement basrd materials salt frost resistance property.Using the method of the fast freeze-thaw test, to study the outward-inside gradient damage law salt in the process of salt-frozen experiment. By studying the pore structure after different cycles of deicer-frost and micro cracks, with mass variation, the cummulative ammount of peeling off, and the relative dynamic elastic modulus retention, to establish the relationship between macro performance degradation and microstructure damage. Experimental results show that during the freezing-thawing thycles, the cummulative ammount of peeling off increasing gradually, the specimen mass and the relative dynamic elastic modulus retention increase at first and then decrease. Gas content is the decisive factors influencing the antifreeze performance. Concrete under the circumstance of chloride salt and frost damage, the air content should not be lower than 4%. Fly ash and slag powder make air content decrease, and the coefficient of air bubble spacing increase, resulting the salt frost resistance of concrete decrease slightly, so the mineral admixture content of antifrost concrete should not be greater than 35%. Bubble spacing coefficient and frost resistance of concrete has good correlation, in this essay, suggesting air content and bubble spacing coefficient should both be considered during the designing of antifrost concrete. The research results of this paper can provide theoretical and pratical significance for the salt frost resistance concrete.Analyzing the corrosion phase and morphology by X-ray diffraction method, scanning electron microscope method and energy spectrum method. The experiment results show that the cement based material composed of dense gel hydration product before freezing-thawing thycles, the chloride and sodium ions diffusion into the substrate after freezing and thawing cycles. The dense hydration product in the pores transforms into a loose rod-shaped morphology. Medium concentration of salt solution has the most destructive effects to cement based material.