Investigation On Evolution Of Pore Structure Of Concrete Under Seawater Environment

Pore structure is one of the most important characteristics of concrete materials, it strongly influences permeability resistance, frost resistance and compression strength. The interfacial transition zone (ITZ) between cement paste and aggregate in concrete is the weakest link, it has low strength and is easy to cause crack. Investigating evolution law of microstructure adopting appropriate characterization methods is beneficial for grasping relationship between performance and composition, structure from different levels so as to evaluate the performance of concrete, it has important significance for making high performance cement-based materials.Pore structure and ITZ in concrete under seawater and freshwater environment are characterized by advanced testing technology, evolution law of pore structure and effects of mineral admixtures on pore structure and ITZ are investigated and mechanism on evolution law of pore structure in seawater environment and improvement mechanism on effects of mineral admixtures on pore structure and ITZ are analyzed in this paper. The results show that:(1) Results of Mercury Instrusion Porosimeter (MIP) and microscope show that at 3d curing age, porosity is lower and there is a higher ratio of fine porosity under seawater environment than freshwater environment. When it comes to 28d, porosity is lower and there is a higher ratio of fine porosity under freshwater environment than seawater environment. At 180d, the change is more obvious.(2) Microhardness of ITZ and SEM test results show that at 3d curing age, microhardness of ITZ in seawater is larger than under freshwater environment, the width of ITZ is smaller under seawater than under freshwater environment, and connection between ITZ and aggregate is more denser. At 28d, microhardness of ITZ in seawater is smaller than under freshwater environment and the width is larger under seawater than under freshwater environment. Meanwhile ITZ becomes looser, and there are some cracks under seawater but it is denser and the porosity is lower under freshwater environment. When it comes to 180d, microhardness of ITZ under seawater is smaller than under freshwater environment obviously, the width is larger than under freshwater environment obviously and there are more cracks and porosity is bigger under seawater environment. Corrosion is more serious under seawater in later curing days.(3)With the adding of slag, silica fume, and metakaolin, porosity decreases gradually, the critical pore diameter, most probable pore diameter and average bubble radius reduce. Pore structure in concrete is optimized, pore size distribution is more reasonable. Microhardness of ITZ increases, the width decreases and ITZ is denser. The mineral admixtures improve pore structure and ITZ, and the effect is as follows: metakaolin>silica fume>slag.(4) At 3d curing age, compressive strength is higher under seawater than under freshwater environment. When it reaches 28d, discipline of the compressive strength has changed. At 180d, compressive strength is obviously higher under freshwater environment than under seawater. With the adding of slag, silica fume, and metakaolin, compressive strength gradually increase, it has been related with improving effect of mineral admixture on pore structure.(5) At 3d curing age, some components under the seawater react with components in concrete and produced undissolved product, it causes promoting hydration and crack "self-closure" effect, so pore structure is more refining under seawater than freshwater environment. At 28d, with the deepening of hydration degree, concrete becomes densification gradually. Interaction corrosion of various ions under seawater making pore structure degraded gradually. When it reaches 180d, corrosion and degradation of pore structure is more apparent. Actually, the mineral admixtures improve pore structure and ITZ mainly through the effects of micro aggregates filling and pozzolanic activity.