RESUMO
Concrete structures can be degraded by exposure to environmental stressors such as freeze-thaw cycling and salt corrosion. Magnesium potassium phosphate cement (MKPC) mortar is useful for the rapid repair of such structures but must acquire environmental resistance rapidly. In this study, the freeze-thaw resistance of MKPC mortar specimens of different hydration ages was tested in water and a 5% Na2SO4 solution. The strength, volume deformation, and water absorption rates were compared with those of full-age MKPC mortar specimens (28 d). The phase composition and microscopic morphology of the MKPC mortar specimens before and after corrosion were observed, and the corrosion-resistance mechanism was analyzed. After 225 freeze-thaw cycles in water and sulfate solution, the strength residual rates of the early-age specimen (1 d) were higher than those of the full-age specimen (28 d). The degree of strength attenuation in the 1 d specimen was lower in the sulfate environment than in the water environment. After 225 freeze-thaw cycles, the volume expansion rates of 1 d specimens in water or sulfate were 0.487% and 0.518%, respectively, while those of 28 d specimens were 0.963% and 1.308%. The comparison shows that the 1 d specimen had significantly better deformation resistance under freeze-thaw than the 28 d specimen. After 225 freeze-thaw cycles, the water absorption rates of 1 d specimens were 1.95% and 1.64% in water and sulfate solution, respectively, while those of 28 d specimens were 2.20% and 1.83%. This indicates that freeze-thaw cycling has a greater effect on the pore structure of fully aged mortar than on early-age mortar (1 d). Therefore, MKPC mortar is suitable for the rapid repair of concrete structures in harsh environments. The results form a theoretical basis for winter emergency repair projects. They also further the understanding of the application of MKPC-based materials in extreme environments.
RESUMO
Concrete facilities in the severe-cold areas of western China (salt lake environments and heavy saline soils) are seriously damaged by the multiple corrosion effects of freeze-thaw cycles and sulfate corrosion. Magnesium phosphate cement (MPC) cement-based material has become an ideal concrete structural component because of its superior performance. Because concrete structural repair materials are used in heavy-corrosion environments, their durability in those environments should also be considered. Regarding the salt-freezing resistance of MPC, the existing studies have all used a NaCl solution as the heat transfer medium. In addition to chlorine salt, sulfate, especially Na2SO4, is also common in typical use environments such as oceans, salt lakes, and groundwater. To evaluate the sulfate freeze-thaw resistance of potassium magnesium phosphate cement (MKPC) mortar, in this study the strength development, weight loss, and water absorption of MKPC mortar specimens subjected to different freeze-thaw cycles were tested and compared with those for Portland cement (P.O) mortar specimens of the same strength grade. The results showed that the P.O mortar specimen completely lost its strength after 75 cycles of rapid water freezing and thawing and 50 cycles of sodium sulfate solution (5%) freezing and thawing. However, the residual strength rating of the MKPC mortar specimen after 75 cycles of water freezing and thawing and 100 cycles of sodium sulfate solution freezing and thawing was higher than 75%. After 50 rapid freeze-thaw cycles in water and a 5% Na2SO4 solution, the P.O mortar specimen's mass loss exceeded the 5% failure standard, whereas the mass loss of the MKPC mortar specimens was much less than 5%. Before the freeze-thaw cycles, the water absorption of the P.O mortar specimen was close to 8 times that of the MKPC mortar specimen, and after 50 water freeze-thaw cycles and 25 sulfate solution freeze-thaw cycles, the water absorption reached 4.88% and 5.68%, respectively. However, after 225 freeze-thaw cycles in water and the sulfate solution, the water absorption rates of MKPC mortar specimens were 2.91% and 2.51% respectively. The test and analysis results show that the freeze-thaw resistance of MKPC mortar was much higher than that of Portland cement mortar specimens. Those results provide a prerequisite for applying and expanding the use of MKPC-based materials in severe-cold areas of western China (salt lake and heavily saline soil environments).