Microstructure characteristics of concrete after erosion of magnesium salts and sulfates

Abstract: Concrete structure is usually affected by sulfate and magnesium salt in soil water environmental. In order to explore anti-erosion properties of cement concrete and high-performance concrete mixed with slag powder after immersed in solution with sulfate and magnesium salt, the study designed three water-binder ratio treatments and two slag powder content treatments. After mixing with water, 1050 specimens (10 mm×10 mm×60 mm) were prepared and incubated in conditions of 20 ℃ and moisture≥9.5% for 28 d. Then, they were immersed in five different solutions with different sulfate and magnesium concentrations for determine anti-erosion coefficient and microstructure of the specimens during erosion. The results showed that: The cement had many pores with large pore size when water-binder ratio was 0.50, and the maximum diameter of pore reached 372.5 μm. The improvement of the pore structure was not obvious when the water-binder ratio decreased to 0.35. When the pure cement concrete specimens was immersed in solution with 2500 mg/L SO42- and 1400 mg/L magnesium2+ for 4 months. The pores of specimens were full of hydration products such as CaSO4?2H2O, 3CaO·Al2O3·3CaSO4·32H2O (AFt), and a small amount of Mg(OH)2 and MgO?SiO2?H2O, which could lead to cracks in concrete, peeling off the side parts, even being crisp and surface slurry loss seriously. After 2 months, the erosion coefficient showed a downward trend with erosion duration and was below 0.85, indicating a limited anti-erosion capacity of silicon concrete. When the concrete was added with slag powder, the pore numbers became small and pore size was small. The largest pore size of concrete with water-binder ratio of 0.35 and 0.30 was only 1/7 and 1/8 times as that of concrete with water-binder ratio of 0.50. Meanwhile, the content of Ca(OH)2 was reduced. The concrete structure became loose, which was more obvious for specimens with low water-binder ratio. After 12 months of erosion in solution of sulfate and magnesium salt (20250mg/L SO42- and 2800mg/L Mg2+), the edge of specimen did not have erosion products and crystals of MgSO4·7H2O, Na2SO4·10H2O, Mg2(OH)3Cl·4H2O, and the corrosion resistance coefficient was still more than 0.85. It suggested that slag powder can significantly improve the anti-erosion properties of high performance concrete formulated by 32.5 silicate cement. Therefore, when cements were used for environment with solutions including 13000 mg/L SO42- and 1400 mg/L Mg2+, or 20250 mg/L SO42- and 2800 mg/L Mg2+, the water-binder ratio of concrete should not be greater than 0.35 and 0.30, respectively, while slag powder content was appropriate from 40% to 45%.The study could provide data support and theoretical basis for concrete preparation and application in irrigation engineering (dam foundation, gates infrastructure and channels etc.) and greenhouse concrete foundation engineering.