Effect of fly ash on the corrosion resistance of magnesium potassium phosphate cement paste in sulfate solution

Abstract In order to explore the effect of fly ash on the corrosion resistance of magnesium potassium phosphate cement (MKPC) paste in sulfate solution, pure MKPC pastes (M0) and MKPC pastes containing 20% fly ash (M1) were prepared and soaked in water and 5% Na2SO4 solution for long-term immersion corrosion test. The strength, volume deformation and water absorption rate of MKPC paste specimens at different soaking ages were measured. The pore structure, phase composition and microstructure of MKPC paste samples were also analyzed. The results show that adding a proper amount of fly ash can improve the strength and the pore structure of MKPC paste hardened for 28 under natural conditions. Compared with those of M0, the residual strength rates of M1 soaked in water and 5% Na2SO4 solution for 360 days both increased by more than 5%. At 360-day soaking age, the volume expansion rates of M1 soaked in water and 5% Na2SO4 solution were two orders of magnitude smaller than those of M0 under the same condition. Therefore, adding a proper amount of fly ash can improve the resistance of hardened MKPC paste to long-term water and sulfate corrosion. The morphological, filling and pozzolanic effects of fly ash enable MKPC paste to have a dense initial structure, which make corrosive media not easy to penetrate, newly generated phases containing sulfate filled the pores of hardened MKPC paste, further making the structure denser and reduce the structural deterioration of hardened MKPC paste under the condition of long-term sulfate erosion.