The Impact of Fly Ash and Steel Slag Powder on Sulfate Erosion Behavior of Magnesium Potassium Phosphate Cement Paste
HOU Yuying1,5, LI Tao1,5, LYU Yin2, CHEN Gang3, HU Xiamin1,5, TANG Lei1,5, YANG Jianming1,4,5,*
1 School of Civil Engineering, San Jiang University, Nanjing 210012, China 2 School of Civil Engineering, Nantong Institute of Technology, Nantong 226001, Jiangsu, China 3 China Construction Eighth Bureau Third Construction Co.,Ltd., Nanjing 210046, China 4 School of Civil Engineering, Yancheng Institute of Technology, Yancheng 224002, Jiangsu, China 5 Jiangsu Engineering Research Center for Low Carbon Materials and Green Structures, Nanjing 210012, China
Abstract: In this work, the corrosion behavior of magnesium potassium phosphate cement (MKPC) specimens modified with fly ash and steel slag powder under long-term immersion in a sulfate solution was investigated. Based on microstructural analysis, using the sulfate ion erosion depth, sulfate ion diffusion coefficient, and strength as characterization parameters, the following conclusions were drawn. Immersed in a saturated 5% sodium sulfate solution, the relationship between the penetration depth of sulfate ions x(mm) and the sulfate ion concentration c(x,t) in MKPC paste specimens follows a second- or third-order polynomial (with a correlation coefficient greater than 0.999). After 360 days of immersion, the sulfate ion concentration inside the MKPC specimens significantly decreased with increasing erosion depth. The erosion depth of the M0 specimen was close to 14 mm, while the erosion depths of the M1 and M2 specimens were approximately 12 mm. The sulfate diffusion coefficient of MKPC specimens was approximately 10-7 mm2/s, which was an order of magnitude lower than that of Portland cement concrete. Under the same conditions, the sulfate diffusion coefficients of the M1 and M2 specimens were 69.7% and 87.5% of that of the M0 specimen, respectively. With the extension of the immersion age, the strength of the MKPC specimens initially increased and then decreased. After 360 days of immersion, the residual flexural and compressive strength of the M1 and M2 specimens were 99.2%, 97.8%, and 97.6%, 96.6%, respectively, which were significantly higher than those of the M0 specimens under the same conditions (92.7%, 94.7%). The experimental results indicate that both fly ash and steel slag powder improve the sulfate resistance of MKPC specimens. Furthermore, the strength and diffusion coefficient of the MKPC specimens followed consistent trends. Microstructural and theoretical analysis confirmed that, under long-term immersion in a 5% sodium sulfate solution, new MgKPO4·6H2O (MKP) crystals continuously formed in the hardened MKPC matrix. The dissolution and phase transformation of MKP, the hydrolysis and carbonation of unreacted MgO, and the crystallization of sulfate-containing salts each had both positive and negative effects on the pore structure of the MKPC matrix, ultimately leading to a gradual deterioration of the MKPC structure.
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2025, Vol. 39 
