Mechanical and microstructure of magnesium potassium phosphate cement with a high concentration of Ni(II) and its leaching toxicity

Magnesium potassium phosphate cement (MKPC) has outstanding prospects for application in the field of solidification/stabilization (S/S) of heavy metals. However, the influences of various heavy metal ions on the strength and microstructure of the MKPC markedly differ, resulting in differences in leaching toxicity with the same mixture ratio. In this paper, the relationships of the strength, micromorphology, pore structure of as-prepared samples, crystallization degree of hydration products, and leaching toxicity of the MKPC with different MgO to KH2PO4 (M/P) mass ratios and Ni(II) content were studied. The results showed that a low M/P ratio is beneficial for reducing the leaching toxicity of the solidified from. Ni(II) significantly affects the compressive strength for specimens with M/P ratios larger than 3:1, and the compressive strength of the specimens decreases with higher Ni(II) content. However, for specimens with lower M/P ratios, such as 2:1, Ni(II) slightly increases the growth rate of the compressive strength of specimens. X-ray diffraction (XRD), the Brunauer-Emmett-Teller method (BET), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) indicated that Ni(II) also reduces the compactness of the MKPC structure and the crystallization degree of the hydration product, K-struvite, especially for specimens with larger M/P mass ratios. The S/S performance of the MKPC for Ni(II) is directly related to the compactness of the structure and the crystallinity of the hydration product. Based on the excellent solidification performance of MKPC for Ni(II), it is expected to have a wide range of application prospects in the field of solidification of Ni(II)-containing wastes. (C) 2020 Elsevier Ltd. All rights reserved.