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

•Ni(II) has an obvious influence on the compressive strength development of MKPC.•Excess Ni(II) will result in a looser structure of MKPC.•The presence of Ni(II) does reduce the quantity and crystallinity of k-struvite.•Lower MgO/KH2PO4 mass ratio has advantageous to S/S treatment of Ni(II).