High-value utilization of boron mud waste: preparation of magnesium oxysulfate cement after primary blank roasting and secondary (NH4)2SO4 roasting

Abstract

Boron mud, as a typical industrial solid waste, calls for value-added use because of environmental problems caused by stockpiling and waste of considerable magnesium resource. In this work, the primary blank roasted boron mud, as well as the secondary (NH4)2SO4 roasted boron mud were used to prepare magnesium oxysulfate (MOS) cement, then, its characterization and mechanical properties were analyzed and compared systematically. Results showed that the primary blank roasted boron mud can replace 45 % of light burned MgO to prepare MOS cement, whose compressive and flexural strength are 59.0 MPa and 8.0 MPa after curing for 28 d. During the secondary (NH4)2SO4 roasting process of boron mud, MgO and Mg2SiO4 in the primary roasted boron mud transform into MgSO4. The secondary roasted boron mud can replace MgSO4 completely, MOS cement rich in 5Mg(OH)2·MgSO4·7 H2O (517) phase can be obtained with compressive and flexural strength of 70.0 MPa and 9.0 MPa after curing for 28 d. When both of the primary blank roasted and secondary (NH4)2SO4 roasted boron mud are used for preparing MOS cement which compressive and flexural strength are higher than the requirements of the national standard, MgSO4 can be completely derived from the secondary roasted boron mud, the primary blank roasted boron mud can replace up to 35 % of light burned MgO at most, and the total utilization ratio of boron mud is 53.3 wt%. This process not only achieves the high-value utilization of boron mud, but also provides a new thought to prepare MOS cement with high mechanical properties.