Preparation of magnesium potassium phosphate cement using by-product MgO from Qarhan Salt Lake for low-carbon and sustainable cement production

Abstract

Herein, to reduce CO₂ emissions and energy consumption and to promote the recycling of waste resources, two types of boron-containing MgO by-products, which were obtained by lithium extraction from Qarhan Salt Lake, China, were used as substitutes for dead-burned MgO to prepare magnesium phosphate potassium cement (MKPC) as a rapid repair material. First, the phase composition and particle-size distribution of the MgO by-product were investigated. The effects of different MgO sources, molar ratio of MgO to KH₂PO₄ (M/P), and curing age on the setting time and mechanical properties of MKPC were then studied. Based on the results, the mix proportion of MKPC was optimized. Finally, X-ray diffractometry, scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), differential thermogravimetric (DTG) analysis, and mercury intrusion porosimetry were used to characterize the phase and microstructure evolution of MKPC prepared with different MgO contents. The results demonstrated that the by-product MgO prolonged the setting time of MKPC to more than 40 min. In addition, in the initial stage of hydration, the compressive strength of the MgO by-product was slightly lower than that of the dead-burned MgO; however, with increasing age, the mechanical properties of MKPC prepared by by-product MgO were excellent (up to 60 MPa). The phase and microstructure results revealed that the main hydration product of MKPC prepared using the three types of MgO was MgKPO₄·6H₂O. Combined with the physical and chemical properties of the raw materials, it was confirmed that the larger particle size and the coexisting impurities from the salt lake were the main reasons for the longer setting time of the MKPC prepared by the by-product MgO. We believe that this research will be of great significance for the preparation of low-carbon, low-cost, and high-performance MKPC materials.

Keywords: Boron-containing by-product MgO; Mechanical properties; Microstructure; Retarding mechanism; Setting time.