Abstract
The aim of this study is to assess the possibility of obtaining phosphate cements based on dolomite calcined at various temperatures with/without quartz sand addition. A lower calcination temperature of dolomite (1200 °C) determines a high increase in the system temperature when calcined dolomite is mixed with KH2PO4 (MKP) solution and also a rapid expansion of the paste. The increase in calcination temperature up to 1400 °C reduces the oxides reactivity; however, for lower dosages of MKP, the expansion phenomenon is still recorded. The increase in MKP dosage increases the compressive strength due to the formation of K-struvite. The mixing of dolomite with sand, followed by thermal treatment at 1200 °C, modifies its composition and reactivity; the compressive strength of phosphate cements obtained by mixing this solid precursor with MKP increases up to 28 days of curing. We assessed the nature of hydrates formed in the phosphate systems studied by X-ray diffraction in order to explain the hardening processes and the mechanical properties of these systems. The microstructure and elemental composition of hardened cement pastes were assessed by scanning electronic microscopy with energy-dispersive spectroscopy. The phosphate cements based on calcined magnesite or dolomite were used to immobilize an industrial hazardous waste with high chromium content. The partial substitution of calcined magnesite/dolomite with this waste determines an important decrease in compressive strengths. Nevertheless, the leaching tests confirm an adequate immobilization of chromium in some of the matrices studied (for a waste dosage corresponding to 0.5 wt % Cr).
Highlights
Magnesium phosphate cement (MPC) hardens due to an acid–base reaction between magnesia (MgO) and phosphate acid or a phosphate salt solution [1,2,3,4]
The usual source of magnesium oxide is magnesite, which is thermally treated at increasing temperatures to obtain caustic calcined magnesite/magnesia (CCM), dead burned magnesite/magnesia (DBM), and fused magnesia (FM)
Deng et al [16] studied the influence of Cr3+ (brought in the system by Cr(NO3)3·9H2O) on the compressive strength, microstructure, as well as leaching toxicity of solidified forms into MPCs based on calcined magnesite and KH2PO4
Summary
Magnesium phosphate cement (MPC) hardens due to an acid–base reaction between magnesia (MgO) and phosphate acid or a phosphate salt solution [1,2,3,4]. Yu et al [14] studied the possibility of using dolomite as raw material to produce magnesium phosphate cement According to these authors, mixing fine dolomite with coarse quartz sand and thermal treatment at relatively low temperatures (1100–1250 ◦C) substantially reduces the amount of free lime, and the MgO obtained has an adequate reactivity vs phosphate salt (NH4H2PO4). Deng et al [16] studied the influence of Cr3+ (brought in the system by Cr(NO3)3·9H2O) on the compressive strength, microstructure, as well as leaching toxicity of solidified forms into MPCs based on calcined magnesite and KH2PO4 According to these authors, the presence of Cr3+ changed the system’s pH and affected the morphology of hydration products; the MPCs leaching toxicity was less than the one assessed for other matrices i.e., geopolymer, calcium aluminum cement, and alkali-activated slag binders. To the best of our knowledge, the immobilization of chromium in phosphate cements based on calcined dolomite has been first reported in this paper
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