*Article* **Phosphate Cements Based on Calcined Dolomite: Influence of Calcination Temperature and Silica Addition**

**Cristina Andreea Vijan, Alina Badanoiu \*, Georgeta Voicu and Adrian Ionut Nicoara**

Department of Science and Engineering of Oxide Materials and Nanomaterials, Faculty of Applied Chemistry and Materials Science, University Politehnica of Bucharest, 1-7 Gheorghe Polizu Street, 011061 Bucharest, Romania; cris\_deea\_vajan@yahoo.com (C.A.V.); georgeta.voicu@upb.ro (G.V.); adrian.nicoara@upb.ro (A.I.N.) **\*** Correspondence: alina.badanoiu@upb.ro

**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).

**Keywords:** phosphate cements; calcined dolomite; quartz sand; calcined magnesite; temperature; K-struvite; industrial waste; chromium
