**1. Introduction**

Magnesium phosphate cement (MPC) hardens due to an acid–base reaction between magnesia (MgO) and phosphate acid or a phosphate salt solution [1–4]. The usual source of magnesium oxide is magnesite (magnesium carbonate), which is thermally treated at increasing temperatures to obtain caustic calcined magnesite/magnesia (CCM), dead burned magnesite/magnesia (DBM), and fused magnesia (FM). CCM has numerous applications such as hydrometallurgy, steel industry, ceramic and cement manufacture, fertilizers, water treatment, etc. [5,6]. DBM and FM are mainly used in the manufacturing process of refractory materials [5], but DBM is also a key ingredient in MPC manufacture.

In the 2014 "Report on critical raw materials for EU", magnesite was identified as a critical raw material [5]. Taking into account this aspect, it is important to find alternative sources of raw materials for the manufacture of MPC.

Dolomite is a sedimentary rock containing calcium and magnesium carbonates [7,8]. Dolomite is an important material in various industries such as the pharmaceutical industry, metallurgy, the production of paper, inorganic binders, concrete, fertilizer, refractory bricks, water treatment, absorption of heavy metals, etc. [8–10].

**Citation:** Vijan, C.A.; Badanoiu, A.; Voicu, G.; Nicoara, A.I. Phosphate Cements Based on Calcined Dolomite: Influence of Calcination Temperature and Silica Addition. *Materials* **2021**, *14*, 3838. https://doi.org/10.3390/ ma14143838

Academic Editor: Rossana Bellopede

Received: 7 June 2021 Accepted: 6 July 2021 Published: 9 July 2021

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The calcination of dolomite is used to transform this mineral into magnesium and calcium oxides. Depending on the experimental conditions, i.e., chemical composition of dolomite, presence and amount of impurities, grain size distribution, decomposition temperature and atmosphere (air, carbon dioxide, nitrogen etc.), the thermal decomposition of dolomite in MgO and CaO can proceed in one or several steps (endothermic processes) [8,11–13].

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). The compressive strengths of phosphate cements pastes prepared with this type of calcined dolomite can reach 22 MPa after 3 h of hardening and 63 MPa after 7 days, with sufficient soundness [14].

MPCs can be used for the immobilization of various types of wastes with heavy metals content such as Ni, Pb, Cr, Cd, etc. [15–19]. Heavy metals are toxic and can cause serious health problems. Chromium (especially Cr (VI)) has an important toxic effect; it can produce skin irritation and ulcerations as well as liver and kidney deficiency and, if inhaled, it increases lung cancer risk [20].

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; however, the MPCs leaching toxicity was less than the one assessed for other matrices i.e., geopolymer, calcium aluminum cement, and alkali-activated slag binders.

Therefore, we assessed in this paper the possibility of producing phosphate cements by replacing calcined magnesite with dolomite thermally treated at various temperatures, with/without quartz sand addition. We also evaluated the efficiency of magnesium phosphate cements (MPC) based on calcined magnesite and magnesium and calcium phosphate cements (MCPC) based on calcined dolomite in order to immobilize an industrial waste with high chromium content. To the best of our knowledge, the immobilization of chromium in phosphate cements based on calcined dolomite has been first reported in this paper.
