# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 1 Introduction
Thus, magnesium phosphate cements are used in various applications, such as the rapid repair of airport runways, bridges, and tunnels [18; 21; 22] but also as bone or dental cement due to their good biocompatibility [23; 24; 25]. MPC hardens due to an acid-base reaction between magnesium oxide and phosphoric acid or phosphate salts in the presence of water [18]. Ammonium dihydrogen phosphate (NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)) and potassium dihydrogen phosphate (KH\({}_{2}\)PO\({}_{4}\)) are the most frequently used phosphate salts for MPC manufacture. [PERSON] et al. [14] prepared an NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\)-based MPC and tested it as a fire-retardant coating for steel structures. The fireproof performance of this coating was adequate, and the use of expanded vermiculite as a lightweight aggregate improved the fire performance of this material. The main drawback when using NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) as a precursor for MPC manufacture is the release of ammonia (toxic gas) both during its preparation and after hardening. Therefore, in a previous study, we used KH\({}_{2}\)PO\({}_{4}\) to prepare magnesium phosphate cement (MPC) using dead burnt magnesia as a precursor and calcium magnesium phosphate cements (CMPC) based on calcined dolomite [16]. The obtained results showed that the coatings based on MPC and CMPC (without borax addition) applied to a steel plate, tested in direct contact with a flame, prevented the temperature of the metal substrate from increasing over 500 \({}^{\circ}\)C. Moreover, during the entire period of the test (45 min), no exfoliation was noticed, i.e., the coatings had good adhesion to the metal substrate. Additionally, although the MPC coating did not crack or delaminate in the area where it was in direct contact with the flame, visible cracks were formed on the adjacent zones during the fire test. In this respect, in this study, we aimed to improve several properties of MPCs based on dead burnt magnesia and KH\({}_{2}\)PO\({}_{4}\), i.e., to increase the setting time of the cement paste as well as its adherence to the steel substrate before, during, and after fire test. Starting from previously obtained results [16] and based on the available information in the literature, in the formulation of MPC coatings studied in this research, other components were added: a styrene-acrylic dispersion (Ac) with the aim of improving the elastic behavior of the MPC coating and at the same time to increase the MPC setting time [26]; fly ash in order to extend the setting time and to improve the MPC adhesion to the substrate [17; 27]; waste glass powder in order to extend the setting time, to improve the mechanical properties and to slightly decrease the melting temperature of the material [15; 28]; and expandable graphite in order to increase the porosity of coating during the fire test [29]. To the best of our knowledge this is a new approach in the attempt to improve the above-mentioned properties. Thus, in this paper we present information regarding the ability of these coatings to prevent the increase in the substrate (carbon steel plate) temperature, along with the influence exerted by high temperatures on the compressive strength of MPCs, and volume and mass changes in correlation with the change of mineralogical composition (assessed by XRD) as well as on the adhesive strength of the coatings to steel substrate.