# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## Materials and Methods  
The materials used in this research were:  
* Magnesia (M) obtained by the thermal treatment of magnesite at 1500 \({}^{\circ}\)C. The magnesia was purchased from Tremag, Tulcea, Romania. The main crystalline component assessed by XRD was MgO (Figure 1). * Fly ash (F), waste resulting from coal combustion in a power plant; the mineralogical composition of this material comprises SiO\({}_{2}\), Al\({}_{2}\)SiO\({}_{5}\), and CaSO\({}_{4}\) (Figure 1). * Waste glass powder (S)--obtained by the grinding of soda-lime glass cultets, up to a fineness corresponding to a Blaine specific surface area of 2858 cm\({}^{2}\)/g. * KH\({}_{2}\)PO\({}_{4}\) (MKP)--chemical reagent ACS grade (Sigma-Aldrich, \(\geq\)99%--Darmstadt, Germany). * Borax (Na\({}_{2}\)B\({}_{4}\)O\({}_{7}\)\(\cdot\)10H\({}_{2}\)O) (B)--chemical reagent ACS grade from Sigma-Aldrich, conc. \(>\) 99.5%. * Styrene-acrylic dispersion Acronal S562 (Ac), from BASF (Ludwigshafen, Germany), solid content 49-51%, particle size range: \(<\)0.1-10 \(\upmu\)m; density 1.04 kg/m\({}^{3}\), and viscosity 800 mPa\(\cdot\)s (23 \({}^{\circ}\)C, 100 1/s). * Expandable graphite (EG) was prepared according to the method proposed by [PERSON] et al. [30]. This method consists of the treatment of graphite with nitric acid and sulfuric acid under continuous stirring for 6 h. Expandable graphite is obtained by filtration, washing the sediment with water, and drying at 70 \({}^{\circ}\)C for 24 h. The graphite was obtained from ACROS ORGANICS (Geel, Belgium). The compositions of the studied MPCs are presented in Table 1.  
Figure 1: XRD patterns of magnesia (M) and fly ash (F). The setting time of MPC was assessed with Vicat apparatus equipped with Vicat needle [31]. The setting time represents the time elapsed from the moment when the MPC components were mixed and the moment when the Vicat needle leaves only a small imprint on the surface of the paste. The compressive strengths were assessed on paste specimens (cylinders with 15 mm \(\times\) 16 mm--diameter \(\times\) height); the pastes were cured the first day in the mold, and after demolding were kept in air at 20 \(\pm\) 2 \({}^{\circ}\)C for 7 days. The compressive strength was determined using a testing machine (Matest, Treviolo, Italy). The adhesion strength was determined on coated plates, i.e., phosphate cement paste was applied on one face of the carbon steel plate (100 \(\times\) 100 \(\times\) 3 mm--length \(\times\) width \(\times\) height), which was previously mechanically treated (grinded) with sandpaper. On the hardened cement coating (average thickness of 2 mm), a square metallic pull-off head (l = 35 mm) was glued with epoxy binder and cured for 1 day in air. The adhesion strength of the cement coating to the steel substrate was assessed by dividing the maximum load recorded with a universal testing machine LFV 300 (Walter + Bai AG Lohningen, Switzerland) using a displacement-controlled test at 1 mm/min (Figure 3) to the cross-sectional area of the specimen. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Specimens** & **Magnexia (M)** & **KH2 PO4 (MKP)** & **Borax (B)** & **Aconal (Ac)** & **Waste Glass** & **Fly ash (F)** & **Expandable** \\  & & & & & & **Graphite (EG) * \\ \hline M & 61 & 30 & 9 & 0 & 0 & 0 & - \\ M\_Ac & 57 & 29 & 9 & 6 & 0 & 0 & - \\ MS\_Ac & 54 & 27 & 7 & 6 & 6 & 0 & - \\ MS\_Ac\_EG & 54 & 27 & 7 & 6 & 6 & 0 & 0.67 * \\ MS\_F\_Ac & 34 & 17 & 8 & 7 & 7 & 27 & - \\ \hline \hline \end{tabular}
*) EG was calculated with reference to M content and was added to the mixture of other components.  
\end{table} 
Table 1: The composition of studied phosphate cements (wt%). Figure 2: Experimental program. The X-ray diffraction (XRD) analyses were performed with an XRD 6000 Shimadzu diffractometer (Shimadzu, Kyoto, Japan), using CuK\(\alpha\) (\(\lambda\) = 1.5406 A) radiation with a scanning speed of 2\({}^{\circ}\)/min.