# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 3 Results and Discussion  
The setting times of the studied magnesium phosphate cements (MPCs) are presented in Figure 4. As can be seen, the setting of M paste is rapid (9 min and 30 s). The presence of the polymer suspension (Ac) in the composition M_Ac, slightly delays the setting of this composition (12 min and 30 s). This delay could be due to the partial adsorption of the polymer suspension (Ac) on the magnesia particles, thus slightly delaying their interaction with phosphate. The partial replacement of magnesia with waste glass powder (MS_Ac) delays the setting time, up to 14 min and 58 s, mainly due to the reduction of magnesia content; it is well known that MPC hardening is due to the reaction of magnesia with a potassium phosphate solution and KMgPO\({}_{4}\)-6H\({}_{2}\)O formation [13; 18; 20]. Furthermore, the increase in the substitution degree of magnesia with fly ash and waste glass powder up to 50% (MS_F_Ac) further delays the setting time of these cements due to the dilution effect as well as the increase in the water-to-solid ratio [34; 35]. The thermal treatment (according to temperature-time curve stipulated in ISO 834:2019 standard) of MPC specimens, previously cured in air for 7 days, determines their contraction (see Figures 5 and 6). The replacement of magnesia with fly ash (F) or/ and waste glass powder (S) reduces this contraction (Figure 6). Figure 4: Setting times of the studied magnesium phosphate cement pastes. Figure 5: Visual aspect of the magnesium phosphate cement specimens before and after thermal treatment: (**a**) M, (**b**) M_Ac, (**c**) MS_F_Ac, (**d**) MS_Ac, and (**e**) MS_Ac_EG. The important mass loss recorded for the cement paste with S and F content, as compared with M paste, can be explained by the processes which take place in these system when the thermal treatment (tt) is applied. According to the XRD patterns presented in Figure 7a,b, in the magnesium phosphate cements M and M_Ac, hardened for 7 days (before tt), the main crystalline hydrate formed is K-struvite (KMgPO4-6H2O), which coexists with MgO (only part of magnesia reacted with the KH2 PO4 solution to form K-struvite). The thermal treatment determined the dehydration of K-struvite and its transformation into KMgPO4[16,36]. Figure 6: Volume and mass changes after the thermal treatment. Figure 7: XRD patterns of the studied composition before and after the thermal treatment (tt): (**a**) M, (**b**) M_Ac, (**c**) MS_Ac, (**d**) MS_Ac_EG, and (**e**) MS_F_Ac. The addition of waste glass powder, polymer suspension (Ac), or expandable graphite (EG) did not modify the nature of crystalline compounds assessed by XRD before and after the thermal treatment (Figure 7c,d). On the XRD patterns of the composition in which magnesia is partially substituted (50 wt%) with waste glass powder and fly ash (MS_F_Ac--Figure 7e), one can assess in the cement paste, hardened for 7 days (before the thermal treatment), the presence of MgO, K-struvite, and SiO2 (from fly ash). The thermal treatment at 1050 \({}^{\circ}\)C determined the of formation forsterite (Mg2 SiO4) in the reaction of MgO with SiO2. At the same time, KMgPO4 resulting from K-struvite dehydration can be assessed on the XRD patterns of this composition (after tt). The identification of forsterite by XRD analysis, only in one sample (MS_F_Ac), could be explained by the higher amount of silica available (both from glass and fly ash); therefore, the amount of forsterite formed is higher (as compared with the other MPCs with only 10% waste glass) and can be identified by this method. At the same time, one cannot exclude the possibility of forsterite formation in the MPCs with S addition, but its amount is probably much smaller (under the detection limit of the diffractometer).