# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 3 Results and Discussion
The compressive strengths of the magnesium phosphate cements hardened for 7 days and after thermal treatment are presented in Figure 8. As can be seen, the addition of the polymer suspension (Ac) does not substantially modify the compressive strength values before the thermal treatment (tt), which is opposite to the substitution of magnesia with waste glass powder (S) and fly ash (F). In this case, the compressive strength decreases by 9% for the composition with 10% S (MS_Ac), and is very small when 50% of magnesia is substituted with S and F. This was to be expected given the fact that the reduction of magnesia content determines the reduction of K-struvite formed by the reaction of MgO and KH2 PO4 solution. K-struvite, the main crystalline hydrate formed in this type of cement, is also the main component that determines the hardening and increase in the compressive strength values. Moreover, due to the presence of porous particles in the fly ash (see Figure 9a-c) and the high content of fine particles (Figure 9d), the water dosage for the pastes with F content increased up to a water-to-solid ratio = 0.38, in order to keep an adequate workability of the fresh paste. The increase in water dosage is detrimental for the compressive strength of hardened paste due to the porosity increase. Figure 8: Compressive strength before and after thermal treatment (tt) performed according to the ISO 834 curve. One can notice in Figure 9, a plerosphere (P), i.e., a hollow round particle which is filled with microspheres (M), and cenospheres (C), i.e., hollow particles (with continuous or porous surface) with different sizes [37]. The thermal treatment (tt) has a different influence on the volume and mass of the MPC pastes (Figure 6) in correlation with the physical and chemical changes which occur in these compositions when the temperature increases. When the main hydrate present in the system is K-struvite (M, M_Ac, MS_Ac, MS_Ac_EG), the tt determines its dehydration--a process that leads to a significantly lower volume of the solid phase and, thus, to a lower compressive strength of the material [38], as can be seen in Figure 8.  
For the composition with fly ash (MS_F_Ac), the compressive strength increases after thermal treatment, most probably due to the formation of a higher amount of forsterite, a compound with good mechanical properties (fracture toughness and hardness) [39]. The direct contact of the MPC coatings applied on steel plates with the flame determines changes in the coating's visual aspect (Table 2) as well as the increase in the substrate (steel plate) temperature (Figure 10). Figure 9: SEM images (**a**–**c**) and particle-size distribution of fly ash (**d**). As can be seen from Table 2, the studied MPC coatings do not present delamination after direct contact with the flame. The visual aspect of coating M did not change during the contact with the flame (in the contact area), but cracks can be visually assessed in this coating, mainly in the adjacent areas of the one which was in direct contact with the flame. For the compositions with an Ac addition, one can notice the darkening of the coating during contact with the flame, most probably due to the burning of this polymer. The addition of waste glass (MS_Ac and MS_Ac_EG) leads to partial melting of the coating (sinterization) in the area where it was in direct contact with the flame; this was to be expected given the melting point of soda-time glass (generally below 1000 \({}^{\circ}\)C) [40]. For the composition with fly ash content (MS_F_Ac), one can also notice a small local swelling of the coating in the area where the flame was in direct contact with the material.