# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 4 Conclusions  
The composition of magnesium phosphate cements (MPCs) can be modified to improve several properties which are important when used as passive fire protection materials. The addition of styrene-acrylic dispersion (Acronal) or /and the glass powder extends the MPC's setting time. The magnesia substitution with waste glass powder and fly ash (50% wt. ), combined with the addition of a polymer suspension (Ac) and an increase in the water-to-solid ratio, determines an important delay of the MPC setting time. The thermal treatment of MPC specimens up to 1050 \({}^{\circ}\)C (applied according to temperature-time curve stipulated in the ISO 834:2019 standard) determines their contraction as well as an important mass loss (25-33%). These changes can be correlated with the modification in the mineralogical composition of the studied MPCs; for the MPCs with waste glass power and without/with the addition of Ac, the main changes assessed by the X-ray diffraction analysis consist of the dehydration of K-struvite--the main reaction product resulting from the reaction of MgO with KH\({}_{2}\)PO\({}_{4}\) solution. When the magnesia substitution rate is 50% wt., due to the presence of both waste glass powder and fly ash in the system, the thermal treatment at 1050 \({}^{\circ}\)C also leads to the formation of forsterite (Mg\({}_{2}\)SiO\({}_{4}\)) in the reaction of MgO (not entirely consumed in the reaction with KH\({}_{2}\)PO\({}_{4}\) solution) with SiO\({}_{2}\). The compressive strengths of magnesium phosphate cements, before and after thermal treatment, are correlated with their mineralogical composition. The reduction of magnesia content in the MPCs (due to partial substitution with fly ash and/or waste glass powder) determines an important reduction in the initial compressive strength (before thermal treatment) correlated with the lower K-struvite content. When the main hydrate present in the hardened paste is K-struvite, the thermal treatment determines its dehydration, a process that decreases the compressive strength of the material. For the compositions with waste glass powder and fly ash, an increase in the compressive strength was recorded after thermal treatment, most probably due to the formation of a higher amount of forsterite, a compound with good mechanical properties. The direct contact with a butane flame of the studied MPC coatings applied on steel plates determines the sharp increase in the steel plate (substrate) temperature in the first 4-6 min, but afterward, the temperature is stabilized around 490-500 \({}^{\circ}\)C; these temperatures are 30% lower compared to the temperature assessed for non-coated steel plates. The adhesion strengths of the studied MPC coatings to the steel substrate were good, and no delamination was visually noticed during and after the contact with the flame. For the composition with fly ash content, the local swelling of the coating in the contact area with the flame determined a drastic decrease in the adhesion strength. The addition of a styrene-acrylic dispersion (Ac) improved the initial elasticity of the MPC coatings (before the fire test), and fewer microcracks were visible on the surface of these coatings compared to the one with plain MPC (without Ac addition). Conceptualization, N.F.C. and A.B. ; Investigation, N.F.C., A.C.B. and R.C.C. ; Methodology, N.F.C., A.B. and G.V. ; Supervision, A.B. and G.V. ; Writing--original draft N.F.C. and  
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