# Fire Behavior and Adhesion of Magnesium Phosphate Coatings for the Protection of Steel Structures
## 1 Introduction  
Fire is essential for human life, but if not properly managed can lead to human life losses and the destruction of important materials. Fire protection in construction is of utmost importance, especially for those with steel structures, given the fact that steel starts to lose its mechanical properties when heated; thus, when the steel temperature exceeds 550-600 \({}^{\circ}\)C, it retains only about 50% of its ambient temperature strength [1; 2; 3]. Various passive fire protection materials can be applied to steel structures in order to improve their fire behavior; among them are cementitious coating mortars, plaster or silicate calcium boards, and intumescent coatings (organic or inorganic) [4; 5; 6]. According to [PERSON] et al. [4], these materials should be cheap, non-hazardous, both during application and in the event of a fire (i.e., they should not emit toxic gases), and should remain undamaged and attached to the steel element as long as possible in order to ensure good insulation and protection. Intumescent organic coatings (based on polymers) can easily be applied to various substrates and present an important swelling when exposed to heat [3]. Although this type of intumescent coating is frequently used today, it has some disadvantages, such as temperature limitation, the release of smoke and toxic fumes during heating, and, in some cases, the potential displacement of the protective char layer formed during the fire [3]. Inorganic intumescent coatings based on alkali silicates can also be used as passive fire protection; the main advantages of these types of materials relate to their good resistance to microorganisms and UV radiation [7], and opposite to the organic intumescent coatings,they can perform at higher temperatures and release non-toxic water vapors during the intumescence process [3; 6]. Intumescent alkali silicate pastes and mortars can also be used to limit the temperature increase in steel structures [5; 8; 9]; these materials, called alkali-activated borosilicate inorganic polymers (AABSIPs), can be obtained by the alkali activation of waste glass powder with a mixture of sodium hydroxide solution and borax. When AABSIPs are heated to temperatures between 400 and 600 \({}^{\circ}\)C or are put in direct contact with the flame, they exhibit an important swelling, i.e., the volume increases between 250% and 430% [5; 10]; these coatings can effectively limit the increase in the carbon steel substrate temperature above that which is considered critical (550-600 \({}^{\circ}\)C) [5]. The main drawback of intumescent materials (organic or inorganic), which could limit their extensive use, is their higher price as compared with cementitious mortars, especially those based on portland cement. In order to enhance the thermal performance (reduce the thermal conductivity) of portland cement concrete/mortar, it is recommended to replace the normal aggregates with lightweight ones such as expanded polystyrene particles, vermiculite, perlite, etc. [4; 11; 12]. Nevertheless, the compressive strength and adhesion to metallic substrates of portland cement mortars with lightweight aggregates are small and decrease with the increase in temperature (although this decrease is smaller as compared with the one assessed for the control mortar--with normal aggregates). For example, for a cement mortar with 10-40% vermiculite, the compressive strength decrease was between 1.7% and 21.41% when the materials were heated to 300 \({}^{\circ}\)C and between 57.06% and 81.03% when the thermal treatment temperature was 900 \({}^{\circ}\)C [12]. In this respect, magnesium phosphate cement (MPC) is considered an interesting candidate for the fire protection of metallic structures [13; 14; 15; 16; 17]. MPCs have a higher adhesion to various types of substrates (including steel) compared to portland cement [16; 17]. Moreover, this type of cement has a rapid setting, high early strength, long-term durability, and good abrasion resistance [17; 18; 19; 20].