Using chemistry to protect structural steel during a fire
By Paul Grad |
A technique for protecting structural steel from fire using the endothermic decomposition of magnesium hydroxide has been developed by a team from the Dept. of Chemical Engineering and Materials Engineering, the University of Auckland (New Zealand; www.auckland.ac.nz). Led by professor Mohammad Farid, the team designed a system consisting of an outer layer of gypsum board and an inner mass of Mg(OH)2. The presence of water molecules in the gypsum (mainly CaSO4 .2H2O) is crucial in establishing its fire-resistance properties. Heat causes the waters of hydration to be released from the gypsum. The dehydration reactions, which occur between 125 and 225°C, are strongly endothermic.
The team’s experimental work shows that the addition of the Mg(OH)2 increased the thermal mass of the system and removed heat through its endothermic decomposition. At 350°C Mg(OH)2 decomposes to MgO and H2O, absorbing 70 kJ/kg. When only MgO is used for protection, the time for the steel to reach 550°C is increased from 50 to 88 min. When MgO is replaced by Mg(OH)2, the time needed for the steel to reach 550°C is increased further to 112 min.
The thermal decomposition of Mg(OH)2 occurs in two stages. The first stage involves the release…