Heat generation in the setting of magnesium oxychloride cements

Abstract

The heats of hardening of magnesium oxychloride pastes and flooring cements were determined by three methods. Two direct methods limited to 18 hours in duration gave heats of hardening of about 190 cal/g of MgO. Temperatures up to 147° C were obtained in the interior of the samples. A heat-of-solution method, applicable only to the pastes, gave 60-day values for the heat of hardening ranging from 135 to 320 cal/g of MgO, depending on the relative concentrations of MgO, MgCl2, and H2O in the pastes. A sample stored for 4 years gave a heat of hardening of 240 cal/g of MgO, as compared with the 60-day value of 215 cal/g of MgO for a different sample of similar composition. The low heats of hardening, corresponding to high heats of solution, obtained for certain of the pastes support the conclusion that free MgO may remain in such pastes. 1. Introduction Magnesium oxychloride cement, also called magnesite cement or Sorel cement, is used extensively for floor surfaces. It is formed by the action of concentrated magnesium chloride solution on active magnesium oxide. Numerous filler materials, such as asbestos fiber, hardwood sawdust, talc, sand, and powdered silica, are incorporated to impart desired physical properties to the finished floor. The compound binding these materials together appears to be the magnesium oxychloride 3MgO.MgCl2.HH2O [I].1 This compound is unstable in contact with solutions containing less than 11 percent of MgCl 2, and consequently the floor must be protected from the action of water. The conditions of mixing and hardening are such that MgCl2 solution is retained in the interstices of the hardened cement. This solution preserves the oxychloride and renders the floor conductive to electricity, a desirable property in locations where sparks of static electricity may create a hazard. There appears to be little or no information in the literature on the amount of heat generated by magnesium oxychloride cement while hardening. Chassevent [2] measured the temperature rise during setting of MgO-MgCl 2-H2O mixtures and found that the temperature rises of the pastes and their 7-day compressive strengths increased together. However, he gave no values for the quantity of heat generated. The work reported in this paper had for its primary purpose the determination of the amount of heat generated by several commercial flooring cements. In addition, similar information was obtained concerning mixtures of MgO and MgCl2 solution without the fillers ordinarily used.