of the mixes was varied and the sand content was reduced in proportion to increases in the fly ash content each fresh mix was transferred to three disposable molds which were 76 mm in diameter and 150 mm deep the contents of the molds were hard in about 30 minutes after standing at room temperature for 3 or 4 days the hardened products were tested without moist curing for compressive strength according to astm c39 83b this procedure involved capping the cylindrical specimens with a sulfur mortar astm c617 84 to assure flat top and bottom surfaces the specimens were then loaded at a rate of 0 34 mpa sec on a calibrated power operated testing machine until they failed under the compressive forces indicated below the magnesium oxide used in the above described procedure was a product which had been obtained by the dead burning of magnesite the product contained about 87 percent mgo and sio 2 cao fe 2 o 3 and al 2 o 3 as major impurities its surface area was about 0 2 to 0 7 square meter per gram it had a mean diameter of 64 u03bcm and a maximum diameter of 149 u03bcm with 75 percent of the particles passing through a 325 mesh screen and 1 percent being held on a 100 mesh screen the fly ash contained approximately 52 percent sio 2 25 percent al 2 o 3 and 13 percent fe 2 o 3 about 74 percent of the particles passed through a 325 mesh screen its specific gravity was 2 46 g cm 3 it complied with astm c618 83 the aggregate used was a combination of fine and.