critical generally expanded perlite is preferred for reasons of availability and economy the amount of lightweight aggregate included may vary from about 20 to about 70 on a dry weight basis in the final product it is particularly preferred to use expanded perlite having particle sizes from about 12 to about 100 mesh in amounts of about 30 40 for very lightweight structural panels of the invention the lightweight aggregate is quite friable and some of it will be shattered by the mixing herein contemplated the panel being formed is provided with a bottom cover sheet and some of the shards of aggregate and any separated fiber shot will collect on the bottom cover sheet it is also preferred that the composition include about 3 25 by weight coarse cellulose fibers to aid flotation and entanglement such fibers generally are about 1 16 1 4 inch in length with some fibers being up to about one inch these are conveniently provided by slushing newspaper or other papers in a 34 slush maker 34 that is suitable coarse cellulose fibers may be made by charging about 10 20 pounds of newspapers or other paper stock for every 100 gallons of water in a high intensity high shear mixer and agitating the mix for a couple of minutes in the mix tank 11 increasingly stiff panels are made as the proportion of cellulose is increased but for ease of cutting with a knife and to avoid jagged edges the amount is preferably about 3.