Abstract: A process for the formation of highly durable cementitious board using lightweight aggregate is disclosed. The process comprises pouring a core mix onto a conveyor, wherein the core mix is comprised of one or more lightweight aggregate filler in the amount of 0.5 to 5 weight percent of the core mix, one or more binders in the amount of 35 to 75 weight percent of the core mix, rheological admixture in the amount of about 0.5 to 5 weight percent of the core mix, surfactant in the amount of 0 to 0.1 weight percent of the core mix, one or more normal weight aggregate filler in the amount of 5 to 50 weight percent of the core mix, and water in the amount of 5 to 20 weight percent of the core mix. The process further comprises passing the core mix under a screed roller to flatten the core mix to produce an extruding board with a desired thickness.