Starch-based lightweight concrete: effect of starch source, processing method, and aggregate geometry

Abstract

One potential new use for starch is in making lightweight (<2 g/cm 3) concrete. Lightweight concrete is important commercially because of its low sound and thermal conductivity and its low plastic, cured, and oven-dried densities. Lightweight concrete was made containing wheat, corn and high amylose corn starches. The starches were used as either hydrated raw starch or aquagels (semi-rigid gels of gelatinized starch that contain mostly water) that were prepared by three different processes to form spherical, angular or cylindrical particles. The moisture content (MC) of the aquagels was highest for cylindrical samples (extrusion process), intermediate for spherical aquagels (batch process #1), and lowest for angular aquagels (batch process #2) and hydrated raw starches. When added to a concrete mixture, some of the moisture contained by the aquagels migrated into the mixture and affected the water:cement ratio ( w/c), which, in turn, affects ultimate strength. The total moisture content (TMC) of the concrete mixture was lowest for mixtures containing aquagels from batch process #2, intermediate for mixtures containing aquagels processed by extrusion or batch process #1, and highest for mixtures containing raw starches. Shrinkage during the curing of concrete test samples was excessive in samples containing hydrated raw starches (1.4%) and aquagels prepared by extrusion (4.2%). The excessive shrinkage resulted in extensive cracking. Only a small amount of shrinkage and no cracking occurred in lightweight concrete containing aquagels prepared by the batch processes. The source of starch had no significant effect on the physical and mechanical properties of the concrete. The starch-based lightweight concrete had a lower compressive strength than samples made of perlite, a commercial lightweight aggregate. The greater strength of perlite-based concrete was likely due to the interfacial bonding that occurs between cement and perlite aggregate, but is absent in starch-based concrete. The size and shape of the starch aquagel significantly affected the cured density and strength of the concrete. Concrete containing small aquagels (0.15–0.84 mm) had a higher cured density and compressive strength than samples containing large (1.4–2.0 mm) aquagels. Concrete samples containing spherical aquagels had a greater compressive strength than samples containing angular aquagels.