A mineral filler is a solid particulate blended into a polymer to change its cost and properties. Calcium carbonate (CaCO3) is the dominant choice because it is abundant, low-cost, bright, and chemically stable. In a plastic compound it displaces a share of the resin, which lowers material cost, and it adds stiffness, dimensional stability, and improved surface finish. Because the mineral is denser than the polymer, higher loadings raise part weight, so formulators balance filler level against the properties they need.

In bio-based and compostable plastics - PLA, PHA, and starch-based blends - the same logic applies, with two extra benefits. Every kilogram of biogenic calcium carbonate filler is a kilogram of polymer you did not have to make, and the mineral itself locks up carbon. When the calcium carbonate comes from a biogenic source such as oolitic aragonite, that stored carbon can strengthen the finished product's carbon story - see carbon-negative filler.

Not on its own - but the claim is judged on the finished product, not the filler. A calcium carbonate filler is inert mineral and is not itself the thing being composted; what matters is whether the whole compounded item still disintegrates and biodegrades within the thresholds of its certification. If a product is sold as compostable, it must still pass ASTM D6400 or EN 13432 as manufactured, at its actual filler loading. Over-filling can slow disintegration and cause a formulation to fail those tests, so compostable claims should be re-verified at the real loading level. For the standards themselves and how the claims work, read the compostable bioplastics guide and compostable vs biodegradable.