Before you get too excited:

- Global production of corn: 1.2 gigatonnes, source https://ipad.fas.usda.gov/cropexplorer/cropview/commodityVie...

- Global production of Portland cement: 4.1 gigatonnes, source https://gccassociation.org/key-facts/

And of course, cornstarch is only about 70% of the weight of corn. IIRC grain production statistics are usually by dry weight, but if we assume wet weight, it's even worse. Even if we completely obliterated the meat and biofuel industries worldwide, we would struggle to meet a quarter of the current demand for cement, which anyway is forecast to increase.