Did Carson say all pyramids? This post from Dec 2014 suggests that at least the very first step pyramid was used exactly for grain storage.
“The Step Pyramid complex at Saqqara Egypt contains grain silos and administrative buildings. Not only was it the first pyramid, it contained the first silos and the first building with columns. It was designed by Imhotep who may have been Joseph of the Bible.”
https://josephandisraelinegypt.wordpress.com/tag/first-pyramid/
About how long would it have taken to build the step pyramid? Is this known? Joseph had seven years, starting immediately [i.e.: as soon as he was put on the job] to store the grain. Where did he keep it while the pyramid was under construction?