The gypsum could actually have helped. Calcium cations can displace both sodium and magnesium ones on the clay particles. Calcium being physically larger can help create greater separation between clay particles which is helpful for building aggregates.

Ya, here in the lower Mondego river delta it’s clay AND calcium concentrations that make water filters die in short order. They do rice here. I can’t imagine gypsum helping anything here.

They still burn the rice fields like morons every autumn, despite the dense clay here… I tried mentioning how even Cambodia prohibited burning and uses enzymes and got blank stares.

Additions like gypsum work in cases like this because they add permanent drainage to the soil. Adding organic matter also adds drainage, but microbial activity removes it over time and you end up with the same soil that you started with.