Skimping on cement is asking for trouble, rain or shine. That mix would have been weak even if it dried. For unpredictable weather, increase the cement ratio and consider a fast-setting additive. I learned the hard way on a garden wall that got caught in a drizzle, now I mix wetter and cover immediately with weighted tarps. Checking the radar for pop-up storms is non-negotiable. Otherwise, you're just building a sandcastle.

From a job last summer, visible mold appeared within 36 hours on a taped pipe... you need to address the source, not just cover it up.