The raw shrimp doesn't produce a pH shift, it produces ammonia. The ammonia is what you want to introduce. How exactly are you currently cycling your tank? If there's nothing but sand (not live, but originally dry sand from a bag) and water in your tank, you're not cycling.
Cycling your tank means that you're building up a population of beneficial bacterial that will take the fish waste (ammonia), turn it into nitrites, and then turn around and turn THAT into nitrates - which in small amounts in not harmful to your inhabitants, and can be dealt with by water changes. The beneficial bacterial will colonize your sand or rock or filter sponges on their own - you don't have to introduce them. BUT, you DO have to introduce an ammonia source to start the whole ball rolling.
Sorry if that's all elementary for you, but from your post it didn't sound like you were really clear on what a cycle is actually doing.
Regarding your cement rock issue... I haven't a clue. From my limited experiences with curing cement, I know there are some serious chemical reactions going on during the curing process. Usually, folks "cycle" their tank with their rock in place as a large portion of the beneficial bacteria will live there, in addition to the sand. Personally, I'm a "play it safe" type of person, so I'd probably choose to let the cement cure first so as not to introduce some unknown into your tank right from the beginning.