In response to a few things...
Concrete is a synthetic rock-like substance composed of cement and an aggregate. Depending on application, the aggregate may be lightweight clay (Illite, Vermiculite), bits of crushed up limestone or old concrete, or Fe and Mg (specifically for high strength apps, like multi-story building foundations).
Cement is the end result of a series of rxns beginning with the extreme heating of limestone and clay. This results in something called a "clinker" which will then be further refined to yield Portland Cement.
When you mix cement with a small amount of very fine aggregate (think powder) and water, you get mortar: perfect for sticking stone and brick together, not great for making 3-D objects. You need a more substantial aggregate, so usually sand or gravel is added to bulk up the strength and structure, giving you a sturdy base for construction once it has fully cured (which could be as long as a year or two!). The amount of water used during this step is also important: more h2o = less viscosity, easier to work with, higher porosity, but less strength.
Limestone is a sedimentary carbonate rock created by organisms that take Ca out of solution and bind it with a carbonate molecule (CO3). In an aquarium since the system is closed, to increase this activity you can pump in C02 and add Kalkwasser treatments, however, if your initial ingredients are poor, you are setting yourself up for disaster.
You can (and should) make your own reef rocks from concrete, since the limestone used for its production is mined from ancient quarries instead of being removed from the oceans and shipped halfway around the world. You can play with different ratios and mixtures, but using 1 part TypeII Portland Cement, 3-4parts finely crushed Oyster shells (or a powdery flour version) and 1 part aragonite or plain old crushed sea shells will yield a realistic looking Fiji-esque reef rock. Use slightly more water than normal since you want your mixture to be porous. Whatever ratio you decide on, you need to cure this rock for a minimum of 6-8 weeks, in freshwater, with weekly or biweekly water changes. No cutting corners here, good things come to those who wait.
Hope this helps
PS- Lava rock, which is probably dark red Scoria is mainly Fe and Mg. It contributes nothing in the way of Ca and will actually hurt your inhabitants over time, since Fe and Mg compete with Ca for binding sites on Oxygen. Take it out of your take. Since these rxns take time to build up in your water, things may seem fine now. In a few months everything will be dead or dying in your tank and you will most likely have to start over from scratch.
Good Luck
