Welcome to the thread,
Lowfi 
Algae blooms are more to do with the calcium hydrates that the cement is made of. Ca(OH)2 (aka lime) has long been used as a soil amendment/fertilizer. Silicates (there are some inherent silicates in cement) may play a small role, as well as phosphates (which are not typically found in cement, so if present, suspect an outside source such as shells or water), but all three substances are generally consumed fairly quickly by the maturing system and don't tend to cause problems for more then a few months, if it ever becomes a "problem".
These are the only things that might leech into your system, and typically stop leeching within 6 months or so.
What you need depends on what "recipe" you want to use. You will need cement - you can use the grout but it has silica sand mixed into it already. Several have used grout and reported no problems. White portland will tend to be stronger and you can add the sand you wish to use.
Sand is really recommended, and if possible, something calcium carbonate based. Try a feed store for chicken grit/ scratch sand, for "feed mixing". This is an excellent and cheap form of calcium based sand.
You also might want sand to cast your rocks into - blasting sand (which is industrial quartz, aka silica sand) works like a charm, and doesn't seem to attract more algae or diatom than rock made with no silica sand, so it is more about preference, IMO, and it is a little cheaper to fill the boxes or totes you use to cast into. It also tends to blend into the surface of the rock, where the calcium based sands can be more noticeable. Many also use salt to fill the casting bins, so the choice is yours.
Next you need aggregates - salt, crushed coral, crushed shell and perlite are all considered to be safe, so use any or all that appeals to you in whatever combo you wish - just keep your cement to aggregate/sand ratio around 1:cement to 3-5: other (at least 1-2 parts of the "other" should be sand of some sort).
You will also need boxes or totes to cast your rock into, something to mix the rock mix in, drop cloths/sheets/plastic to protect surfaces, small hand tool for mixing the rock and a measuring device for the ingredients and rubber gloves to protect your skin.
This should be enough to get you started
If you have questions, feel free to post - someone will most usually reply
HTH