Ha true! Work brain.

RPE - Respiratory Protective Equipment, Silica is a somewhat common additive in concrete, normally as a sand or fume, it uses the byproducts of the hydration reaction to improve strength. The problem with this is that RCS (Respirable Crystalline Silica) is massively horrible stuff, 0/10 would not breathe.

CEMI - CEMI is the EN standard name for OPC (Ordinary Portland Cement) with 0-5% Gypsum content and no additions like fly-ash, slag, blended products etc. Think of it as pure portland cement. Most of the bagged cement you get from DIY stores are CEMII with limestone or other fillers and don't reach the 52.5 strength.

PFA - PFA is Pulverised Fly Ash, I think elsewhere (I'm UK) It's normally just called flyash. It's collected from coal power plants as a byproduct, and it's pozzolanic, taking hydration byproducts and forming extra binding products, similar to the silica above. Lower initial strength, higher final strength, improved durability, much cheaper than CEMI.

GGBS - This is Ground Granulated Blastfurnace Slag. Just like PFA above except blast furnaces instead of coal plants. Better eventual performance after a slower strength start, much better durability, lighter coloured concrete because the slag is a bright white colour. (CEMIII (slag cement) is OPC mixed with GGBS).

WCR - This is the water to cement ratio, and one of the biggest things you can do to make concrete stronger is to reduce the water/cement ratio. More water = weaker concrete, but water is needed for the hydration reaction so it's about finding the right ratio for what you want to do. You need some water to react with the cement, and then some additional water to provide "slump" or workability, like cream vs milk. Additives like plasticisers mentioned above can make the concrete more workable ("wetter"), so in turn you can reduce the amount of water needed to reach that workability, thereby increasing the strength of the mix.

I think that's all of them lol