Re-using the yeast cake?

[Toxxyc]

OK I've got a question. A few weeks ago I made a rum wash, 20 litres. I used 1kg molasses and 2kg brown sugar into 20l of solution, so not very strong. Into that I pitched 15g of S-04 beer yeast (had some on hand) as well as a packet (20g) of bread yeast.

I wrapped the whole thing in my half electric blanket, set it to hot (around 32°C) and left it there. In a week it was done fermenting. I let it sit for another week and then poured the wash off into a sanitized cube, sealed it and set it aside for stripping later.

Not wanting to waste the yeast colony in the fermenting container, I pre-mixed a few kilos of brown sugar in water and after cooling, poured that into the container straight onto the yeast cake. I've done this with beer many times and it usually results in a faster, stronger fermentation as there's a crapload of yeast in the cake, as well as a lot of dead yeast that works as nutrients for the live yeast. Sounds good?

Nope. I set this whole thing in the electric blanket again and set the temp up to 32°C again. The first few days went well and I saw a lot of CO2 escape from the osmosis cap. It slowed down and stopped completely after about 6 days so I removed the blanket and set it aside to let the yeast drop out a bit, thinking I'll run the sugar and the molasses washes together in the kettle in a single strip.

Stripping day came and I opened the molasses wash, gave it a quick taste to confirm there's no sugars left and it was dry and tart, exactly like it should be. All good.

Opened the sugar wash, gave it a taste and it was sickly sweet. Obviously, fermentation wasn't complete. I figured it must then be a pH thing and as I know a sugar wash can get relatively acidic, I poured in a tablespoon of sodium bicarbonate I had on hand. Off topic, but in hindsight I should have known this was a BAD idea. I got a HUGE reaction and the wash came spewing out of the container like mad. LOL.

Cleanup done and all that and now fermentation is going again. I wrapped it in the blanket again, and it's fermenting, but SUPER slow. I'm guessing (after some Googling) that the Bicarb simply didn't have enough power to raise the pH enough in order to let the fermentation complete.

So now, because I don't want to spend the next 6 weeks waiting for this thing to finish, how can I speed this process up? I don't think we get oyster shells here, as it's South Africa and this place is a bit of shithole when it comes to lots of things. Can I use a swimming pool alkalinity booster like soda ash (Na2CO2)? Or is there a better way to boost this thing to completion?

PS: I just want it to finish as I got better yeasts to use and I need the vessel it's fermenting in. Otherwise I would have just left it to do it's thing.