PH after Acid wash

[chem geek]

I wanted to chime in but was not able to when I first saw this post. Indeed, fresh plaster surfaces can take a long time to cure as the slack lime (Calcium hydroxide) in the cement converts to calcium carbonate through hydration and CO2 absorption. That is why the pH rises, the plaster surface is literally sucking CO2 out of the water. This is also why some also strenuously argue for the "acid-start" process as they say the low pH drives more CO2 out of solution (assuming your carbonate alkalinity is high enough).

It's not carbon dioxide, but primarily bicarbonate that reacts as follows:

Ca(OH)2 + HCO3(-) ---> CaCO3(s) + H2O + OH-
Calcium Hydroxide + Bicarbonate Ion ---> Calcium Carbonate + Water + Hydroxyl Ion

An acid start is not the best way to cure plaster since it inhibits the above process and results in a lot more plaster dust as a result. In any startup method, the calcium hydroxide forms from the curing process as follows (using the most common component of klinker):

2(Ca3O•SiO4) + 7H2O ---> 3CaO•2SiO2•4H2O + 3Ca(OH)2 + heat
Tricalcium Silicate + Water ---> Calcium Silicate Hydrate + Calcium Hydroxide + heat

So what really gets sucked into the plaster the most is water. In the much better bicarbonate startup (see this link), there is very little plaster dust and little calcium increase because the calcium stays in the cement (plaster) and instead the hydroxide exchanges with carbonate. This process occurs near the surface of the plaster to form a hard surface, but calcium hydroxide remains in the depths of the plaster. This is why even years later if there is a loss of integrity at the surface of the plaster, calcium hydroxide can leak out and produce more calcium carbonate. This occurs most especially if there are voids from a poor plaster job as these can form calcium nodules (see Calcium Nodules in Pools).