Looks like they’re about 20% silica, but most of the mentions I found were people on grow forums saying it could be used as fertilizer. There’s a reference on a gardening equipment site that’s trying to sell them, but it’s mostly pushing them as aeration - it mentions it takes 3-5 years for them to fully break down. That’s a bit longer-term than I’m looking for, but thanks. 
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Potassium silicate is the cheapest option. It is approved for organic gardening as a foliar spray only for some reason. It will definitely raise you pH very high.
If you want to stay away from the extra potassium and your pH is already low, look into calcium silicate. It’s a mined mineral called Wollastonite.
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