No reagents were used
Well:
mixing NaOH with Na2SiO3
(yes, NaOH is there mostly to keep the pH high, but for me technically these are reagents, just like water is in this case).
What was the temperature, was the mixture covered? Such concentrated solutions don't dry easily, they often build a layer of a solid on the surface, which makes drying even slower. Doesn't mean the idea suggested by P is wrong, just the time scale (especially when comparing 48h with 10h) doesn't make it plausible for me.