One complicated silicic acid system, containing Al3+, Mg2+, Fe2+, and Fe3+, is prepared through acidolysis of blast furnace slag. By investigating its gel time concerned with pH value, compared with that of ordinary silicic acid system, it is shown condensation reaction existing and efficiently controlling gelling characteristics of complicated silicic acid system by regulating pH values. The results are validated by FT-IR spectra analysis. With nonionic surfactant employed, the gel time is increased from 11 to 312 h, more suitable to control gel time than anionic surfactant. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 1339-1343, 2011