Synthetic crystalline calcium silicate hydrates Ca5Si6O16(OH)2·4H2O and Ca6Si6O17(OH)2 which are identical to the silicate minerals 11Å tobermorite and xonotlite respectively show Mg2+ uptake in aqueous solutions. When placed in solutions initially of 100-1000 ppm in Mg2+, 11Å tobermorite takes up a maximum of 3.5 wt% of Mg after equilibration for 15 days at 25°C whereas the uptake level in xonotlite is as high as 3.7% after equilibration for 7 days. However, Mg2+ uptake by both the phases did not reach an equilibrium. The studies of magnesium uptake appear to be useful in delineating the mechanisms of ion exchange in calcium silicate minerals. The uptake appears to be a combined effect of reversible ion exchange on broken bonds and outer planar surfaces and irreversible reaction of Mg2+ with the calcium silicates which led to Ca release. These mechanisms were corroborated by the morphological and chemical studies of the exchanged products, MgxCa5-xSi6O16(OH)2·nH2O and MgxCa6-xSi6O17(OH)2 using a transmission electron microscope (TEM) with energy dispersive spectrometry (EDS).
All Science Journal Classification (ASJC) codes
- Building and Construction
- Materials Science(all)