Impact of minor iron content on crystal structure and properties of porous calcium silicates during synthesis

We investigate the impact of minor iron content on the crystallization behavior and thermal stability of hydrothermally treated calcium silicates for insulation at high temperatures. The impurity content of the raw materials (micro silica and quicklime), particularly iron, varies greatly with the production site and method. In this work the porous calcium silicate is synthesized with a Ca/Si molar ratio of 1. To systematically study the iron impact on crystallization during synthesis of calcium silicates, we introduce different amounts of iron (Fe/Si = 0 to 0.013) into the reactants. Using Fourier transform infrared spectroscopy we find a pronounced decrease in the number of 3-bridging oxygen coordinated Si sites in the calcium silicate with increasing iron content. The X-ray diffraction results revealed that the fractions of tobermorite, calcite, and amorphous phases increase with iron content, whereas the fraction of xonotlite phase decreases. These phase changes affect the compressive strength and thermal shrinkage of the calcium silicates. The compressive strength decreases with increasing iron content until amorphous phase becomes dominant, and then it increases again. The shrinkage increases from 1.3% at 1050 °C for the reference sample, to 30.4% for the sample with Fe/Si = 0.01. We discuss why such a small amount of iron can greatly affect the crystal structure of calcium silicates and deteriorate the physical performances of final products. Therefore, the iron contamination of raw materials should be avoided to ensure high temperature stability of porous calcium silicates.