Influence of KOH sol. on the Formation of Tobermorite Phase at Room Temperature

Abstract

The authors have studied the reaction in the systemKOH-CaO-Silica gel-H2Oat 20±1°C and the solid phase formed by the reaction.The mixture of CaO and SiO2 gel of the mol ratio CaO/SiO2=2.5 was added to KOH solution whose concentration was changed up to 50g/l. After keeping 95 days with occasional agitation the reaction products were subjected to chemical and X-ray analyses and to the observation under electron microscope.The results obtained are summarized as follows:(1) The solid reaction product came into sight was identified as tobermorite phase which appeared under electron microscope as films or crumpled foils. In general, the individual crystals were smaller than 5μ, which grow comparatively well in the direction of thickness with increasing concentration of K2O, although this effect was not so pronounced as that of Na2O, which had been reported elsewhere.(2) With increasing concentration of K2O the concentration of CaO found in the liquid phase decreased, whereas that of SiO2 increased. The change of CaO was by far larger especially up to about K2O≤5.86g/l. From this point the decrease became slow and the concentration dropped below 0.1gCaO/l. The amount of CaO entered into the reaction to form solid phase was therefore, increased with increasing K2O concentration up to about the same limit.(3) A part of Ca2+ in tobermorite is very likely to be replaced by K+ whose amount seems to be governed by the concentration of K2O in the liquid phase.In the case of 13.78gK2O/l or more in the final concentration the mol ratio K2O/SiO2 of the solid reaction product came up to 0.22. It was also found that there exist a relation(CaO/SiO2 mol. ratio)+(K2O/SiO2 mol. ratio)=1.2-1.3in the solid reaction product.(4) Like NaOH the KOH-solution acts as an accelerator in the formation of tobemorite phase from Ca(OH)2 and silica gel.In the first place there would be the combination of KOH with silica gel to form K2O-SiO2-nH2O, which then acts upon Ca(OH)2 to precipitate tobermorite phase liberating K2O passing again into the liquid phase, so that the reactions are repeating round on the same way. In the mean time the replacement of Ca2+ and K+ would take place.The chemical equation may be written as follows;Si(OH)4+2KOH=K2O⋅SiO2+3H2O SiO2+2KOH=K2O⋅SiO2+H2O}…(1)K2O⋅SiO2+xCa(OH)2+mH2O=zK2O⋅xCaO⋅SiO2⋅yH2O/solid reaction product (tobermorite)+2(1-z)KOH+(x+z+m-y-1)H2O…(2)where, z was estimated to≤0.22 in the present experiment.