Synthesis of Ca 3 SiO5
1) Solid state sintering calcium carbonate and silica are mixed and heated at high temperature (1450°C to 1650°C), following the global reaction :
SiO2 + 3 CaCO3 => Ca 3SiO5 + 3 CO2.
The reaction can be divided into 3 main steps
a) Decarbonation of CaCO3 according to : CaCO3 => CaO + CO2
b) Formation of Ca 2SiO4 as intermediate compounds according to : 2 CaO + SiO2 => Ca 2SiO4
c) Formation of Ca 3SiO5 according to: Ca 2SiO4 + CaO => Ca3SiO5
This later reaction has been largely studied and occurs through a diffusion controlled process according to the Ginstling-Brounshtein equation.
When liquid phase occurs in the system (i.e. industrial clinkering), the reaction is facilitated and is achieved at temperature around 1450° C and short time (1/2 hours). In the absence, or when only a small quantity of liquid phase is present, the temperature required for the reaction is higher (1600°C). In this case, it is possible to add mineralizers ( i.e. fluorine CaF2) to decrease the working temperature.
2) Co precipitation of calcium salt (nitrate or oxalate) and colloidal silica. Solution of calcium salt ( nitrate for example) is mixed with silica. 3 Ca (NO3)2 + SiO2 + H2O -=> Ca 3Si (OH)5 + HNO3 Nitric acid is evaporated at low temperature (300°C) and an amorphous mass is obtained. Finally, the product is heated at 1450°C.
3) Sol gel processing method The method is mainly based on the polymerization reaction, i.e. hydrolysis and poly-condensation of metal alkoxides. For the majority of them, the simultaneous reaction of hydrolysis and polycondensation lead to the formation of particles which undergo agglomeration. These alkoxides cannot be used for further polymerization and formation of a continuous polymeric network. Silicon alkoxides such as Si(OC2H5)4 (also called TEOS) show a particular behaviour because they hydrolyse more slowly and incompletely to produce soluble species: by polycondensation reaction, these species give rise to the formation of an extended polymeric network. For single component oxide materials, the alkoxide are utilized by first partially hydrolyzing the alkoxide.
M(OR)x + y H2O à M(OR)x-y (OH) y + y ROH
The partially hydrolysed species are the allowed to link forming M-O-M bond by polymerrisation or condensation reaction. If an OH group was to react with an OR group, then an other alkohol ROH would be produced by formation of the M-O-M bond . When more than a single species is involved, two types of reaction are possible. Those where all of the cationic constituents are supplied as alkoxide (Tetraethoxysilane TEOS and calcium alkoxyde ) and there where for different reasons (costs, availability...) some components are added as soluble salts ( TEOS and calcium nitrate).