Reactions during Autoclaving of AAC Blocks

In the first phase, a topochemical reaction takes place during which the lime and silica acid are the reactants. 

Water doesn't happen within the actual rejection; it’s only built into the new formulations that are formed. 

These reactions occur within the border areas of the raw materials, starting at the active spots (e.g. on the edges) of a crystal.

The second phase of the reaction will mainly involve the solvent phase where the lime and silica acid, both of them are dissolved in water, will react into insoluble calcium silicate hydrates. 

These hydrates will form a structure.

Looking at this in a little more detail from when the green blocks enter the autoclave, the main reactions that occur are broadly as follows:

Over 2 hours or so, as the pressure and temperature increase, the normal cement hydration products that formed in the green state progressively disappears and the pulverized fly ash becomes reactive. 

C-S-H(I) forms, partly from silica derived from the pulverized fly ash. 

As more pulverized fly ash reacts, calcium hydroxide from the lime and from cement hydration is gradually used up by the continued formation of C-S-H(I).

With continued autoclaving, 1.1 nm tobermorite starts to crystallize from the C-S-H(I); the total proportion of C-S-H(I) declines and that of 1.1 nm tobermorite gradually increases. C-S-H(I) is therefore mainly an intermediate compound. 

The final hydration products are then principally: 1.1 nm tobermorite Possibly some residual C-S-H(I).

Chemistry of AAC Blocks

From the start to the end the simplified chemical reactions look as follows:

Before Autoclaving:

CaO+H20 → Ca(OH)2 + 65.2 KJ/mol

3 Ca(OH)2 + 2Al + 6 H2O → Ca3(Al(OH)6)2 + 3H2

After Autoclaving:

6SiO2 + 5Ca(OH)2 → 5CaO. 6SiO2 · 5H2O

This is the final Tobermorite or Hydrated Calcium Silicate C5S6H5.