Skip to main content

Reactions during Autoclaving of AAC Blocks

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.


  1. Your blog contains lots of valuable data about Commercial Paving Companies in Toledo. It is a factual and beneficial article for us. Thankful to you for sharing an article like this.


Post a Comment

Popular posts from this blog

Recipe Calculation for AAC Blocks

Recipe is nothing but knowing the dry weight (batch weight) of one batch or one cake. To find out dry weight, we should know the desired dry density of final block. Let us work on the density of 600 kg/m³. Batch Weight = Density x Mould volume Density = 600 kg/m3 Dimensions of the mould : Length = 4840 mm = 4.84 M Width = 1250 mm = 1.25 M Height = 670 mm = 0.67 M Volume of the mould: 4.84 M x 1.25 M x 0.67 M = 4.0535 M³ Batch Weight = 600 x 4.0535 = 2432 kg (We have taken mould height 670 mm, which is actually depth of the mould. Generally we maintain the cake height equal to mould level. But in some AAC factories, cake height may be 650 mm or 680 mm depending on the parameters like quantity of returns required, sinking of cake, uneven surface of the cake, etc., In this case cake height should be taken to calculate the volume.) Batch weight or Dry weight consists of binders (Cement and Quicklime), Fly ash or Sand, returns and anhydrite. Usually th

What is Aluminium Powder? Why it is used in AAC Plants?

  Aluminium Metal powder or paste Aluminium powder, which is commonly used as a reaction agent in AAC Plants, is a fine granular powder made up of Aluminium Metal. Process of Manufacture: The aluminium powder is usually manufactured in many forms such as flaky particles, granulated powder (atomised aluminium) etc. There are different processes one can use for the production of aluminium powder. The metal is melted in furnaces, and the temperature is maintained around 720 to 760 C. Atomised Aluminium is produced by blasting the stream of molten Aluminium into small articles by air jet. For this purpose, an atomiser is employed which consists of a straight tube with lower end dipped in molten metal and upper end terminating as a little orifice. A jet of hot air under pressure is passed through armular opening near the top which impinges on a stream of molten Aluminium drawn by suction through the orifice. This results in the formation of small particles of Aluminium. Th

What is AAC? What are the properties of AAC? How AAC blocks are produced?

  Definition of AAC Blocks   AAC Blocks   A - Autoclaved A - Aerated C - Concrete   Autoclaved aerated concrete is a lightweight concrete containing uniformly distributed voids that is subjected to high-pressure steam curing.   Production of AAC Blocks   This is a light- weight building material produced by autoclaving a set mix of fine siliceous materials such as ground silica sand or flyash and the binders like Portland cement and lime. Lightness is achieved by incorporating a large proportion of closed microscopic pores in the slurry with the help of entraining or foaming agent. The basic raw materials are fine-ground sand or fly ash, cement, quicklime, anhydrite or gypsum, water, and aluminum  powder or paste as a rising component. A suitable mixture of these raw materials is poured into a mould, and the rising process starts due to the development of hydrogen gas formed by the reaction of aluminium in the alkaline suspension. After 20 to 40 minutes the maximum rising