Process for producing autoclaved aerated concrete using silica raw materials having higher solubility than quartz

1 . A process for producing an autoclaved aerated concrete comprising the steps of:

a) preparing a suspension comprising a silica source and a calcium oxide source in water, wherein the silica source comprises at least 50 wt % of soluble silica species;

b) preparing a green porous solid from the suspension of step a);

c) steam curing the green porous solid from step b) at a temperature of from 100° C. to 170° C. in the presence of saturated water steam; and

d) recovering the autoclaved aerated concrete,

wherein the autoclaved aerated concrete has a content of tobermorite of at least 12.5 wt % and a content of amorphous calcium silicate hydrate (CSH) phases of at least 30 wt %, based on the total weight amount of autoclaved aerated concrete.

2 . The process according to claim 1 , wherein the silica source comprises the soluble silica species and quartz in a weight ratio of the soluble silica species to quartz of 50:50 to 100:0, based on the total weight of the silica source.

3 . The process according to claim 1 , wherein the soluble silica species is selected from cristobalite, tridymite, stishovite, fly ash, amorphous silica species, siliceous earth, silica glass, alkali-silica glass and water glass, and mixtures thereof.

4 . The process according to claim 3 , wherein the amorphous silica species are particles or porous aggregates of amorphous silica.

5 . The process according to claim 1 , wherein the calcium oxide source is selected from cement, burnt lime, hydrated lime, and mixtures thereof.

6 . The process according to claim 1 , wherein the calcium/silicon molar ratio (C/S molar ratio) of the raw materials mixture of step a) is from 0.50 to 0.80.

7 . The process according to claim 1 , wherein the suspension further comprises up to 25 wt % of recycled pulverized autoclaved aerated concrete, based on the total amount of solid material in the suspension, which is counted to the silica source in accordance with its molar amounts of silicon.

8 . The process according to claim 1 , wherein the autoclaved aerated concrete has a content of residual quartz of not more than 20 wt %, based on the total weight amount of autoclaved aerated concrete.

9 . The process according to claim 1 , wherein the autoclaved aerated concrete has a density of from 200 to 800 kg/m 3 .

10 . The process according to claim 1 , wherein the autoclaved aerated concrete has one of the following properties:

a compressive strength of at least 1.5 MPa for a density of from 205 to 250 kg/m 3 ,

a compressive strength of at least 2.0 MPa for a density of from 255 to 300 kg/m 3 ,

a compressive strength of at least 2.5 MPa for a density of from 305 to 350 kg/m 3 ,

a compressive strength of at least 3.0 MPa for a density of from 355 to 400 kg/m 3 ,

a compressive strength of at least 3.7 MPa for a density of from 405 to 450 kg/m 3 ,

a compressive strength of at least 4.5 MPa for a density of from 455 to 500 kg/m 3 ,

a compressive strength of at least 5.3 MPa for a density of from 505 to 550 kg/m 3 ,

a compressive strength of at least 6.3 MPa for a density of from 555 to 600 kg/m 3 ,

a compressive strength of at least 7.5 MPa for a density of from 605 to 650 kg/m 3 ,

a compressive strength of at least 8.8 MPa for a density of from 655 to 700 kg/m 3 , or

a compressive strength of at least 12.1 MPa for a density of from 705 to 800 kg/m 3 .

11 . The process according to claim 1 , wherein the autoclaved aerated concrete has one of the following properties:

for density class 0.35 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 2;

for density class 0.40 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 2;

for density class 0.45 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 2;

for density class 0.50 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 4;

for density class 0.55 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 4;

for density class 0.60 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 4;

for density class 0.65 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 6;

for density class 0.70 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 8; or

for density class 0.80 kg/dm 3 the autoclaved aerated concrete meets compressive strength class 8,

wherein the density classes and compressive strength classes are categorized according to DIN 20000-404.

12 . The process according to claim 1 , wherein the autoclaved aerated concrete shows a peak of wollastonite (CaSiO 3 ) formation at a temperature of 835-843° C. in differential thermal analysis (DTA) in accordance with DIN 51004.