Concept of Grooved AAC Block

The manufacturing process of AAC block is totally different from that of clay brick.

For casting a clay brick, typically a mold of size 210 (length)×110 (width)×68 (height) mm³ is used. The bottom surface of the mold contains a circular or rectangular raised pattern or design called emboss. As a result, the circular or rectangular frog/deboss is formed on the final clay brick surface. The frog is basically an indentation on the brick surface. In contrast to this, AAC block of size 600 (length)×200 (width)×150 (height) mm³ is manufactured by wire cutting of a huge soft green cake cast in a mold of size 4200 (length)×1200 (width)×600 (height) mm³ as discussed in Chapter 1. Thus, the final AAC block surface is smooth and there is no scope to form a frog on its surface. However, producing the grooved AAC block was found to be practically feasible in the existing manufacturing set-up through mold modification method. The mold modification is simple, economical and can be easily implementable in the existing AAC manufacturing industry.

Three different miniature molds of internal dimension 220×120×140 mm³were fabricated to produce two blocks each of plain, single-groove and double-groove type. The

mild steel plates of 5 mm thickness were used to fabricate the mold. The photograph of mold is shown in Figure 5.1.

(a) (b) (c)

Figure 5.1 Photograph of miniature molds for producing (a) plain AAC block, (b) single geoove AAC block and (c) double groove AAC block

For the molds producing grooved blocks, holes of 30 mm diameter were made in the plates using stamping operation. The holes were for inserting steel cores. Fine grade materials were used to produce the AAC blocks in the following way:

(1) Initially, a slurry was prepared using 38% water, 14% cement, 7% lime, 0.03%

aluminium powder and rest crushed fine sand by weight.

(2) The slurry was poured into oil treated miniature molds. The molds for producing grooved blocks contained 30 mm diameter cores spanning the entire width. Only 60% of the mold volume was filled with slurry to allow for the expansion of the slurry during casting process.

(3) After pouring, slurry was allowed to pre-cure and rise for about 45 h for getting a green cake with sufficient strength. During this time, the aluminum powder reacts with calcium hydroxide formed as a reaction between cement and water. As a result, hydrogen gets released and the pores are filled up with air, causing the expansion/rise of the slurry. The volume increases with increase in the amount of aluminum powder.

(4) After pre-curing, the top surface of the green cake was levelled by removing excess material. In case of grooved blocks, the core was removed gently, producing holes in green cakes. Green cake was taken out from the mold and wire-cut to make blocks of 70 mm height. As a result, two blocks could be prepared from each mold. Blocks from the molds with cores contained semi-circular grooves on one face.

(5) After wire-cutting, the blocks were placed in an autoclave chamber for hydrothermal treatment at the steam temperature and pressure of 200 ºC and 1.3 MPa, respectively. The temperature and pressure for hydrothermal treatment adopted was the same as used for manufacturing the conventional AAC blocks. Finally, the hardened plain as well as grooved AAC blocks were obtained after 18 h of autoclaving. The photograph of different stages to produce the grooved AAC blocks using the miniature is shown in Figure 5.2.

Figure 5.2 The photographs showing the different stages of manufacturing of grooved AAC block: (a) pouring of slurry mix into the miniature mold, (b) expansion/solidification of slurry in the miniature mold, (c) removal of core from mold leaving the hole inside the block and (d) levelling the top surface of green cake by removing extra material

Although AAC blocks are usually of size 625×250×200 mm³, for the ease of testing and comparison, the sizes of AAC blocks in this study were of the clay brick size (220×120×70 mm³). As per the I.S. 1077, clay bricks of 7090 mm height have a rectangular or circular frog of 1020 mm deep on one of its flat surfaces. Therefore, the AAC blocks with circular groove of depth 15 mm at one of its faces were used. The designations for plain

(a) (b)

(c) (d)

block, single-grooved block and double-grooved block are PB, SGB and DGB, respectively.

The different grooved AAC blocks used in this study are depicted in Figure 5.3.

Figure 5.3 AAC blocks used in the study: (a) plain block, (b) single-grooved block and (c) double-grooved block

All the stages (pouring, rising, cutting and autoclaving) in production of grooved AAC block using the miniature mold were performed in an industry, manufacturing conventional AAC blocks following the usual industrial practice as discussed in Chapter 1.

The moisture contents and the dry density of the final blocks were tested on each of six PB, six SGB and six DGB specimens. The moisture contents of PB, SGB and DGB blocks were found to be 8%, 9.4% and 9.2%, respectively, while the average dry densities of PB, SGB and DGB blocks were 658.2 kg/m³, 665.3 kg/m³ and 668.24 kg/m³, respectively. The density of lower block was found to be marginally higher (49%) than that of upper block. However, the effect of groove on the moisture content and dry density of the block is very less.