ACCOA - PRODUCTION DESCRIPTION

ACCOA Production 'Process'       ACCOA Production 'Pictures'       ACCOA AAC Panel Profiles

1 Introduction 2 Production and Structure of the Material 3 Environmental Implications

4 Quality Control

5 Energy Consumption

6 Byproducts

7 ACCOA Products

1 Introduction

1.1 History of AAC

In the early 1920’s Dr. Axel Eriksson invented gas concrete. At that time Dr. Eriksson was assistant professor for Building Techniques at the Royal Institute of Technology in Stockholm. Later this new and specific lightweight concrete was called Autoclaved Cellular Concrete or Autoclaved Aerated Concrete (AAC).

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1.2 Worldwide Usage

Aerated concrete has been available in most European countries for the past 50 years. It has gained in popularity throughout the rest of the world and now has a large presence in the Far and Middle East and, to a lesser extent, in Australia and South America. The manufacture of AAC is also on the increase in China and plants are under construction in India. The world’s current greatest producer of AAC is Russia. Undoubtedly the areas of massive growth potential are China, India and North America.
Currently, some 300 manufacturing locations produce in excess of 25 million yards3.

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1.3 Classification of ACC

AAC can be subdivided into classes according to characteristic strength as shown in the table below.
Property Low Medium High
Compressive Strength (psi) 260 260-580 580
Dry Density (lbs/ft3) 12.5-25 18.7-37.45 31.2-62.4

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1.4 Principles of Control

QUALITY CONTROL
In-house testing of the product(s) following the national guidelines. Written manual is reviewed annually and as needed.

QUALITY ASSURANCE
Testing of the product(s) by an independent, nationally recognized institute.

IN HOUSE CONTROL
Will always include the following material properties and performance:
- Density
- Compressive Strength
- Dimensional tolerance
- Dry shrinkage
- Corrosion protection of the reinforcement

In addition, depending on design rules used, the following properties shall be controlled:
- Modulus of rupture
- Thermal conductivity
- Modulus of elasticity
- Thermal conductivity
- Yield stress of reinforcing steel
- Strength of welded joints


If required by design rules the following performance tests shall be carried out:
- Bond strength
- Full-scale bending tests of units intended for transverse loading
- Full-scale tests of units intended for axial load

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1.5 Advantages

AAC has the following properties:
Lightweight – easily handled, speed of construction, reduced transportation cost
Good Thermal Insulation - reduced energy costs and build costs
Easy to work (workability) - can be worked using conventional wood working tools, equipment costs are low.
Fire resistant
Sound Adsorption
Does not decay and is resistant to termites.
Compare the above to wood.

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2 Production and Structure of the Material



2.1 Raw Materials Selection

AAC is a calcium silicate product and therefore requires sources of calcium and silica.

Sources of silica

Sand which requires grinding to produce a fine material
Pulverized fuel ash - pfa (waste product from power stations)
Slag - (GGBS, Ground granulated blast-furnace slag), a waste product from the steel industry

Sources of Calcium
Cement
Lime
Anhydrite/Gypsum

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2.2.1 Sand Grinding

Sand is fed into the ball mill at a rate of approximately 10 tons per hour to provide the “fineness” of sand that we require. The feed rate is adjusted dependent on the initial grading of the sand.

To ensure we achieve the correct solids to water ratio the sand entering the mills via conveyors is weighed. The moisture content of the sand is also measured and the results computed to ensure that the mills are fed with the correct quantity of “dry” sand. Water at a fixed rate is added to the mills together with the sand.

The ball mills are rubber lined and are filled to approximately 1/3rd of the volume with hardened steel balls. As the mills revolve “lifters” carry the steel balls until they reach a position at which they cascade down crashing onto the balls at the bottom of the mill. This action results in the sand being ground in addition to the natural revolving of the steel balls.

The resultant sand slurry is pumped to holding silos via a density meter that measures the density of the slurry.

During the grinding process heat is generated which would result in the slurry being too hot to use in the process. Chilled water is therefore added to the mill at a temperature of 42.8oF. Cooling of the slurry after milling is also done.

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2.2.2 Returns

Returns slurry is generated from the material that is cut of the molds during the cutting process. The waste material cut from the molds falls into a channel or ditch that has constantly circulating returns slurry being pumped from the collecting tank.

The density of the circulating returns slurry is constantly monitored and when the desired density is achieved, the slurry is pumped into the returns slurry holding tanks. The desired quantity of returns slurry is then pumped into the pre-mixer.

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2.3.1 Pre-Mixing

The pre-mixer is fitted with an agitator and is therefore a mixer in itself and is mounted on strain gauges that measure the amounts of sand slurry and returns slurry. Pre-measured quantities of cement and gypsum are then added.

When predetermined quantities (by mass) have been reached, the transfer stops and the agitator mixes the products. The resultant mix is then dropped into the mixer.

The purpose of the pre-mixer is to increase the production rate by mixing some of the materials prior to the main mixing cycle. This enables the total time for the mixing cycle to be accomplished in 3.5 minutes, which is one of the fastest mixing cycles in the world.

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2.3.2 Mixing

The contents of the pre-mixer, sand/return slurry, cement, and gypsum that have been thoroughly mixed are dropped into the main mixer. The desired quantity of lime is then dropped into the mixer. As soon as the lime enters the mixer a reaction occurs, generating heat as the lime slakes.

When only 20 seconds of the mixing cycle remain, the desired quantity of Aluminum slurry is dropped into the mixer. Certain conditions have to be met before the aluminum can be added to the mixer; namely a mold has to be present under the mixer in readiness to receive the mix. With the lime and aluminum now present, the mix will thicken very quickly. To prevent situations where it would become necessary to “dig” out the mixer, mixes will be dumped if required. The mix can now be cast into the mold.

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2.4 Casting

The steel mold that has been sprayed with a release agent (oil) will receive the contents of the mixer. The mix has to be of the correct consistency so that it flows out of the mixer easily and distributes in the mold evenly. The mix at this stage will occupy approximately 50% of the volume of the mold.

The mold is transferred in the pre-curing room via the transfer trolley that is fitted with vibrating pokers. To eliminate any entrained air that is a detriment to the compressive strength and finished quality the pokers are lowered into the mix as the mold is transferred in the pre-curing area.

As the reaction occurs, the hydrogen gas pumps up the mix so that it increases in volume by some 100% when the reaction is complete. This is known as the “blow period” of time. The product at this stage is composed of approximately 50% air pores, 30% smaller pores and 20% solid material.

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2.5 Pre-Curing (Blow and Set)

The completion of the reaction in which the Hydrogen is produced is known as the blow time. The blow is identified when small blisters appear on the top of the cake that rupture and become small fissures. This is probably one of the most critical parts of the process. If on completion of the reaction the mix ahs not hardened sufficiently, the mix will collapse and will be a reject. However, if the mix hardens prior to the completion of the reaction, the gas pressure will result in the cracking of the cake.

After the blow the mold remains in the pre-curing areas so that it hardens to a predetermined penetration or resistance that is known as the set period of time. Although a number of methods are available to determine the hardness. We will employ a fine rod fitted with a thermocouple. This will measure the cake temperature as well as the as the resistance as it is inserted into the “green concrete.”

The cake must be sufficiently hard prior to transfer to the cutting line. If it is too soft, the cake will crack during transfer and de-molding. If the cake is too hard, the cutting wires will be broken. Once the set period has been reached, a window of approximately 15 minutes is available before the cake is too hard too cut.

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2.6.1 Cake Tilting

When the cake is ready for cutting, the Tilt Crane is positioned over the mold and smoothly lifts the entire mold assembly off the rails in the pre-curing area. The assembly is turned through 90° and transported to the cut line where it is positioned on the traveling carriage. The side clamping arrangement is now released and the mold lifted clear of the “green concrete.” All of these actions have to be performed smoothly and accurately to ensure that the concrete is not damaged.

The purpose of tilting the cake is to ensure that the blocks or panels that are cut by tensioned wires are dimensionally accurate. This is achieved by having short, highly tensioned wires fitted on the cutting machines. If the cake were not tilted, the cutting wires would have to be much longer, resulting in inaccuracies. The finish of the products should also be superior.

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2.6.2 Cutting

CUT METHOD RESULT
1st Cake is pushed through the blades Sides are trimmed off to give the length of the block or the width of a panel.
2nd Cake is pushed through the wires The block height or panel thickness is cut.
3rd Wire frame is pushed down through the cake. Cuts the block thickness or the panel length.

Upon completion of the cutting, a hood that sucks off the top waste material is lowered onto the cake. The side waste is also removed at this point with the waste material falling into a ditch. The cake travels to a position where a crane that off loads the cut cakes onto bogies lifts it. These bogies carry three cut cakes and transport them into the Autoclaves.

NOTE: The purpose of transferring the cake from one carriage to another under the third cutting machine is to maintain the cycle speed. After transfer, the first carriage returns to the position under the tilt crane in readiness for the next cake. At this stage in the process the product, whether it is a block or a panel, has now been formed and is ready for cooking in the Autoclaves.

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2.7 Autoclaving

What occurs in the autoclaves? Curing of cement based material at normal temperature and pressure it is essentially the binder that react to hold the aggregates together. The reaction products form a cement gel with a high specific surface, as found typically in ordinary concrete. If curing takes place at high temperature and high pressure in saturated steam, part of the silica material reacts chemically with the calcium ingredients such as the lime and the lime liberated by the hydration of cement. The reaction products are of a micro-crystalline structure with a much lower specific surface than that obtained by normal curing.

Each of the eight autoclaves holds 18 mixes or 107 cubic yards of material. Since the autoclaves are fitted with doors at both ends, each autoclave can be loaded and unloaded simultaneously. Six bogies, each holding three molds fill an autoclave and when full, the doors are shut and locked in a closed position. Steam flows into the autoclaves under controlled conditions until the pressure reaches the operating conditions of 170 psi with a temperature of approximately 360°F. Three factors that affect production output are the type of product cooked, the time required to raise the pressure and the time that the pressure remains constant.

SAFETY CONSIDERATIONS: 1 psi of pressure on the autoclave doors exerts a force of over 3 tons and with an operating pressure of 170 psi the energy release should failure occur would be devastating. A sophisticated system of interlocks is fitted to each autoclave to ensure that they are operated safely and a high degree of training for the operators is necessary.

BLOCKS PANELS
Time required to fill/empty: 1hour 30 min 1 hour 30 min
Time required to raise pressure: 2 hours 4 hours
Time at pressure: 8 hours 16 hours
Time required to blow down: 1 hour 30 min 1 hour 30 min
Total time: 13 hours 23 hours

On completion of curing in the autoclaves the products are unloaded and travel to the packaging part of the operation.

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2.8 Reinforcing

Metal bars or mesh are placed inside the AAC material to resist the internal tensile stresses. The reinforcement consists of smooth, hot rolled and/or cold worked steels that conform to relevant national codes. The two types of reinforcing mesh produced are non-structural or transportation mats that are fitted in large wall panels and structural mats which are in floor, roof and wall panels. Depending on the design, different diameter steel wire is used by being fed into a machine for cutting to the required lengths. The cut lengths are then fed into an automatic welding machine that welds together the wires in a design configuration that considers the loading that is being applied to the panel. After welding a protective coating is applied to the mesh.

Due to its porosity at as comparatively low alkalinity, AAC does not provide the corrosion protection to the steel reinforcement like dense concrete. The reinforcement is therefore protected by a suitable surface coating. A water-based acrylic solution for the coating is increasing within the industry due to the ease of cleanup as well as being environmentally accepted. The bonding properties of AAC and reinforcing steel are also strongly influenced by the protective coating. The anchorage capacity provided by the bond between AAC and reinforcement is determined by a pull out test. A slip of 0.1mm is considered failure.

After application of the coating it is allowed to dry; the mesh is then loaded into a holding frame fitted with bayonet-type fixings. The fixings extend to the bottom of the mesh to ensure that the mats are rigidly held. The frame is placed over the cast AAC mold and the mesh is lowered into the cast mold. The bayonet fixings are removed when the mix has set.

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2.9 Structure of the material

DEFINITION—Autoclaved Aerated Concrete AAC is a cellular lightweight concrete in which the lightness is obtained by the formation of microscopic gas bubbles produced by a chemical reaction within the mass during the liquid or plastic phase. The air bubbles are uniformly distributed and are retained in the matrix upon setting and hardening to produce a cellular structure.

AAC is produced by high-pressure and steam-cure in autoclaves. It is factory made and available in pre-cast reinforced units such as wall, floor, roof, panel and lintel. Unreinforced masonry units (blocks) are also produced for laying in mortar or for gluing together. The density of the material is much lower than conventional concrete because of its cellular nature. The compressive strength of AAC is generally 10% lower than perpendicular to the direction of the rise. AAC achieves its final strength during the autoclaving process without further curing.

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3 Environmental Implications

RAW MATERIAL CONSUMPTION
The main raw materials used in the production of AAC have been stated. Keeping in mind that AAC contains a pore content of 80%, 4 yds 3 of material can be produced out of 1 yd3 of raw materials. With the low consumption of raw materials, this contributes to their conservation when compared to other construction materials. AAC can utilize industrial waste such as pfa slag etc that would otherwise have to be stockpiled or disposed of, contributing to ecological relief.

RAW MATERIALS NON-TOXIC AND ABUNDANT
The raw materials used in the process are found in almost unlimited quantities throughout the world and are non-toxic.

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4 Quality Control

SAND
Sand must be free from stone, clay, salty impurities and organic substances. It must have a silica content over 80%, ideally over 90%. At least 80% must be quartz. It is wet milled.

CEMENT
Portland cement type 1/type 2 complying to ASTM-C150-97a 7 day compressive strength over 4500 psi 28 day compressive strength over 6500 psi

LIME
High Calcium burnt lime, known as quicklime. Temperature rise after 2 minutes of being added to water in the range 30 to 50°C. Final temperature increase in the range 45 to 65°C.

ANHYDRITE/GYPSUM
Both can be used to retard the reaction of the quicklime, but due to its anhydrous properties, Anhydrite is preferred. Gypsum is added toe cement clinker during the grinding stage to control the setting time of the cement.

ALUMINUM
Very finely divided flaked aluminum is used to create hydrogen gas to raise the cake.

RETURNS
Returns are generated when the waste material is cut from the cake and mixed with water. A return contain quantities of partially hydrated cement and lime and is used to replace sand slurry up to 25% to achieve an increase in compressive strength. The density of the returns will be in the region of 1600g/1 with a moisture content in the region of 35 to 45%.

MIX RATIO
Mix ratio is described as the ratio of dry aggregate to the total binder content. Dry aggregate is the total of the dry weight of sand plus the dry weight of returns. Total binder is total weight of cement, lime, and anhydrite/gypsum. Mix ratio should be in the range 2.6:1 to 4.0:1. Mix ratio has a direct effect on compressive strength. A decrease would result in an increase in compressive strength.

WATER/SOLIDS RATIO
Water solids ratio is described as the ratio of the total water in the mix to the total dry solids. Total water is the amount of water carried in the slurries plus any water added directly to the mixer. Total dry solids are the amount of dry aggregate and binder. The ratio should be in the range 0.350 to 0.600. Water/Solids ratio has an inverse relationship with compressive strength. An increase in the ratio would result in a decrease in compressive strength.

MIX CONSISTENCY
This is a measure of the “flow ability” of the mix and is similar to a slump test on concrete. It is checked immediately after the mix has finished pouring.

BLOW/SET RATIO
The Blow to Set ratio should always be greater than 1 to 3. i.e. if the blow time is 15 minutes the set time should be greater than 45 minutes. A ratio of less than 1 to 3 indicates that the set is starting at the bottom of the cake before the reaction is complete and that stresses are building up in the cake that can lead to cracking.

POUR TIME
Pour time is the time the mix pours into the mold and is used as the basis for all other timings relating to that mix. Pour temperature is taken immediately after the mix pours into the mold. Variations will result in changes to the blow, set and ultimate product density and compressive strength. Pour height is taken immediately after the mix pours into the mold. Variations will result in changes to the blow, set and ultimately product density and compressive strength.

BLOW TIME
Blow time is the time between the mix pouring and the aluminum reaction reaching completion i.e. the blow. Variations will affect set time and firmness at the cutter and are useful early indicators. Blow temperature is recorded immediately after the mix blows. Variations will affect set time and firmness at the cutter, and are useful early indicators. Blow height is recorded immediately after the mix blows. Variations will affect density and compressive strength and are useful early indicators.

SET TIME
Set time is the time between the mix pouring and the mix setting, i.e. being at a suitable firmness for cutting. Variations create problems at the cutter.

SINK
This is the difference between the blow height and the set height. The set height should always be lower than the blow height, producing a “positive” sink. It will normally be in the region of 1 to 4 inches.

RATE OF RISE
This is performed to assess the reaction rate of the aluminum. On completion of the pour, the height is measured, and is measured every 30 seconds for the first 6 minutes and thereafter every minute until blow. When performed with consistent slurry densities and binder contents, it can also be used to ascertain the yield of aluminum.

DIMENSIONS
On a regular basis, and especially after wire changes, checks are made by measuring all dimensions on each block/panel in a cake. This ensures that dimensionally incorrect material is not produced. Measurements are performed using a steel rule or steel tape measure that has been checked for accuracy against a calibrated device that can be traced to a National Standard. All measuring equipment must be checked on a regular basis for continuing conformity (usually every 3 months) and records kept for Quality Assurance accreditation.

Finished Product Testing

'HOT' TESTS
Increase in size during autoclaving if the lime is unsound. (Unsound lime is a function of the granular size after calcining and milling.) These checks are therefore important not only as a record for Quality Assurance but also as a check on the quality of the lime.

DRY DENSITY
This is the density of a sample of the product in an oven-dried condition. It is one of the criteria the product must conform to.

COLD STRENGTH
This is the compressive strength achieved by the product in a cold, air-dried condition. This test is normally used as pass/fail criteria for product strength.

SOAKED STRENGTH
This is the definitive strength test for the product and all product must achieve the stated strength in the soaked state. “Soaked” refers to a product that has been immersed in water for a period of 16 hours.

ISO 9002
The ISO 9002 standard is a Quality Assurance standard, not a Quality Control standard. It is a monitoring system for the quality control procedures and all operations of the business. To achieve certification for ISO 9002 a series of manuals will cover all aspects of the plant from raw material intake to the shipping of the finished product. At all times the contents of these manuals must be adhered to. Training schedules and records will be kept for all personnel on site.

We are quality people, with a quality plant, supplying a quality product. The processes that are monitored and the data that is collected by the Honeywell Plant Scan system will be used to substantiate the statements in the manual that pertain to all aspects of the manufacturing process—from raw material intake to the shipping of the finished product.

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5 Energy Consumption

AAC requires a modest amount of energy for production and due to a high insulating value, it reduces the energy needed for the heating and cooling of buildings, reducing pollution. In addition due to a low weight less volume of transportation is required, further reducing emissions to the atmosphere. The amount of energy consumed including that to process raw materials is approximately 1000 MJ/m3. This is 2 to 3 times lower than other building materials is. Gas emissions from steam generation in the production of AAC are therefore relatively low. Besides the gas emissions other byproducts are produced from AAC production.

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6 Byproducts

The byproducts produced are as follows: 1. Unhardened AAC mixture (returns) 2. Hardened AAC waste 3. Condensation from the autoclaves In modern production processes the above materials are either used in the process or up-graded/reworked to marketable products. All of the returns generated in our process will be used within the process and generated when the cake undergoes the cutting process or is rejected material at the mixing stage. Some of the hardened waste material (finer particles) will be reused in the process but other avenues where we can sell the waste are being explored.
The autoclave condensate is being pumped directly to the sewage system after the heat it contains has been utilized to heat the boiler feed water. The system will allow us at some stage to re-use the condensate should we require it.

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7 ACCOA Products

7.1 Product Lines

ACCOA has the capability to produce the following products:
1. Blocks (Solid & Cored)
2. U blocks
3. Lintels
4. Wall panels (Horizontal & Vertical)
5. Floor panels
6. Roof panels


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7.2 Packaging and Shipping

Blocks will be packaged on wooden pallets and shrunk wrapped.
Wall Panels will be packed as followed:
Panels up to 8’ in length will be on 4’ long pallets. Panels over 8’ will be on 8’ long pallets.
The pallets are 2’ wide and will carry four 6", three 8”, two 10" or two 12" thick panels.
Floor and roof panels will be laid flat on the pallets.

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