Calcium Silicate Bricks or Sand Lime Bricks for Masonry Construction

Calcium silicate bricks are made of sand and lime and popularly known as sand lime bricks. These bricks are used for several purposes in construction industries such as ornamental works in buildings, masonry works etc.

Sand lime bricks are popularly used in European countries, Australia and African countries. In India, these bricks are widely used in Kerala state and its usage is regularly growing.

Materials Used for Sand Lime Bricks

The materials listed below are used for the production of calcium silicate bricks.

• Sand
• Lime
• Water
• Pigment

Sand

Calcium silicate bricks contains high amount of sand is about 88 – 92%. It means the properties of these bricks depends upon the characteristics of sand used.

So, the sand used shall be well graded and should not contain any impurities like organic matter, soluble slats etc. the finely divided clay may be present but it is up to 4% only which helps the brick in pressing and provides smoother texture.

Lime

Lime content in calcium silicate bricks varies from 8 to 12%. The lime used shall be of good quality and high calcium lime.

Water

Clean water should be used for preparing calcium silicate bricks. Sea water or water containing soluble salts or organic matter more than 0.25% are not suitable.

Pigment

Pigments are generally used to give color to the bricks. They are added to the sand and lime while mixing.

Total weight of brick contains 0.2 to 3 % of pigment quantity. Different pigments used to get different colors are tabulated below:

Pigment	Color
Carbon black	Black, grey
Iron oxide	Red, brown
Chromium oxide	Green
Ochre	yellow

Manufacturing of Calcium Silicate Bricks

In the first step, suitable proportions of sand, lime and pigment are taken and mixed thoroughly with 3 to 5 % of water. Then paste with moldable density obtains.

The mixture is molded into bricks using rotary table press which uses mechanical pressure to press the bricks. The pressure of pressing varies from 31.5 to 63 N/mm2.

In the final stage, bricks are placed in autoclave. Autoclave is nothing but a steel cylinder with tightly sealed ends. The diameter of autoclave is bout 2m and length is about 20m.

After placing bricks in this closed chamber saturated steam pressure is released which is about 0.85 to 1.6 N/mm2. The temperature inside the chamber is raised and the reaction process begins.

Silica content in sand and calcium content in lime reacts and forms crystal like compound called calcium hypo silicate. This process is done for 6 to 12 hours. Finally, the obtained bricks are transported to work place.

Advantages of Calcium Silicate Bricks

There are many advantages of calcium silicate bricks when used in masonry construction, and they are:

• Mortar required for providing plaster on calcium silicate bricks is very less.
• Color and texture of these bricks is uniform.
• Compressive strength of sand lime bricks is about 10N/mm2. So, they are well suitable for multi storied buildings.
• For constructions in clay soils, these bricks are more preferable.
• Trouble of Efflorescence does not arise in the case of sand lime bricks.
• Not only bricks, blocks and tiles can also be made using calcium silicate.
• Sand lime bricks provides more comfort and accessibility for architects to attain desired shape and designs.
• These bricks have accurate shape and size with straight edges.
• Solar heat effect is reduced on exposed walls made of calcium silicate bricks.
• Colored sand lime bricks do not need any finish to the wall, so, cost reduces.
• These bricks have great fire resistance and water repellant properties.
• Calcium silicate brick walls resists noise from outside.
• Cost of construction gets reduced about 40% of total cost due to following factors.

i. Wastage of calcium silicate products is very less.

ii. Less quantity of mortar is needed.

iii. Thickness of wall can be reduced when constructed using these bricks because of high compressive strength.

Disadvantages of Calcium Silicate Bricks

In some conditions, calcium silicate bricks are not suitable and their disadvantages are:

• If clay is available in plenty, clay bricks are more economical than calcium silicate bricks.
• These are not suitable for laying foundation, because they cannot provide resistance to water for longer period.
• They cannot also resist fire for longer periods so, they are not suitable for building furnaces etc.
• Abrasion resistance of these bricks are very less so, they cannot be used as paving materials.

Types of Bricks – Their Field Identification, Properties & Uses

For a brick masonry construction, it is desirable to know different types of bricks, their field identification, properties, uses and suitability for different construction works.

A brick is an important construction material which is generally available in rectangular shape manufactured from clay. They are very popular from olden days to modern days because of low cost and durability.

Types of Bricks – Their Properties and Uses:

Based on the manufacturing process, bricks are broadly classified into two types, 1. Sun-Dried or unburnt bricks 2. Burnt bricks

Sun-Dried or Unburnt Bricks

Sun-dried or unburnt bricks are less durable and these are used for temporary structures. Unburnt bricks preparation involved in 3 steps they are preparation of clay, molding and drying. After molding, bricks are subjected to sunlight and dried using heat from sun. So, they are not that much strong and they also have less water resistance and less fire resistance. These bricks are not suitable for permanent structures.

Burnt Bricks

Burnt bricks are good quality bricks but however they also consist some defected bricks. So, burnt bricks are classified into four types and they are

• First class bricks
• Second class bricks
• Third class bricks
• Fourth class bricks

First Class Bricks

First class bricks are good quality bricks compared to other classes. They are molded by table-molding and burnt in large kilns. So, these bricks contain standard shape, sharp edges and smooth surfaces. They are more durable and having more strength. They can be used for permanent structures. However, because of their good properties they are costly than other classes.

Second Class Bricks

Second class bricks are moderate quality bricks and they are molded by ground-molding process. These bricks are also burnt in kilns. But because of ground molding, they do not have smooth surfaces as well as sharp edges. The shape of bricks also irregular due to unevenness in ground. These also will give best results in strength and durability. Smooth plastering is required on the brick structure.

Third Class Bricks

Third class bricks are poor quality bricks which are generally used for temporary structures like unburnt bricks. These are not suitable for rainy areas. They are ground-molded type bricks and burnt in clamps. The surface of this type of bricks are rough and they have unfair edges.

Fourth Class Bricks

Fourth class bricks are very poor quality bricks and these are not used as bricks in the structure. They are crushed and used as aggregates in the manufacturing of concrete. They are obtained by over burning, because of this they gets overheated and obtains brittle nature. So, they can break easily and not suitable for construction purpose.

Identification of Bricks Quality at Construction Site

To build a good quality structure, observing quality of materials is important. Here we discuss about how good bricks are identified at construction site.

• The color of bricks should be bright and uniform.
• They should be well burned and having smooth surfaces and sharp edges.
• Thermal conductivity of bricks should be less and they should be sound proof.
• They shouldn’t absorb more than 20% by weight when we placed it in water.
• When we struck two bricks together, ringing sound should be delivered.
• Structure of bricks should be homogeneous and uniform.
• The bricks should not break when we dropped it form 1m height.
• There should not be any scratch left on the brick when we scratched with finger nail.
• There should not be any white deposits on brick, when we soaked it in water for 24 hrs.

Properties of Bricks

Following are the properties of bricks which represents the importance of bricks in construction. i. Hardness ii. Compressive strength iii. Absorption

Hardness of Bricks

A good quality brick will have resistance against abrasion. This property is called hardness of brick which helps to give permanent nature of brick structure. Because of this property bricks do not damaged by scraping.

Compressive strength of Bricks

Compressive strength or crushing strength is the property of brick which represent the amount of load carried by brick per unit area. According to BIS the minimum compressive strength of brick should be 3.5N/mm2. Crushing strength of bricks reduced when they are soaked in water.

Crushing strength of Bricks	Grades
7 – 14 N/mm2	Class A
>14N/mm2	Class AA

 

Absorption of Bricks

Bricks are generally absorbs water but having limits. Absorption limit percentage by weight for different classes of bricks is tabulated below.

Class of Bricks	Water Absorption % by weight
Heavy duty bricks (special made)	Only 5%
First class	20%
Second class	22%
Third class	25%

Uses of Different Types of Bricks

Bricks are widely used in construction industry for different purposes as following.

• Good quality bricks (1st and 2nd class) are used in the construction of buildings, tunnels, pitching works etc.
• 3rd class and unburnt bricks are used for temporary structures.
• 4th class bricks are used as aggregate for making concrete.
• Bricks are also used for architectural purposes to give aesthetic appearance to the structure.

Manufacturing of Bricks for Masonry Construction – Methods and Process

The process of manufacturing of bricks from clay involves preparation of clay, molding and then drying and burning of bricks. The bricks are building materials which are generally available as rectangular blocks. The bricks do not require any dressing and brick laying is very simple compared to stone masonry.

Site selection for manufacturing of bricks

For the manufacturing of bricks, the site should be selected based on some important considerations such as:

• The ground should be of plain surface.
• The site should be connected with communicating roads for transporting materials etc.,
• Good brick earth should be easily available.
• The site should offer all facilities to the workers.

Manufacturing process of bricks

There are four different operations are involved in the process of manufacturing of bricks:

1. Preparation of clay
2. Molding
3. Drying
4. Burning

1. Preparation of clay for brick manufacturing:

Preparation of clay for bricks manufacturing is done in six steps: Unsoiling of clay We need pure clay for the preparation of bricks. The top layer of soil may contains impurities, so the clay in top layer of soil about 200mm depth is thrown away. This is called unsoiling. DiggingAfter the removal of top layer, the clay is dug out from the ground and spread on the plain ground. Cleaning In this stage, the clay is cleaned of stones, vegetable matter etc. if large quantity of particulate matter is present, then the clay is washed and screened. The lumps of clay are converted into powder with earth crushing rollers.Weathering The cleaned clay is exposed to atmosphere for softening. The period of weathering may be 3 to 4 weeks or a full rainy season. Generally, the clay is dug out just before the rainy season for larger projects. Blending If we want to add any ingredient to the clay, it is to be added in this stage by making the clay loose and spread the ingredient over it. Then take small portion of clay into the hands and tuning it up and down in vertical direction. This process is called blending of clay. Tempering In this stage, water is added to clay and pressed or mixed. The pressing will be done by cattle or with feet of men for small scale projects, pug mill is used as grinder for large scale projects. So, the clay obtains the plastic nature and now it is suitable for molding.

2. Molding of clay for brick manufacturing

In the molding process, prepared clay is mold into brick shape (generally rectangular). This process can be done in two ways according to scale of project.

• Hand molding ( for small scale)
• Machine molding ( for large scale)

Hand molding of bricks

If manufacturing of bricks is on a small scale and manpower is also cheap then we can go for hand molding. The molds are in rectangular shape made of wood or steel which are opened at the top and bottom. The longer sides of molds are projected out of the box to serve it as handles. If we take durability in consideration steel molds are better than wooden molds. In hand molding again there are two types and they are

1. Ground molded bricks
2. Table-molded bricks

Ground molded bricks

• In this process of ground molding, first level the ground and sand or ash is sprinkled over it.
• Now place the wet mold in the ground and filled it with tempered clay and press hard to fill all corners of the mold. Extra clay is removed with metal strike or wood strike or with wire.
• The mold is then lifted up and we have raw brick in the ground. And again wet the mold by dipping it in water and repeat the same process. The process of dipping mold every time to make bricks is called slop molding.
• Sometimes, the inside surface of mold is sprinkled with sand or ash instead of dipping in water this is called sand molding
• Frog mark of bricks are made by using a pair of pallet boards. Frog mark means the mark of depth which is placed on raw brick while molding. The depth may be 10mm to 20mm.
• Frog mark stats the trademark of manufacturing company and also it is useful to store mortar in it when the bricks is placed over it.

Table molded bricks

• This process is similar to ground molding process, but here the bricks on molded on the table of size 2m x 1m.
• Ground molding is economical when compared to table molding.

Machine molding of bricks

The bricks required are in large quantity, then machine molding is economical and also saves more time. Here also we are having two types of machines,

1. Plastic clay machines
2. Dry clay machines

Plastic clay machines This machines contain an opening in rectangular shape and when we place the tempered clay in to this machine it will come out through this opening. Now, the rectangular strips coming out the opening are cut by wires to get required thickness of brick. So, these are also called wire cut bricks. Now these raw bricks are ready for the drying process. Dry clay machines Dry clay machines are more time saving machines. We can put the blended clay into these machines directly without tempering. Means tempering is also done in this machine by adding some water. When the required stiffness is obtained the clay is placed in mold and pressed hard and well-shaped bricks are delivered. These are called pressed bricks and these do not require drying they may directly sent to burning process.

3. Drying of raw bricks

• After molding process the bricks contain some amount of moisture in it. So, drying is to be done otherwise they may cracked while burning. The drying of raw bricks is done by natural process.
• The bricks are laid in stacks. A stack consists 8 to 10 stairs. The bricks in these stacks should be arranged in such a way that circulation of air in between the bricks is free.
• The period of drying may be 3 to 10 days. It also depends upon the weather conditions.
• The drying yards are also prepared on higher level than the normal ground for the prevention of bricks from rain water.
• In Some situations artificial drying is adopted under special dryers or hot gases.

4. Burning of bricks

• In the process of burning, the dried bricks are burned either in clamps (small scale) or kilns (large scale) up to certain degree temperature. In this stage, the bricks will gain hardness and strength so it is important stage in manufacturing of bricks.
• The temperature required for burning is about 1100oC. If they burnt beyond this limit they will be brittle and easy to break. If they burnt under this limit, they will not gain full strength and there is a chance to absorb moisture from the atmosphere.
• Hence burning should be done properly to meet the requirements of good brick.