Cement and cement additives

Overview of cement and types of cement, its manufacturing process and mineral additives

Are you looking for cement and cement additives? Cement is a binding agent that hardens and bonds to other materials to hold them together. It is made through a strictly controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.

To produce cement, limestone, shells and chalk or marl are used in combination with shale, clay, slate, blast furnace slag, quartz sand and iron ore. These ingredients form a stone-like substance at high temperatures, which is transformed into a fine powder that we commonly call cement.

The cement manufacturing process can be briefly described below.

1. Extraction of raw materials from the quarry
2. Grinding, dosing and mixing
3. Preheating phase
4. Furnace phase
5. Cooling and final grinding
6. packaging and shipping

Raw materials such as calcium, silicon, iron and aluminum are used in the production of cement. To meet the demand for these materials, limestone, clay and sand are mainly used. Limestone is used to meet calcium needs, and small additions of sand and clay meet silicon, iron and aluminum needs. In addition, many other raw materials are used in cement production. The addition of these materials depends on production requirements.

After collecting the raw materials, they are dosed and ground. Generally, the limestone and clay content is 80% and 20% respectively. The materials are then pre-kneaded to improve process productivity and ensure environmentally friendly production. Furthermore, the hot gases that come out of the oven are used during pre-kneading.

The kiln is a rotating kiln in which the manufacturing process takes place. At this stage (oven stage), a paste of material is prepared after heating the raw material. The temperature in the central part of the kiln is around 1000 °C and in this area limestone decomposition occurs. CaO remains after the reaction if CO ₂ was released.

CaCO ₃ = CaO + CO ₂

The lower part of the oven is heated to approximately 1500 – 1700 °C. The reaction of lime and clay in this area leads to the formation of calcium aluminates and calcium silicates. This process creates aluminates and silicates from calcium clinker. They are hard and resemble small stones measuring around 5 to 10 mm. In this process the following reaction can be observed.

The following reaction can be observed.

2CaO + SiO ₂ = Ca2SiO ₄ (Declare silicate (C ₂ S))

3CaO + SiO ₂ = Ca3SiO ₅ (Tricalcium silicate (C ₃ S))

3CaO +Al ₂ Ó ₃ = Approx. ₃ Al ₂ Ó ₆ (Dicalcium Aluminate (C ₂ A))

4CaO +Al ₂ Ó ₃ + Faith ₂ Ó ₃ = Approx. ₄ Al ₂ Faith ₂ Ó ₁₀ (Tetracalcium aluminum ferrite (C ₄ AF))

Then the cooling process begins. It is cooled with air and soil to produce cement powder. In the final grinding step, 2-3% gypsum is added as a retarding agent.

The following figure shows the components of the cement manufacturing process.

TYPES OF CEMENT
Cement is categorized according to different standards and developments depending on its use and specifications. Guidelines considered by the American Society for Testing and Materials (ASTM) are discussed here. ASTM-C150 (Standard Specification for Portland Cement) and C595 (Standard Specification for Mixed Hydraulic Cement) specifications are as follows.

Standard Specification for Portland Cement ASTM C150

Description/use of cement type

INormal Type

Type II Moderate sulfate resistance

Type II (MH) Moderate hydration pressure and moderate sulfate resistance

Type III High initial strength

Type IV Low Heat Hydration

Type V High sulfate resistance

Standard Specification for Mixed Hydraulic Cement ASTM C595
cement

Type Description/Use

IL type Portland limestone cement

IS Type Portland Slag Cement

IP Type Portland Pozzolanic Cement

Type IT Ternary Mixed Cement

Additionally, there is a performance-based standard to categorize cement ASTM C1157

Description/use of cement type

GU type general purpose

Type HE, high initial strength

Type MS Moderate sulfate resistance

Type HS High sulfate resistance

Type MH Moderate water balance

Type LH Low heat of hydration

Additionally, there are other cement categorizations depending on the type/application. They are mainly based on the intended purposes of the construction. The following types can be identified most commonly.

1. Ordinary Portland Cement (OPC)
2. Portland pozzolanic cement (PPC)
3. Fast hardening cement
4. Fast-setting cement
5. Lowe Thermal Cement
6. Sulfate resistant cement
7. Blast Furnace Slag Cement
8. Cement with high alumina content
9. White cement
10. Colored cement
11. Air-entraining cement
12. swollen cement
13. Hydrographic cement
14. Wall cement

Different standards must be observed for each type of cement. The following table from BS 5328 Part 1 contains this information.

Cement is classified in accordance with BS 5328 Part 1 based on its compressive strength measured after 28 days.

There are five classes, namely 22.5, 32.5, 42.5, 52.5 and 62.5.

In addition, there are two intermediate strength classes, 37.5 and 47.5, for the combination of Portland cement mixer complying with BS 12 with granulated ground blast furnace slag (GGBS) complying with BS 6699.

CEMENT TESTING

The physical and chemical properties of cement are generally tested before use. A series of tests and test sections can be based on the relevant specifications. The following physical properties are commonly tested.

1. Air content
2. fineness
3. Compressive strength
4. Heat of hydration
5. Initial and final setup time
6. solidity
7. consistency

Additionally, the chemical composition of the cement is also checked to ensure it is within limits. The relationship between the lime content and the proportion of silica, aluminum oxide and iron oxide is normally checked. The Cement Testing article provides a more detailed description of cement testing

MINERAL OR SUPPLEMENTAL ADDITIVES
Mineral additives are called cementitious additives (pozzolans). These are finely ground silicate substances which, as such, do not have cement-like properties, but at normal temperatures they chemically react with calcium hydroxide, which is released during the hydration of Portland cement, thus forming substances that are poorly soluble with the cement. . -similar properties. This effect is called the pozzolana effect. These substances are most commonly used to make concrete mixes more economical, reduce permeability, increase strength, or affect other properties of concrete. They can be used individually or in combination with Portland or mixed cement or as a partial additive to Portland cement.

Pozzolanic soil can be divided into two groups

1. Natural pozzolan clay
2. Artificial pozzolan soil

Natural pozzolan includes clay, slate, opal gravel, diatomaceous earth, as well as volcanic tuff and humanite. The most commonly used artificial pozzolans are fly ash, blast furnace slag, silica fume, rice husk ash, metakaolin and surkhi. Pozzolan is added to concrete as an additive or substitute for cement. Pozzolan is generally used as a cement substitute at 10-50%. Pozzolan reduces expansion caused by alkali-aggregate reaction or alkali-silica reactivity in concrete. This expansion can be controlled by adding pozzolans in the range of 5-35% of the cement mass, depending on the type of aggregate and the alkali content of the cement.

Advantages of mineral additives (pozzolans)

1. Improved processability due to less water
2. Lower heat of hydration
3. Improve resistance to salt and sulfate attacks from soil and seawater
4. Reduced susceptibility to dissolution and leaching of calcium hydroxide
5. Reduce permeability
6. Lower costs

The most commonly used pozzolanic material is fly ash compared to other materials. Therefore, it is good to have some knowledge about artificial pozzolans.

FLY ASH

Fly ash or pulverized fuel ash is the residue from the combustion of pulverized coal, which is collected from the combustible gases of thermal power plants by mechanical dust collectors or electrostatic precipitators or separators. Fly ash is also a combination of calcium, aluminum and silicon oxides, just like cement, but contains significantly less calcium oxide.

The particle size of fly ash is in the range of 1-100 microns (0.1 mm) and the average size is about 20 microns, which is the average particle size of Portland cement. Fly ash can be used as a raw material in many cement-based products. Some of the most common uses include pouring concrete, cinder blocks, and building bricks.

There are two different types of fly ash. They are Class F and Class C fly ash. Class F fly ash contains particles coated with a type of molten glass. It is able to resist the risk of expansion and sulfate formation. Class C fly ash also resists expansion caused by chemical attack. Class C fly ash is most commonly used for structural concrete. Typically, Class F fly ash is used in a dosage of 15 to 25 percent of the mass of the cementitious material, while Class C fly ash is used in a dosage of 15 to 40 percent.

Fly ash can be used as an additive or partial substitute for cement. Generally, fly ash is used in the following three ways.

1. Partial replacement of cement. The ideal amount of pozzolan replacement is generally between 10 and 30 percent.
2. Partial replacement of aggregates. Fly ash can be used as a substitute for sand. Although there are positive effects such as initial strength, it is not economical
3. Simultaneous replacement of cement and fine aggregate.

BENEFITS OF FLYING ASH

Fly ash is a good substitute and replacement material for concrete when you compare the pros and cons. In particular, low carbon emissions, which are a prerequisite for sustainable development, can be considered as one of the key factors to take into account when planning the future. The following main benefits can be achieved using fly ash.

1. Different target times are generated
2. Cold resistance
3. High increase in resistance (depending on application)
4. Can be used as an additive
5. Considered a material that does not shrink
6. Improve workability
7. Reduces cravings, permeability and bleeding
8. Reduces heat of hydration
9. The water-cement ratio can be reduced (compared to a mix without fly ash)
10. Reduce CO ₂ Emissions

DISADVANTAGES OF FLYING ASH

Fly ash has some disadvantages. These materials may not be suitable, especially for smaller construction projects. When concrete mixed with fly ash is removed, special regulation is required in the concrete mixing plant. In these cases the costs would be higher. The following factors can be considered as main factors.

1. Slow gain in strength
2. Seasonal restrictions
3. Increased need for air-entraining agents
4. Increase in salt deposition due to higher proportion of fly ash

GROUND BLASTING FURNACE SLAG (GGBS)

It is an industrial waste created during the production of iron and used to improve the properties of concrete. With the addition of GGBS, improvements in processability, strength and durability can be expected.
As in cement, GGBS mainly contains oxides of calcium, silicon, aluminum and magnesium. The content of these materials is comparatively less compared to Portland cement. Particle sizes range from 0.1 to 40 micrometers and specific surface area ranges from 400 to 600 m ² /kg.

GGBS can be added to concrete in the concrete mixing plant. According to studies, GGBS can replace 30 to 85 percent of the weight of cement. However, the proportion is usually in the range of 40 to 50 percent.

The following key aspects can be identified as the main advantages of using GGBS.

1. Increase strength and durability
2. Reduces voids in concrete and therefore reduces permeability
3. Improved processability
4. Low heat of hydration
5. Low temperature rise
6. Avoid cold joints
7. Significantly reduces the risk of damage caused by alkali-silica reactions
8. Provides greater resistance to chloride penetration, reducing the risk of corrosion of reinforcement
9. Offers greater resistance to sulfate attack and other chemicals
10. Makes concrete more chemically stable
11. Does not produce carbon dioxide, sulfur dioxide or nitrogen oxides

There are some disadvantages of GGBS. However, these can be overlooked compared to the list of benefits

1. GGBS cement hardens slower than normal OPC
2. Slow gain in strength

silica

Silica fume is a light-dark gray cementitious material composed of at least 85% ultrafine, amorphous, non-crystalline spherical silica particles. It is a byproduct of the production of silicon metal or the production of ferrosilicon alloys. Due to its chemical and physical properties, it is a very reactive pozzolan. Concrete with silica fume can have very high strength and good durability.

Silica fume is a very thin material, about 1/50th the size of regular Portland cement. The average particle size varies between 0.1 and 0.3 micrometers and the minimum specific surface area is about 15,000 m ² /kg.

The extreme fineness, large surface area and high content of amorphous silicon dioxide give silica fume extremely pozzolanic properties. Reducing bleeding and segregation in fresh concrete and improving the strength and durability of hardened concrete are some of the main benefits of silica fume. Low porosity combined with fineness reduces permeability. Furthermore, the following advantages and disadvantages can be noted.

1. Processability: Due to the fineness of the material, a more cohesive mixture is created. Therefore, there is a greater demand for water. However, this can be reduced by adding a suitable flow agent.
2. Segregation and bleeding: Silica fume significantly reduces bleeding.
3. set time; Adding a small amount (250 – 300 kg/m3) does not have a significant impact on setting time. However, increasing the content may result in a delay in setup time.
4. It can be used with fly ash or blast furnace slag to develop strength at a young age.
5. Longer mixing time required
6. Increasing chloride permeability
7. Increase plastic shrinkage
8. Improves adhesion to steel
9. Significant reduction in alkali-silica reactivity
10. Offers excellent resistance to sulfate and seawater attack
11. Reduce steel corrosion

Other pozzolans are also used in the construction industry. In addition to the artificial pozzolans discussed above, rice husk ash, metakaolin and surkhi are also used in construction works.

REFERENCE

Internet and books