Cement and refractory's

By Inderjit Singh

Cement and Refractory:

(Syllabus)

Introduction, Types of Portland cement, BIS specification of cement Raw materials Manufacture, Reactions in the Kiln, Mixing of Additives ,Introduction to various parameters for testing of cement ,Brief introduction of acid resistant cement ,waterproof cement ,white cement

Introduction

Cement may be described as a material possessing adhesive and cohesive properties and capable of binding materials like stones, bricks, building block etc.The principal elemental composition of cement used for constructional purposes are Ca, Al and Si.

Cement have the property of setting and hardening under water by virtue of certain chemical reaction.

Portland Cement

First invented in 1824 by Joseph Aspdin. It resembles stones found near Portland.It is greenish –grey colored extremely finely ground powder which turned into solid material on reaction with water. It si obtained by calcining an intimate & properly proportional mixture of clay and limestone at 1500oC followed by addition of gypsum as additive.It is most important and reliable cementing material used for construction work.

PORTLAND CEMENT:

• PORTLAND CEMENT A cement is a binder, • a substance that sets and hardens

independently, and can bind other materials together.

• Material with adhesive and cohesive properties

• Any material that binds or unites essentially like glue

Cement Chemical Composition

• Cement Chemical Composition Cement Compound Weight Percentage Chemical Formula Tricalcium silicate 50 % Ca 3 SiO 5 or 3CaO . SiO 2 Dicalcium silicate 25 % Ca 2 SiO 4 or 2CaO . SiO 2 Tricalcium aluminate 10 % Ca 3 Al 2 O 6 or 3CaO . Al 2 O 3 Tetracalcium aluminoferrite 10 % Ca 4 Al 2 Fe 2 O 10 or 4CaO . Al 2 O 3 . Fe 2 O 3 Gypsum 5 % CaSO 4 . 2H 2 O

Functions of the ingredients of cements:

• Functions of the ingredients of cements Lime is the principal constituent of cement. Its proportion must be properly regulated. However excess of lime reduce the strength of cement . On the other hand presence of lesser amount of lime than required also reduce the strength of cement and make it quick setting. Silica imparts strength to cement. Alumina makes the cement quick setting. (due to excess weakness of cement increased) Gypsum helps to retard the setting action of cement. It actually enhance the initial setting time of cement. Iron oxide provides color, strength and hardness to the cement

PRODUCTION STEPS:

• PRODUCTION STEPS Raw materials are crushed, screemed & stockpiled. Raw materials are mixed with definite proportions to obtain “raw mix”. They are mixed either dry (dry mixing) or by water (wet mixing). Prepared raw mix is fed into the rotary kiln. As the materials pass through the kiln their temperature is rised upto 1300-1600 ° C. The process of heating is named as “burning”. The output is known as “clinker” which is 0.15-5 cm in diameter . Clinker is cooled & stored. Clinker is ground with gypsum (3-6%) to adjust setting time. Packing & marketting.

REACTIONS IN THE KILN:

• REACTIONS IN THE KILN ~ 100 ° C→ free water evaporates. ~ 150-350C °→ loosely bound water is lost from clay. ~ 350-650 ° C → decomposition of clay→ SiO 2 & Al 2 O 3 ~ 600 ° C → decomposition of MgCO 3 → MgO & CO 2 (evaporates) ~ 900 ° C → decomposition of CaCO 3 → CaO& CO 2 (evaporates

SETTING AND HARDENING OF CEMENT:

• SETTING AND HARDENING OF CEMENT INITIAL SETTING : Hydration of C 3 A, C 2 S, C 3 S Gel Formation of C 4 AF 3CaO.Al 2 O 3 + 6 H 2 O 3CaO.Al 2 O 3 .6H 2 O 2CaO.SiO 2 + 4 H 2 O 2CaO.SiO 2 .4H 2 O 3CaO.SiO 2 + 6 H 2 O 3CaO.SiO 2 .6H 2 O 4CaO.Al 2 O 3 .Fe 2 O 3 + 7 H 2 O 3CaO.Al 2 O 3 .6H 2 O + CaO.Fe 2 O 3 .H 2 O FINAL SETTING AND HARDENING : Hydrolysis of C 3 S, C 2 S 2[3CaO.SiO 2 ] + 6H 2 O 3CaO.2SiO 2 .3H 2 O + 3Ca(OH) 2 2[2CaO.SiO 2 ] + 4H 2 O 3CaO.2SiO 2 .6H 2 O + Ca(OH) 2 When cement is mixed with water to a plastic mass called cement paste, hydration reaction begin, resulting in the formation of gel and crystalline products

ROLE OF CEMENTING MATERIALS

• ROLE OF CEMENTING MATERIALS 1) GYPSUM : INITIAL SETTING TIME RETARDER C 3 A + 6H 2 O C 3 A.6H 2 O + HEAT C 3 A + x H 2 O + y CaSO 4 . z H 2 O C 3 A.yCaSO 4 .z H 2 O Insoluble calcium sulphoaluminate Quick hardening property Gypsum 2) Silica / CaO: Imparts srengths 3) Alumina : Imparts strengths, Makes the cement quick setting 4) Fe 2 O 3 : Imparts Grey color , strength and hardness 5) SO 3 : Imparts soundness 6) Alkalies (MgO, Na 2 O, K 2 O): Lower the clinkering temperature

Refractories

Firebricks for furnaces and ovens. Have high Silicon or Aluminium oxide content.Brick products are used in the manufacturing plant for iron and steel, non-ferrous metals, glass, cements, ceramics, energy conversion, petroleum, and chemical industries.

Refractories

• Used to provide thermal protection of other materials in very high temperature applications, such as steel making (Tm=1500°C), metal foundry operations, etc.

• They are usually composed of alumina (Tm=2050°C) and silica along with other oxides: MgO (Tm=2850°C), Fe2O3, TiO2, etc., and have intrinsic porosity typically greater than 10% by volume.

• Specialized refractories, (those already mentioned) and BeO, ZrO2, mullite, SiC, and graphite with low porosity are also used.

Refractory Brick

• Any material can be described as a ‘refractory,’ if it can withstand the action of abrasive or corrosive solids, liquids or gases at high temperatures. The various combinations of operating conditions in which refractories are used, make it necessary to manufacture a range of refractory materials with different properties. Refractory materials are made in varying combinations and shapes depending on their applications. General requirements of a refractory material are:

• Withstand high temperatures• Withstand sudden changes of temperatures• Withstand action of molten metal slag, glass, hot gases, etc• Withstand load at service conditions• Withstand load and abrasive forces• Conserve heat• Have low coefficient of thermal expansion• Should not contaminate the material with which it comes into contact

• Fireclay Refractories• Firebrick is the most common form of refractory material. It is

used extensively in the iron and steel industry, nonferrous metallurgy, glass industry, pottery kilns, cement industry, and many others.

• Fireclay refractories, such as firebricks, siliceous fireclays and aluminous clay refractories consist of aluminum silicates with varying silica (SiO2) content of up to 78 percent and Al2O3 content of up to 44 percent.

• The table shows that the melting point (PCE) of fireclay brick decreases with increasing impurity and decreasing Al2O3. This material is often used in furnaces, kilns and stoves because the materials are widely available and relatively inexpensive.

• High alumina refractories • Alumina silicate refractories containing more than 45

percent alumina are generally termed as high alumina materials. The alumina concentration ranges from 45 to 100 percent. The refractoriness of high alumina refractories increases with increase in alumina percentage. The applications of high alumina refractories include the hearth and shaft of blast furnaces, ceramic kilns, cement kilns, glass tanks and crucibles for melting a wide range of metals.

• Silica brick • Silica brick (or Dinas) is a refractory that contains at least 93 percent

SiO2. The raw material is quality rocks. • Various grades of silica brick have found extensive use in the iron and

steel melting furnaces and the glass industry. • Advantages are

• The outstanding property of silica brick is that it does not begin to soften under high loads until its fusion point is approached. This behavior contrasts with that of many other refractories, for example alumina silicate materials, which begin to fuse and creep at temperatures considerably lower than their fusion points.

• High resistance to thermal shock (spalling)• High refractoriness. • Flux and slag resistance• Volume stability

• Magnesite • Magnesite refractories are chemically basic materials,

containing at least 85 percent magnesium oxide. They are made from naturally occurring magnesite (MgCO3).

• The properties of magnesite refractories depend on the concentration of silicate bond at the operating temperatures. Good quality magnesite usually results from a CaO-SiO2 ratio of less than two with a minimum ferrite concentration, particularly if the furnaces lined with the refractory operate in oxidizing and reducing conditions.

• The slag resistance is very high particularly to lime and iron rich slags.

• Chromite refractories • Two types of chromite refractories are distinguished:• Chrome-magnesite refractories, which usually contain 15-35

percent Cr2O3 and 42-50 percent MgO. They are made in a wide range of qualities and are used for building the critical parts of high temperature furnaces. These materials can withstand corrosive slags and gases and have high refractoriness.

• Magnesite-chromite refractories, which contain at least 60 percent MgO and 8-18 percent Cr2O3. They are suitable for service at the highest temperatures and for contact with the most basic slags used in steel melting. Magnesite-chromite usually has a better spalling resistance than chrome-magnesite.

• Zirconia refractories • Zirconium dioxide (ZrO2) is a polymorphic material. • It is essential to stabilize it before application as a refractory, which is achieved

by incorporating small quantities of calcium, magnesium and cerium oxide, etc. Its properties depend mainly on the degree of stabilization, quantity of stabilizer and quality of the original raw material.

• Zirconia refractories have a very high strength at room temperature, which is maintained up to temperatures as high as 1500 oC. They are therefore useful as high temperature construction materials in furnaces and kilns.

• The thermal conductivity of zirconium dioxide is much lower than that of most other refractories and the material is therefore used as a high temperature insulating refractory.

• Zirconia exhibits very low thermal losses and does not react readily with liquid metals, and is particularly useful for making refractory crucibles and other vessels for metallurgical purposes. Glass furnaces use zirconia because it is not easily wetted by molten glasses and does not react easily with glass.

• Oxide refractories (Alumina) • Alumina refractory materials that consist of aluminium oxide

with little traces of impurities are known as pure alumina. • Alumina is one of the most chemically stable oxides known. It is

mechanically very strong, insoluble in water, super heated steam, and most inorganic acids and alkalies.

• Its properties make it suitable for the shaping of crucibles for fusing sodium carbonate, sodium hydroxide and sodium peroxide.

• It has a high resistance in oxidizing and reducing atmosphere. Alumina is extensively used in heat processing industries. Highly porous alumina is used for lining furnaces operating up to 1850oC

• Monolithics • Monolithic refractories are single piece casts in the shape of equipment, such as a ladle

as shown in the figure. They are rapidly replacing the conventional type fired refractories in many applications including industrial furnaces. The main advantages of monolithics are:

• Elimination of joints which is an inherent weakness• Faster application method• Special skill for installation not required • Ease of transportation and handling • Better scope to reduce downtime for repairs• Considerable scope to reduce inventory and eliminate special shapes • Heat savings• Better spalling resistance • Greater volume stability • Monolithics are put into place using various methods, such as ramming, casting,

gunniting, spraying, and sand slinging.