Chemical Composition of Cement

Chemical Composition of Cement

Chemical composition of cement: The raw materials used for the manufacture of cement mainly include lime, silica, alumina and iron oxide. These oxides interact with each other in the kiln at high temperatures to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing various properties of cement; In addition to cooling rate and grinding fineness. Estimated oxide composition limit of ordinary Portland cement.

Oxide	Per cent content
CaO	60-67
SiO2	17–25
Al2O3	3.0–8.0
Fe2O3	0.5–6.0
MgO	0.1–4.0
Alkalies ( K2O, Na2O)	0.4–1.3
SO3	1.3–3.0

Identification of the major compounds of cement is largely based on Bogie's equations and is therefore called "bogus compounds". Four compounds are generally considered as the major compounds.

Name of Compound	Formula	Abbreviated Formula
Tricalcium silicate	3 CaO.SiO2	C3S
Dicalcium silicate	2 CaO.SiO2	C2S
Tricalcium aluminate	3 CaO.Al2O3	C3A
Tetracalcium aluminoferrite	4 CaO.Al2O3.Fe2O3	C4AF

It should be noted that for simplicity brief notation is used. C means CaO, S means SiO2, Al2O3 for A, Fe2O3 for F, and Hbfor H2O. The equations suggested by Bogue for calculating the percentage of major compounds are given below.

C3S = 4.07 (CaO) - 7.60 (SiO2) - 6.72 (Al2O3) - 1.43 (Fe2O3) - 2.85 (SO3)

C2S = 2.87 (SiO2) - 0.754 (3CaO.SiO2)

C3A = 2.65 (Al2O3) - 1.69 (Fe2O3)

C4AF = 3.04 (Fe2O3)

In addition to the four major compounds, there are several smaller compounds in the kiln. Two small oxides referred to as alkaline and Na2O in cement are of some importance and expressed in terms of Na2O. These alkalis basically react with activated silica in aggregates and produce unlimited swelling type alkali-silica gel under favorable conditions of moisture and temperature in voids and crevices and further, it causes disruption and pattern cracking. is.

Tricellium silicate and disilium silicate are the most important compounds responsible for strength. They consist of 70 to 80 percent cement. The average C3S content in modern cement is about 45 percent and the C2S content is about 25 percent.

The calculated amount of compounds in cement also varies for relatively small changes in the oxide composition of raw materials. To manufacture cement of a determined compound composition, it becomes necessary to closely control the oxide composition of the raw material.

SO3 also appears in cement analysis which comes from adding gypsum (4-6)% during clinker grinding. The Iraqi and British specification for common high rapid Portland cement stated that SO3 content should be between (3–2.5)% according to the type of cement and C3A content.

The percentage of MgO in cement that comes from magnesia compounds in raw materials. The maximum extent to control the expansion from hydration of this oxide in hardened concrete is approximately (4–1)% and 5%.

Increasing the lime CaO content beyond a certain value makes it difficult to combine with other compounds and free lime will be present in the clinker which causes uneasiness in the cement.

The insoluble residue is a part of cement non-soluble in hydrochloric acid HCl and arises from non-activated silica mainly for cement compounds dissolved in this acid hence it expresses the completeness of chemical reactions inside the rotary kiln.

loss on ignition

Loss on ignition is calculated by heating the cement sample to 900 - 1000 ° C until a constant weight is achieved. The weight of the sample is reduced due to heating. A high loss on ignition may indicate hydration and carbonation, which may be due to improper and prolonged storage. Iraqi specification limits are 4% for the normal and rapid hardening of Portland cement.

Cement Setting and Hardening

Setting process: The process of losing plasticity over time and becoming dense, which is made by mixing cement and water. This occurred in the initial setting and the final setting of two stages.

Hardening process: The process of making hard cement paste, which loses elasticity, and its strength increases over time.

Factors such as fineness and type of cement, mixture of gypsum, age, water content, temperature, and humidity affect the setting and hardening of Portland cement.

In setting concrete work it is important to keep fresh concrete plastics for sufficient time which helps in the processes of transport, molding and compaction.

There are four stages during the installation of cement, first: when mixing the cement with water, rapid heat development, lasting for a few minutes, this heat development is probably due to the reaction of the cement compounds, the heat. Development stops quickly. Second: This phase is called the "dormant period" and lasts for hours (1–4). Additionally, cement particles begin to form an initial layer of hydration products. Bleeding and sedimentation are seen at this stage. Third: The evolution of the next heat is due to the dissolution of the weak gel that was previously formed on the surfaces of C3S crystals so water will reach these surfaces and form a new gel. This phase lasts for about 6 hours. Forth: In this phase, cement hardens and begins to gain strength.

False and flash settings

False setting, freshly mixed Portland cement paste, is the rapid development of hardness in the mortar, or occurs a few minutes after mixing the cement with water without too much heat to the concrete. This can be controlled by re-mixing without additional water or loss of strength. The reasons for the false setting are:

1. Drying of Gypsum: When gypsum (CaSO4.2H2O) is gridding with hot clinker it damages 75% of its water and formation (CaSO4.1 / 2 H2O) and if the temperature of the clinker loses the gypsum Is, all the water in its structure and construction is destroyed. CaSO4. CaSO4 reactions cause rapid setting when water is added to the cement.

2. Poor storage: Alkalis in cement react with carbon dioxide to form alkali carbonate, which reacts with calcium hydroxide to form CaCO3 which leads to the establishment of cement paste.

3. Activate effective C3A exposed to humidity: During poor storage, water adheres to the surface of cement particles and during rapid mixing of these active surfaces with water within minutes.

Flash setting is the rapid development of hardness in freshly mixed Portland - cement paste, mortar, or concrete. Further mixing cannot overcome this hardness, and a large amount of heat is generated in this process. This was caused by the rapid reaction of aluminates - when insufficient sulfate is present to prevent this, a small amount of gypsum (CaSO4 · 2H2O) with a cement clinker retains the hydration reaction of tridinium aluminate so that calcium silicate is first set. Can be

Sound of cement

This refers to the ability of a hardened cement paste to maintain its volume after setting without a destructive expansion. This destructive expansion is caused by excessive amounts of lime (CaO), (SO3) or magnesia (MgO).

The hydrating speed of burnt CaO or MgO is slow, CaO or MgO cement starts to hydrate after hardening and causes expansion and cracking of the hardened cement.

When added in very high amounts, the gypsum continues to react with calcium aluminate hydrate to form calcium sulfolate hydrate, the amount of which increases 1.5-fold compared to gypsum and causes the hardened cement paste to crack.