CEMENT CHEMISTRY

Cement is mainly of two kinds depending on the way it is set and hardened. Hydraulic cement hardens due to the addition of water, while non-hydraulic cement hardens by carbonation with the carbon present in the air. This is the reason that the non-hydraulic cement cannot be used underwater.

The following steps (lime cycle) are followed to produce Non-hydraulic cement:

1. Calcinations: Lime is produced from limestone at over 825°C for about 10 hours. (CaCO3 → CaO + CO2)
2. Slaking: Calcium oxide is mixed with water to make slaked lime. (CaO + H2O → Ca(OH)2)
3. Setting: Water is completely evaporated.
4. The cement is exposed to dry air and it hardens after time-consuming reactions. (Ca(OH)2 + CO2 → CaCO3 + H2O)

On the other hand, hydraulic cement is mainly made up of only silicates and oxides:

1. Belite (2CaO·SiO2);
2. Alite (3CaO·SiO2);
3. Tricalcium aluminate/ Celite (3CaO·Al2O3)
4. Brownmillerite (4CaO·Al2O3·Fe2O3)

The ingredients are processed in cement plants. The chemistry of the reactions is still a subject of research.

EXTRACTION AND PROCESSING

Raw materials are extracted with the aid of blasting when necessary. Extraction done of hard rock’s such as limestone, slates, and some shale for the manufacture of cement. Some minerals are mined by underground methods too. Softer rocks such as clay and chalk can dig directly by excavators. Now these materials are transported to the crushing plant by trucks, conveyor belts, railway freight cars, or ropeways. They also can be transferred in a wet state or semi-liquid state by pipeline

MANUFACTURING OF CEMENT

There are four stages in the manufacture of mostly used Portland cement:

(1) Crushing and grinding the raw materials, 

 (2) Blending the materials in the correct proportions, 

 (3) Burning the prepared mix in a kiln, and 

 (4) Grinding the burned product, known as “clinker”. This clinker is added with some 5% of gypsum (this helps to control the time of set of the cement). 

CRUSHING AND GRINDING

All materials except soft ones are first crushed, mostly  in two stages, and then ground, usually in a cylindrical ball, rotating or tube mills containing a charge of steel grinding balls. This grinding process is done either wet or dry, depending on the process. For dry grinding the raw materials first need to be dried in cylindrical, rotary dryers. Soft materials are broken down by intense stirring with water in wash mills, resulting in fine slurry, which is passed through screens to discard oversize particles. 

BLENDING

After the crushing and grinding, the chemical composition required for particular cement is obtained by selective quarrying. In this process the control of the raw material is essential.  Finer control is attained by collecting material from two or more batches. In the dry process these mixtures are stored in silos. Slurry tanks are used for the wet processes. Vigorous circulation induced by compressed air for thorough mixing of the dry materials in the silos. In case of the wet process, mechanical means or compressed air or both can be used for stirring in the slurry tanks. 

BURNING

The Initial kilns where cement was burned in batches were bottle kilns. Secondly it is followed by chamber kilns and at last by continuous shaft kilns. The shaft kiln is still used in some countries in a modernized form.  Predominantly the burning has been done by the rotary kiln. For the wet process plants these kilns measure up to 200 meters (660 feet) long and six meters in diameter. But the kiln is shorter for the dry process. 

These kilns are made up of steel, cylindrical shells lined with refractory materials. The raw material feeds, or poured at the upper end, which moves slowly down the lower end of the kiln. Pulverized coal, oil, or natural gas is used as the fuel which is injected through a pipe. The temperature ranges from about 1,350 to 1,550 °C (2,460 to 2,820 °F) at the firing end. Temperature depends on the raw materials which are being burned. The burned product emerges as small nodules of clinker from the kiln. These clinkers pass into coolers to be cooled. The clinkers are then instantly ground to cement or stored in stockpiles for later use.

For control of the burning process now-a-days modern cement plants are well equipped with elaborate instrumentation. Raw materials in some plants are sampled automatically, and a computer calculates and controls the raw materials mix composition. The largest rotary kilns have output more than 5,000 tons per day.

GRINDING

Grinding done in horizontal mills to a fine powder of the clinker and the required amount of gypsum. The materials pass straight through the mill (open-circuit grinding). The coarser material separated from the ground product and returned to the grinding mill for further grinding (closed-circuit grinding). 

CEMENT TESTING

In national cement specifications, various tests are laid down to control the fineness, soundness, setting time, and strength of the cement. These tests are described below.

FINENESS

Previously the fineness was controlled by sieve tests. But now more sophisticated methods are broadly used on the Blaine Air permeability apparatus. The fineness test of the cement is done to determine the particle size of the cement. As the rule of thumb, the finer the cement particle is the best quality cement and on the other hand the coarser the particle is not considered to be that good.

SOUNDNESS

Soundness of cement means the ability to resist volume expansion. The testing of the soundness of cement is very important to ensure that the cement should not show any subsequent expansion, which determines the quality of the cement. The unsoundness of the cement occurs mainly due to adding an excess lime with acidic oxide at the kiln. The soundness of cement is determined by two methods. These two methods are the Le-Chatelier method and the autoclave method. In the soundness test a specimen of hardened cement paste is boiled for a fixed time so that if there is any tendency to expand, it can be detected. Once it sets, cement must not undergo any appreciable expansion, which could disturb a mortar or concrete. 

SETTING TIME

Setting time is essential for cement to determine its quality. It should not set too rapidly or too slowly. The setting and hardening of cement is a continuous process, but two setting points are noted for test purposes The Initial setting time (the interval between the mixing of the cement with water) of cement is when the cement paste starts losing its plasticity and final setting time (the set cement has acquired a sufficient firmness to resist a certain defined pressure) of cement is when the cement paste completely loses its plasticity. For OPC, usually the initial minimum setting time value at ordinary temperatures is 30 to 45 minutes and for final setting time is no more than 10 to 12 hours. The device which is used to determine the initial setting time of cement is called a Vicat apparatus. 

STRENGTH

The tests to measure the strength of the cement are usually made on a mortar common mixture of one part cement to three parts sand with a defined quantity of water. For masonry cement, compressive strength is ranging between 33 – 53MPa (megapascals) at 28 days, 22- 27 MPa (megapascals) at 7days and 16 -27MPa (megapascals) at 3days of curing after casting. Compressive strength of ordinary Portland cement (OPC) represented as OPC33, OPC43 & OPC53. Compressive strength test, measures the maximum amount of compressive load a material can bear before fracturing. The test piece, usually in the form of a prism, cube or cylinder is compressed between the platens of a compression-testing machine by a gradually applied load. In this type of testing of cements, a minimum strength at 3 and 7 days and sometimes 28 days is specified. In case of a rapid-hardening Portland cement test 1 day is required. For high-alumina cement, tests 1 and 3 days are required.