Cement

In the wet process the raw materials are made into a slurry with 30 to 40 percent water. This water must be evaporated in the kiln, so the process needs a very long kiln and uses much more fuel (about 5000 to 6000 kJ per kg of clinker, against about 3000 to 3500 kJ in a dry process kiln with a preheater). It is therefore costly, and the dry process, which is more energy-efficient, has replaced it.

Tricalcium aluminate in clinker reacts almost at once with water, which would make the cement set in a few minutes (flash set) and leave no time to mix, place and compact the concrete. Gypsum (3 to 5 percent) reacts with it and forms a coating of insoluble ettringite on the aluminate grains, which slows the setting.

OPC hydrates rapidly and gives out a large amount of heat. In dams, thick piers and foundations the heat cannot escape quickly from the interior, so the inside becomes much hotter than the outside. The difference in temperature causes tensile stress and the concrete cracks.

The pozzolana in PPC reacts with the calcium hydroxide released during hydration and forms extra C-S-H gel:

This makes the concrete denser and less permeable and removes the soluble calcium hydroxide, so sea-water sulphate and chloride ions cannot enter easily and attack the cement or corrode the steel bars.

Definition. Cement is a fine grey powder of calcium silicates, aluminates and ferrites, made from limestone, clay and other minerals, which acts as a binder: when mixed with water it sets and hardens into a hard, stone-like mass that holds sand, bricks and stones together.

Gypsum, \(\mathrm{CaSO_4\cdot2H_2O}\), is added (3 to 5 percent) to the cooled clinker during finish grinding to control the setting time. Without it the tricalcium aluminate would hydrate at once and the cement would set too quickly (flash set) to be used. Gypsum reacts with the aluminate to form ettringite on the grain surface, which delays hydration:

Clinker is the hard, dark grey nodules (small pellets) formed when the raw meal is heated to 1400 to 1500 °C in a rotary kiln. It consists of tricalcium silicate (C₃S), dicalcium silicate (C₂S), tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF).

Use. Clinker is the intermediate product of cement. It is cooled and ground with 3 to 5 percent gypsum to give Portland cement, and with gypsum and pozzolana to give PPC.

The production of cement involves four steps:

The flow sheet below shows the stages from the raw materials to the finished cement.

Merits.

Demerits.

Pozzolanic materials (fly ash, volcanic ash, slag) contain reactive silica and alumina. They have no binding property themselves, but at ordinary temperature they combine with the calcium hydroxide set free during hydration of OPC and give more of the strong, stable C-S-H gel:

Draw the flow sheet diagram given in this page on the chart paper and write the public and private industries of Nepal (Hetauda, Udaypur, Himal, Arghakhanchi, Sagarmatha, Maruti, Jagdamba, Hongshi Shivam and others).

The production of Portland cement involves the following stages:

The dry process is the most common method today because it saves energy. Its steps are shown in the flow sheet.

In the wet process the raw materials are mixed with water to make a slurry. It blends well but needs more energy to evaporate the water. The flow sheet is:

Growth. The cement sector of Nepal has grown greatly, in production capacity and in exports. The total yearly production capacity is about 22 million tons, more than 50 cement companies are in operation, supporting about Rs 300 billion of investment and about 15,000 jobs. Exports of cement and clinker rose to Rs 3.85 billion in the fiscal year 2023/24, almost three times that of the year before, helped by an 8 percent cash incentive for exporting mine-based products and power tariff waivers of 1 to 15 percent for high-consuming manufacturers.

Demand. Large infrastructure projects are expected to raise the demand to about 26 million tons a year by 2024/25. Actual output has lagged behind the capacity, so some imports were needed, but Nepal is becoming more self-sufficient and a possible exporter in the regional market.

Industries.

Challenges. Energy shortage, outdated equipment, high cost of imported coal and clinker, regulatory barriers, export restrictions (especially with India), poor infrastructure, oversupply of capacity, labour shortage, environmental impact and financial instability.

Prospects. Growing demand from roads, housing and hydropower projects, together with modern dry-process plants and more PPC, can make the industry stronger and more eco-friendly.