Industrial Giants: Cement Production and Construction Materials Manufacturing
The unprecedented production volumes of chemical substances around the globe prompted a fresh exploration for the subsequent installment in the "Big Chemistry" series. In this quest, a compelling article was discovered discussing the world's most predominantly produced chemicals. Among these, a surprising front-runner emerged: sulfuric acid.
Produced in quantities of approximately 265 million metric tons annually and 36 million tons in the United States alone, sulfuric acid deemed itself the undisputed champion when it comes to chemical production, mainly due to its extensive uses in fertilizers, metal processing, and various industrial applications.
Nevertheless, a substantial contender emerges if we broaden the discussion to chemically produced materials of colossal consumption: Portland cement – the cornerstone of concrete – reigns supreme, garnering around 4.4 billion tons in annual production, significantly surpassing the production volume of sulfuric acid [5]. Despite not often being recognized as a traditional 'chemical' in industry reports, the manufacturing process of Portland cement invariably entails heating and chemical transformation.
Defining the Basics
For the uninitiated, it's crucial to demarcate the distinction between cement and concrete. Although frequently used interchangeably in layman's terms, cement and concrete signify entirely distinct entities. Concrete refers to the completed building material, while cement merely constitutes the integral glue that adheres gravel and sand together, creating a cohesive mass suitable for construction purposes.
The origins of cement can be traced back to ancient times, with the Romans earning a sizable share of the credit for their innovative use of pozzolana – a silicate-rich volcanic ash – in cement-based construction. Yet, the application of cement predates the Romans, reaching back at least to ancient Egypt, if not further [5].
Although many types of cement exist, the focus here will predominantly revolve around Portland cement, given its nearly exclusive production today. The name "Portland" was bestowed upon it by its inventor, Joseph Aspdin, who appraised the cured substance as resembling the famous limestone from the Isle of Portland off the coast of Dorset in the English Channel.
The initiation of Portland cement manufacturing begins with the extraction of its primary raw material, limestone. Quarrying, crushing, conveying, and storing vast amounts of limestone take center stage in the cement production process, with limestone deposits being relatively abundant and easily accessible due to their prevalence as a sedimentary rock that forms between 20% and 25% of all sedimentary rocks on Earth.
The Turning and Burning Process
A crucial piece of cement production infrastructure, the rotary kiln, is an enormous tube extending up to seven meters in diameter and approximately 80 meters in length, suspended at a slight angle by a series of supports. The slow rotation and slight slope of the kiln foster a gradual migration of raw materials along its length, subjecting them to escalating temperatures.
The rotary kiln's lower end houses a burner, while the exhaust gases exit from the upper end via a riser pipe that channels the heat through a series of preheaters. These preheaters systematically raise the temperature of the raw materials, driving off water in the process.
Upon entering the kiln, the raw materials embark on a journey through various temperature zones, resulting in the decomposition of limestone into calcium oxide (lime) at a temperature of 500 °C. As the heat reaches temperatures between 800 - 1,000 °C, the calcium oxide reacts with silicate to produce corresponding calcium silicate minerals, such as belite and alite.
The Final Product: Clinker
Near the bottom of the kiln, the calcium silicates release water, resulting in the formation of calcium hydroxide. Simultaneously, further reactions occur within the temperature range of 1,400 – 1,600 °C, which transform the belite and alite into more stable calcium silicates. The finished product, clinker – small, gray lumps formed through partial fusion – drops from the bottom of the kiln.
Blistering air then rapidly cools the clinker, reducing its temperature to approximately 100 °C. The exhaust from the cooler re-enters the kiln, preheating the rawmix, while the now-cooled clinker is combined with a minimal amount of gypsum and subjected to grinding in a ball mill. The resulting fine gray powder is either bagged or transported in bulk containers for distribution by road, rail, or cargo ship.
The Cure: The Transformation of Clinker into Cement
Most cement is delivered to concrete plants, which are strategically distributed due to the perishable nature of the products they produce. Concrete plants procure immense quantities of rock, pulverize it accordingly, and stockpile the aggregate until required.
Upon receiving an order for concrete, the plant combines aggregates (gravel and sand), any admixtures required by the customer (such as plasticizers, retarders, accelerants, or reinforcements like chopped fiberglass), and the prescribed amount of cement from storage silos. In some cases, water is added to the mix at this stage; otherwise, the dry mix is loaded into the mixer truck, which adds water later.
Hydration, the process of combining water with the calcium silicates in the cement, commences upon the addition of water, marking the initiation of the cement's cure. Although the initial curing process concludes within 24 hours, the maturation process continues over an extended period, characterized by the concrete exhibiting enhanced strength and durability.
References:[5] International Cement Review (2019). Jobs 2019: The world’s top 10 chemicals by volume. Available online: https://www.cement-review.com/analysis/article/jobs-2019-worlds-top-chemicals-volume
- The manufacturing process of Portland cement, a significant player in the chemical industry, involves its primary raw material, limestone, and the utilization of a rotary kiln, a critical piece of production infrastructure.
- The initiation of hydration, a critical process in cement production, occurs when water is added to the combination of aggregates, admixtures, and cement at a concrete plant, marking the commencement of the cement's cure and the eventual transformation into a durable and versatile building material often used in sports facilities and infrastructure, alongside its role in medical-conditions treatments, lifestyle improvements, and technologically advanced endeavors like aerospace and nuclear engineering.
