Calcium oxide, a common chemical compound that appears as a white crystalline solid at room temperature, is easily attainable through the thermal decomposition of limestone. This was how the material, also known as quicklime, was acquired for human use, after ancient peoples burned limestone in a kiln to take advantage of its structural properties.
The name quicklime is fitting in many ways, since the substance forms rapidly after heating limestone and reverts back to its original state if it remains exposed to air. Similarly, if calcium oxide is mixed with water, it becomes slaked lime, a viscous, slurry material highly suitable for mortar applications. Lime plastering can be found in in the Egyptian pyramids, which are also composed of limestone blocks. Millennia after the pyramids were erected, the Roman Empire extensively used lime-based mortars in its architecture. There is even lime in the Great Wall of China.
Calcium Oxide in Warfare
As time progressed, the peaceful use of quicklime remained, but the chemical found a new application that satisfied a different goal. Evidence suggests that armies during late BCE hurled calcium oxide at their enemies. Generally thrown by hand, it didn’t even matter if the vessel carrying the quicklime directly hit its target, since the powdery substance could spread rapidly through the force of the wind.
The calcium oxide wasn’t really a form of direct assault, but an advanced smokescreen that would significantly stun the opposing forces. Upon exposure, many soldiers underwent choking and even suffocation, and their eyes were especially susceptible to the forces of calcium oxide. The moisture in the eyes would turn the quicklime into slaked lime upon contact, and the resulting mortar material in the soldiers’ eyes would practically blind them.
During the Middle Ages, many took advantage of this weapon, using it against naval forces. Carried by the speed and distance of a catapult, trebuchet, or some other kind of ballista, calcium oxide could easily affect every crewmember of a ship. This was detailed by Thirteenth Century writer Giles of Rome:
“there used to be a large number of pots filled with ground quicklime, which are to be thrown from aloft into the enemies’ ships. When the pots are thrown with force and shatter on impact, the powder rises in the air and enters the enemies’ eyes and irritates them so greatly that, nearly blinded, they cannot see. This situation is very dangerous in naval warfare because fighting men in such war see themselves threatened with death from every quarter. Wherefore, if the eyes of the fighting men in such a battle are so irritated by powdered lime that they cannot see, they can easily either be slain by their enemies or submerged in the water.”
Another dreadful maritime war application of calcium oxide might have been its inclusion in the secret recipe of Greek Fire, a mysterious substance still not entirely understood. Greek Fire was an incendiary mixture used by Eastern Roman Emperors to face against naval power, and it first appeared in the Seventh Century CE to defend Constantinople against an invading fleet. This material was basically liquid fire; it was hurled at enemies through siphons, burning on contact. As it was inextinguishable, the chemical fireball spread quickly, even continuing to burn on water (and it should sound familiar to Game of Thrones fans).
Calcium Oxide in Industry Today
Today, the primary uses of calcium oxide differ from its history in warfare and more closely resemble its original application. Like in the pyramids and the Great Wall, quicklime is still a strong component of different materials. Lime is a common chemical used in iron and steel manufacturing, acting as a flux in purifying steel in electric arc furnaces (EAF) and basic oxygen furnaces (BOF). In addition to removing impurities, calcium oxide reduces refractory wear and gunning, and can provide a foaming slag for long arc operation.
As for structural purposes, as stated in ASTM C5-18 – Standard Specification for Quicklime for Structural Purposes, “quicklime can never be used as such for structural purposes; it must always be slaked first.” Because the purity and quality of slaked lime is greatly dependent on how it is prepared from quicklime, this process needs to be conducted properly, with a speed that depends on the specific lime used. The resulting slaked lime is used for mortar, plaster, and cement in buildings and other structures.
Calcium oxide (and calcium hydroxide) is also an important chemical for raising the pH of potable water and wastewater during its treatment. However, there are different methods used to utilize quicklime during the different stages of water softening, neutralization, and stabilization, so it is important to adhere to the types of lime recommended by ASTM C1529-19 – Standard Specification for Quicklime, Hydrated Lime, and Limestone for Environmental Uses. In addition, the amount of quicklime added to water varies by country due to federal regulations. In the United States, the use of quicklime and hydrated lime in the water supply service is standardized by AWWA B202-2019 – Quicklime And Hydrated Lime.
Quicklime is also an important chemical for interacting with soil. One aspect of this is agricultural lime, which is added to crops to provide nutrients and control the pH for plants to easily absorb nutrients readily available in the soil. Another aspect is soil stabilization, a process by which quicklime and hydrated lime are added to soils to render them suitable for load-bearing applications, such as highway construction. Guidance for quicklime soil stabilization is addressed in ASTM C977-18 – Standard Specification for Quicklime and Hydrated Lime for Soil Stabilization.
Quicklime and slaked lime also play a significant role in processes found in the pulp and paper industry. For the many uses of these chemicals, ASTM C110-20 offers standard test methods for Physical Testing of Quicklime, Hydrated Lime, and Limestone.