Lime as a Building Material in the 18th Century

Lime has long been a fundamental component in construction and masonry, valued for its versatility and durability. It is distinguished by its characteristic white color and caustic properties, producing a sharp sensation on the tongue. Lime is one of the most infusible substances in nature, resisting fusion under ordinary temperatures. Though only slightly soluble in water, it dissolves more readily in cold water than in hot.

 

Natural Occurrence and Composition

In nature, lime is seldom found in a pure state but is most commonly encountered in combination with acids, particularly carbonic acid. It frequently appears as calcium carbonate in various forms such as limestone, chalk, and marble. Additionally, lime is a major component of the Earth’s crust and occurs in both sulfate and carbonate forms as a constituent of numerous mineral substances.

 

To obtain pure lime, commonly referred to as quicklime (calcium oxide, CaO), raw mineral sources such as limestone, chalk, or oyster shells must be subjected to a process known as calcination. This involves heating the material to high temperatures, typically between 900°C and 1,000°C (1,650°F–1,800°F), to drive off carbon dioxide (CO₂), leaving behind the highly reactive quicklime. When water is added to quicklime, it undergoes an exothermic reaction, producing slaked lime (calcium hydroxide, Ca(OH)₂), which is then used in construction.

 

Lime in Construction and Mortar Production

During the 18th century, lime played a crucial role in masonry, serving as the primary binding agent in mortar, plaster, and whitewash. Builders recognized that not all limestone deposits produced lime with equal cementitious properties. The quality of lime depended on the presence and proportion of various impurities, such as iron, silica, and magnesium, which influenced its performance.

• Iron content: Limestones rich in iron compounds exhibited a deep brown or red coloration. When burned, they produced a lime with a yellowish hue.
• Silica content: If the limestone was hard enough to scratch glass and did not readily effervesce in acid, it contained significant silica, affecting its setting properties.
• Magnesium content: Limestones that effervesced slowly in acid and turned the solution milky indicated the presence of magnesium, producing dolomitic lime, which had distinct properties compared to high-calcium lime.
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The chemical setting process of lime mortar was understood even in early construction practices. Initially, slaked lime mixed with sand and water formed a plastic, workable paste. Over time, exposure to air allowed the mortar to reabsorb carbon dioxide, gradually converting the calcium hydroxide back into calcium carbonate in a process known as carbonation. This chemical transformation increased the mortar’s strength, often surpassing that of the stones it bonded.

 

Types of Lime-Based Binders

By the 18th century, builders distinguished between two main types of lime-based binding materials:

1. Air Lime Mortar:
• Composed of slaked lime and sand.
• Sets slowly by absorbing carbon dioxide from the air.
• Used primarily for above-ground masonry, where prolonged exposure to air allowed for gradual hardening.
2. Hydraulic Lime and Water Cement:
• Contained naturally occurring clay or silica impurities, enabling it to set under water.
• These materials were crucial for marine construction, bridge building, and damp environments where standard lime mortar would weaken.
• The properties of hydraulic lime anticipated the development of modern Portland cement in the 19th century.

Sir Humphry Davy, an early 19th-century chemist, noted that lime mortars function by forming carbonates over time, strengthening their bond through continued exposure to atmospheric carbon dioxide. He observed that lime mortars could regain over 60% of their original carbonic acid content within a few years, illustrating their long-term durability.

 

Conclusion

Lime was an indispensable building material in the early 18th century, forming the foundation of masonry construction. Its chemical properties, influenced by natural impurities and calcination processes, determined its effectiveness in different applications. The distinction between air lime and hydraulic lime foreshadowed advancements in cement technology, paving the way for the durable building materials of later centuries.

 

Sources and References

• Davy, Humphry. Elements of Agricultural Chemistry in a Course of Lectures. Longman, 1813.
• Cowper, A. D. Lime and Lime Mortars. Stationery Office, 1927.
• Taylor, F. W. The Chemistry of Cements. Academic Press, 1964.
• Ashurst, John, and Nicola Ashurst. Practical Building Conservation: Mortars, Plasters, and Renders. English Heritage, 2011.
• Schofield, Jane. Lime in Building: A Practical Guide. The Building Conservation Directory, 2003.