Concrete does not contain pure lime, or calcium hydroxide (\(\text{Ca(OH)}_2\)), as a raw ingredient, but its chemistry is built upon calcium compounds derived from limestone. The binding agent in concrete, Portland cement, is manufactured almost entirely from pulverized limestone, which is calcium carbonate (\(\text{CaCO}_3\)). Concrete is the final composite material, while cement is the fine gray powder that acts as the glue holding the structure together. Lime-based compounds are central to the material’s function, but they are chemically transformed during manufacturing and hardening.
The Essential Ingredients of Concrete
Concrete is a composite material created by mixing three main components: aggregate, water, and Portland cement. The aggregate, which consists of coarse materials like gravel or crushed stone and fine materials like sand, provides the bulk and structural integrity of the final product. Aggregates account for approximately 70% of the concrete’s volume and 80% of its weight.
Portland cement acts as the hydraulic binder, meaning it hardens when combined with water, even underwater. The cement paste, formed by mixing cement and water, coats the surface of the aggregates and fills the spaces between them. This paste is what chemically reacts and solidifies, binding all the materials into the hard, durable mass known as concrete.
The water added to the mixture is a reactant in the setting and hardening process, not simply a vehicle for mixing. The correct proportion of water to cement is important because high water content can decrease the final strength of the concrete. The cement component is the source of the calcium compounds often mistaken for raw lime in the final mixture.
Tracing Lime’s Path to Cement
The primary raw material for Portland cement is limestone, a sedimentary rock rich in calcium carbonate (\(\text{CaCO}_3\)). To create the cement binder, this limestone is quarried, crushed, and then ground into a fine powder. This powder is then mixed with other materials like clay, shale, and iron ore, which provide the necessary silica, alumina, and iron oxides.
The mixture undergoes a high-temperature process called calcination, which is the chemical transformation that brings lime into the picture. In large rotary kilns, the raw materials are heated to temperatures reaching approximately 1,450 degrees Celsius. At this intense heat, the calcium carbonate (\(\text{CaCO}_3\)) thermally decomposes, releasing carbon dioxide (\(\text{CO}_2\)) gas and leaving behind calcium oxide (\(\text{CaO}\)), which is commonly known as quicklime or free lime.
This quicklime then reacts with other components, such as silica and alumina, to form complex calcium silicates and aluminates. These new compounds fuse into small nodules called clinker. The clinker is cooled and ground into the fine gray powder known as Portland cement, often with gypsum added to control the setting time. The resulting cement powder contains calcium-silicate compounds derived from the original limestone, but it is not pure calcium oxide or hydrated lime.
The Chemistry of Concrete Hardening
The final chemical action that hardens concrete is the hydration reaction, which occurs when Portland cement is mixed with water. The calcium silicate compounds in the cement react with the water in a process that generates heat. This reaction forms two main calcium-based products that fill the space between the aggregate particles.
The most significant product is Calcium Silicate Hydrate (CSH) gel, which is primarily responsible for the concrete’s strength and durability. The CSH gel grows and interlocks as the concrete cures, creating a dense, solid matrix that binds the aggregates together. The compounds responsible for early strength development, specifically tricalcium silicate, begin this process rapidly.
The second product formed during the hydration reaction is calcium hydroxide (\(\text{Ca(OH)}_2\)), known as slaked or hydrated lime. This byproduct does not contribute significantly to the material’s strength but provides the concrete with a highly alkaline environment, typically maintaining a pH of 12.5 or higher. This high alkalinity is important for protecting any embedded steel reinforcement from corrosion. The calcium hydroxide created during hydration is the only instance where “lime” is present in the finished, hardened concrete structure.