High-calcium hydrated lime, or calcium hydroxide (\(\text{Ca}(\text{OH})_2\)), is a versatile white powder with broad industrial utility. It is produced by adding water to quicklime (calcium oxide), which is derived from heating limestone. The resulting fine, dry powder is highly alkaline, typically reaching a \(\text{pH}\) of around 12.4 in solution. Its low cost and strong chemical reactivity make it an effective reagent, allowing it to act as a modifier, binder, or neutralizer in numerous large-scale processes.
Essential Role in Construction and Masonry
Hydrated lime improves the physical characteristics of cement-based mixtures used in mortar, plaster, and stucco. The fine, smooth powder acts as a plasticizer, significantly enhancing the workability of the mix and making it easier to spread and shape during construction.
The addition of calcium hydroxide also increases the water retention of the mortar, which is important for proper curing. By holding moisture longer, the lime ensures that the Portland cement fully hydrates, leading to a stronger final product and a better bond with masonry units.
Lime-based mortars provide a unique self-healing capability known as autogenous healing. When micro-cracks form, rainwater dissolves free calcium hydroxide, which is deposited in the crack as the water evaporates. This dissolved lime reacts with atmospheric carbon dioxide (\(\text{CO}_2\)) through carbonation, reforming calcium carbonate (\(\text{CaCO}_3\)).
This reaction fills and seals the micro-cracks over time, preventing moisture penetration and prolonging the structure’s life. Carbonation also allows wall systems to “breathe,” permitting trapped moisture to escape as vapor. This breathability prevents the buildup of dampness and internal pressure that can damage less permeable materials.
Environmental Applications for Water and Air Quality
The strong alkaline nature of hydrated lime is widely leveraged in environmental protection and treatment processes. In water treatment, it neutralizes acidic wastewater streams from industrial facilities. The calcium hydroxide raises the effluent’s \(\text{pH}\) level, ensuring discharged water meets strict environmental regulations before returning to natural waterways.
This \(\text{pH}\) adjustment is also instrumental in removing heavy metals, such as lead and arsenic, from contaminated water. Increasing the \(\text{pH}\) causes dissolved metal ions to precipitate as insoluble hydroxides, which are then easily filtered and removed. Lime is also used in water softening to remove hardness-causing minerals like calcium and magnesium, preventing scale buildup in industrial equipment.
In air quality control, hydrated lime is a primary reagent in flue gas treatment systems. Industrial exhaust gases from power generation and manufacturing contain acidic pollutants like sulfur dioxide (\(\text{SO}_2\)) and hydrogen chloride (\(\text{HCl}\)). The lime is injected into the flue gas stream as a dry powder or slurry.
The calcium hydroxide neutralizes these acidic gases through an acid-base reaction. It reacts with sulfur dioxide to form solid byproducts, such as calcium sulfite and calcium sulfate, which are collected and disposed of. These dry scrubbing methods achieve high desulfurization efficiencies, making hydrated lime a tool for reducing industrial air pollution and controlling acid rain.
Improving Soil Structure and Stability
Hydrated lime plays a dual role in ground modification for geotechnical engineering and agricultural soil health. For geotechnical applications (roadbeds, foundations, runways), it stabilizes soil by rapidly reducing moisture content and lowering the plasticity index of soft, wet clay.
The calcium ions displace water molecules and exchange with ions on the clay particles, causing flocculation. This results in a more stable, drier, and workable soil base. Over time, the calcium hydroxide reacts with silica and alumina in the clay through a pozzolanic reaction. This forms cementitious compounds, significantly increasing the soil’s long-term strength and load-bearing capacity.
In agriculture, liming adjusts the \(\text{pH}\) of acidic fields. The lime neutralizes excess soil acidity, unlocking essential nutrients like nitrogen, phosphorus, and potassium that are less available at low \(\text{pH}\) levels.
The calcium content also promotes better soil aggregation, improving the overall soil structure. This enhanced structure allows for better water infiltration and aeration, leading to healthier root systems and greater crop yield.