Kalkwasser

Kalkwasser is a saturated aqueous solution of calcium hydroxide (Ca(OH)₂), commonly known as limewater. It is produced by dissolving hydrated lime (CaO·H₂O) or quicklime (CaO) in water, often under controlled temperature and agitation to reach maximum solubility. The resulting solution contains free calcium hydroxide and, if calcium carbonate (CaCO₃) is present in the lime, a low concentration of dissolved calcium carbonate. The pH of commercial kalkwasser typically ranges from 12.5 to 13.5, making it a strong base.

In the paper and pulp industry, kalkwasser is employed as a “softening agent” during the bleaching process. It neutralises acidic inter‑sludge pH, stabilises lignin‑catalysed radicals, and helps to balance the pH before the addition of chlorine‐based bleaching agents. The calcium hydroxide precipitates as calcium carbonate when it comes in contact with carbon dioxide in the air, thereby reducing the acidity of the washing stages and recovering valuable calcium for reuse.

Besides papermaking, kalkwasser is utilized in water treatment to raise pH, precipitate metal ions as hydroxides, and control equilibrium between carbonate species. In construction, it serves as a gypsum‑free plaster for interior walls and as a quicklime‑based grout for masonry. It is also applied in green chemistry as a source of calcium in the synthesis of fine chemical intermediates.

The handling of kalkwasser requires care because of its high alkalinity. Contact with skin and eyes can cause severe irritation or burns, and inhalation of dust from the dry lime used to prepare the solution poses health risks. Personal protective equipment such as gloves, goggles, and respirators is recommended during preparation and application.

From an environmental perspective, kalkwasser is considered relatively benign when used responsibly. Its high pH can neutralise acidic effluents, contributing to the protection of aquatic ecosystems. However, surplus calcium carbonate precipitates can form scale in piping and water‑handling equipment, potentially increasing maintenance requirements. Recycling of lime from calcination processes and the use of renewable sources for quicklime production can mitigate the overall carbon footprint associated with kalkwasser production.