Hydrated Lime

Ca(OH)₂ content answers “how many grams of active material are there”. BET specific surface area and pore volume answer “how quickly can that material be reached”. Standard hydrate typically sits at 18–22 m²/g; high-surface grades reach 35–45 m²/g with pore volume above 0.1 cm³/g.

The difference shows up wherever contact time is short. In a flue gas duct the sorbent meets the gas for one or two seconds; what reacts in that window is the outer shell of the particle, and the core travels to the filter unused. Double the surface area and removal efficiency rises appreciably at the same stoichiometric ratio.

In aqueous systems with long contact time the relationship reverses. A milk of lime tank stirs for minutes and the particle has time to dissolve completely; there the surface area difference is largely erased and what matters is insoluble residue and particle size distribution. There is no single “best lime”, only the best one for a given process.

Dry sorbent injection starts from a theoretical Ca/S ratio, or Ca/(S+2Cl) when all acid gases are counted. Practice runs at 1.5–2.5. That excess is not chemistry but physics: the reaction happens at the gas-solid interface and only a limited amount of acid gas reaches the interior of the particle.

The real lever is temperature. Bring the approach to saturation temperature down to 15–20 °C and a film of moisture a few molecules thick forms on the particle surface, at which point HCl and SO₂ capture jumps. In flue gas treatment plants it is common to add duct cooling and cut the dose by a third with the same sorbent.

The bag filter sets the limit: thicken that moisture film and the cake sticks, differential pressure climbs and cleaning frequency rises. This is why 140–180 °C is the typical operating window, and why sorbent selection is made together with it — optimising dose without watching filter behaviour is half a job.

The solubility of Ca(OH)₂ in water is only 1.73 g/L at 20 °C, and it falls rather than rises with temperature. This retrograde solubility has two consequences: the alkalinity that saturated limewater can carry is low, and dissolved lime re-precipitates as scale wherever lines warm up.

That is why water and wastewater treatment works in practice with milk of lime at 5–20% solids; what travels through the pipe is a suspension, not a solution. Choosing the concentration is a hydraulic decision, not a chemical one: above 20% the slurry is abrasive and blocking, below 5% it inflates storage and agitation volume.

Saturators producing saturated limewater are still preferred for drinking water remineralisation and low-flow dosing points. The advantage is a clear, non-blocking solution and fine dose control; the drawback is large equipment volume and limited capacity. Flow rate and target alkalinity increase decide between them.

CL 90-S is added to cement mortar not to gain strength; lime's strength contribution is limited and slow. Lime changes the water retention and workability of the fresh mortar: applied on absorbent brick, aerated concrete or a dry block, the mortar does not lose its water in an instant, cement hydration is not cut short and adhesion holds.

Once hardened, two properties stand out: vapour permeability and autogenous healing. Lime mortar carbonates with atmospheric CO₂ at crack faces and closes hairline cracks on its own. That is why restoration work on the building materials side prefers lime: letting a historic wall manage its moisture balance beats sealing it in with cement render.

A typical mix is 1:1:6 by volume (cement:lime:sand). Raising the lime fraction improves workability and water retention but lowers early strength and changes shrinkage behaviour. The right ratio is set by the absorbency of the wall material and the exposure class, not by a product leaflet.

Stripping is the separation of the bitumen film from the aggregate surface under the action of water, and it is most pronounced on siliceous, acidic aggregates. Applied to the aggregate at 1–1.5% of dry weight, hydrated lime lets Ca²⁺ ions bond with the carboxylic acids in the bitumen; the bitumen that would otherwise let go is chemically anchored to the surface.

Liquid amine additives do the same job chemically but degrade over time at storage and mixing temperatures. Lime does not degrade. It also stays in the mastic as filler: it slows oxidative ageing of the bitumen and adds rutting resistance. In road and asphalt work the TSR measured to AASHTO T283 is typically carried above the 0.80 threshold.

The application method changes the result. Dusting dry lime onto dry aggregate is the weakest route; part of the powder leaves through the stack without ever touching a surface. Applying dry lime or lime slurry to aggregate at around 3% moisture gives a markedly more even coating — the same dose, different performance.

Hydrated lime is a fine, light and cohesive powder. Aerated bulk density runs 350–450 kg/m³, settled 500–650 kg/m³. Sizing a silo on the aerated figure overstates stock capacity; setting up weighing on the settled figure introduces error during filling. Both numbers have to be used together.

Cohesion is the source of arching and line blockage. Cone angle, aeration pad layout, vibration and feeder type are chosen as a set. Free moisture drifting past 2% triggers all of these problems at once, which is why moisture is an operating parameter in the specification rather than a cosmetic line.

Fineness is likewise equipment-driven. Dry sorbent injection and asphalt filler want the fine fraction; in milk of lime preparation excessive fineness raises viscosity and pump load. A residue of ≤ 7% on 0.090 mm is a practical balance between the two ends, though a narrower band can be defined for special grades.

Hydrated lime left exposed reacts with atmospheric carbon dioxide and a layer of calcium carbonate forms on the grain surface. That layer both consumes active Ca(OH)₂ and closes access to the hydroxide beneath it. A rising CO₂ figure in the analysis is therefore not a simple impurity indicator but a direct performance indicator.

The ≤ 4% CO₂ limit EN 459-1 sets for CL 90-S describes the value at dispatch; it is not an open-ended field guarantee. In a closed silo or a sealed big bag, 6–12 months is untroubled in practice; a torn sack, an open silo hatch or a pallet left in the rain shortens that to weeks.

Moisture is a separate problem. Hydrated lime does not react with water — it is already hydrated — but moisture causes caking and loss of flow. Caked material cannot be dosed without screening and makes the feeder rate fluctuate. The storage rule is simple: closed, dry, first in first out.

Under CLP, Ca(OH)₂ is classified as Skin Irritation 2 (H315), Serious Eye Damage 1 (H318) and specific target organ toxicity STOT SE 3 (H335). It is neither flammable nor explosive; the real risk is to the eye, because the saturated solution sits at pH 12.4 and corneal damage develops quickly.

The EU indicative occupational exposure limit is 1 mg/m³ for respirable dust (8 h TWA) and 4 mg/m³ for inhalable dust. Enclosed transfer, local extraction at the filling point and a P2/P3 filter mask keep operation below these values in practice. An eyewash station belongs in the dosing area.

The product is registered under REACH. Drinking water applications call for conformity with EN 12518 (calcium hydroxide for water intended for human consumption); in food and sugar the E526 additive purity is a separate criterion and separately documented. The point of use, not the product, decides which document is needed; our technical team will map the required document set to the application.