Calcium chloride dihydrate, CaCl₂·2H₂O, CAS 10035-04-8, EINECS 233-140-8, molecular weight 147.01 g/mol, is supplied as white deliquescent flakes, prills, pellets, or powder with a theoretical calcium chloride content of 75.49% by mass. Commercial technical-grade material is normally controlled at 74.0–77.0% CaCl₂ with a CaCl₂·2H₂O assay ≥98.0%. It differs from anhydrous calcium chloride CAS 10043-52-4 and calcium chloride hexahydrate CAS 7774-34-7 in hydration state, dissolution exotherm, dusting behavior, and gravimetric dosing mass. Commercial forms include technical, food-grade FCC/E509, and pharmacopoeial USP/Ph. Eur. grades, with package formats of 25 kg HDPE bags and 500–1000 kg flexible intermediate bulk containers. Because the saturated solution has an equilibrium relative humidity near 30% at 25 °C, moisture-tight storage is required in humid plant environments.

Specification Boundaries Across Technical, Food, and Pharmaceutical Grades

Vendor certificate-of-analysis boundaries vary by grade. Technical-grade flake for de-icing, dust control, and oilfield use is commonly controlled for water-insoluble matter, alkali chlorides, sulfate, and magnesium chloride. Food-grade material is evaluated under Commission Regulation (EU) No 231/2012 for E509 calcium chloride and FDA 21 CFR §184.1193; pharmacopoeial material is tested by USP or Ph. Eur. monographs. Representative technical-grade release limits are listed below. Food and pharmaceutical grades may impose tighter limits on heavy metals, arsenic, and fluoride.

Prilled technical material typically has a granule size distribution within 0.2–2.0 mm, while pelletized de-icing product may fall between 2.0–8.0 mm. Food-grade material is commonly assayed as CaCl₂·2H₂O with a range near 99.0–107.0%; exact specifications should be aligned with the applicable monographic edition.

On road maintenance and dust-abatement lines, calcium chloride dihydrate is metered through prewetting or spreading equipment as a hygroscopic brine former rather than as an inert abrasive. The CaCl₂–water eutectic is approximately -51 °C at 29.5 wt% CaCl₂; practical de-icing response in truck-mounted spreaders is commonly cited in vendor technical bulletins to -32 °C, because mechanical removal and dilution reduce the equilibrium freezing point achieved at the ice surface. Because dihydrate dissolution is exothermic but less strongly exothermic than anhydrous CaCl₂, prewetting systems often use a 32–35 wt% brine prepared in insulated tanks and delivered through stainless steel strainers at 0.4–1.0 L/m². Dust-control applications dilute the brine to 0.5–2.0 L/m² per pass, with frequency adjusted by surface moisture and vehicle count; published data for this specific configuration is limited.

What Limits the Addition Rate in Reinforced Concrete?

When calcium chloride dihydrate is added to portland cement paste, the calcium ion adsorbs onto C₃S and C₃A hydration products, shortening the induction period and modifying early ettringite morphology. The material is classified as a Type C accelerating admixture under ASTM C494/C494M-19. A typical dosage is 1.0–2.0% CaCl₂·2H₂O by mass of cement. At 2.0% addition, the chloride ion introduction is approximately 0.96% by mass of cement, calculated from the 48.2% chloride ion content of the dihydrate; this exceeds the 0.15% water-soluble chloride ion limit in ACI 318-19 Table 19.3.2.1 for reinforced concrete exposed to chlorides and the 0.06% limit for prestressed concrete. The product is therefore restricted to plain concrete or to mixtures where a corrosion-inhibiting admixture is used and chloride ion content is verified by ASTM C1218/C1218M-17. In ready-mixed plants, flash setting is observed at dosages above 2.5%, and dry flake addition can produce rapid slump loss when added to hot mix water above 35 °C. Pre-dissolving the dihydrate to a 25 wt% brine and injecting after batching water through a 2 m³ twin-shaft mixer at 30 rpm reduces set-time variation in comparison to dry addition.

Dense, solids-free brines are prepared from calcium chloride dihydrate for completion and workover operations where hydrostatic pressure must be controlled without suspended weighting solids. The dihydrate is dissolved in field mixers, filtered through diatomaceous earth units to ≤10 µm clarity, and density is verified by ASTM D4052-22 using a laboratory densitometer. A single-salt calcium chloride brine reaches saturation at approximately 11.6 lb/gal (1.39 g/cm³) at 20 °C, roughly 38–40 wt% CaCl₂. Lower-density annulus packer brines at 10.0–11.0 lb/gal (1.20–1.32 g/cm³) are prepared by dilution and re-filtered. Gravimetric batching requires a correction factor of 1.325 relative to anhydrous calcium chloride for equivalent CaCl₂ mass, because the dihydrate contributes hydration water. The concentrated chloride brine is corrosive to carbon steel in aerated surface tanks; continuous nitrogen blanketing and filming-amine corrosion inhibitor addition are used to control pitting in API 5CT casing handling circuits.

When Storage Conditions Drive Surface Moisture Uptake and Caking

Moisture uptake is governed by the equilibrium relative humidity of the saturated calcium chloride solution, approximately 30% at 25 °C. Above this threshold, the dihydrate forms a surface brine film that cements adjacent particles into agglomerates and blocks rotary valves or auger metering units. In warehouse environments with RH greater than 60%, open storage of unlined bags can gain 1–3% mass within 24 h; storage is therefore specified in sealed HDPE liners with desiccant, and pneumatic transfer lines are purged with dried air at a dew point of ≤-40 °C. If caked material is encountered, lump crushing through a jaw crusher with 5 mm gap followed by screening over a 2 mm mesh restores flowability, but the crushed product should be re-analyzed for assay before batching because surface moisture may alter the CaCl₂ concentration. The bulk density of prilled dihydrate is approximately 0.95–1.10 g/cm³; feed hoppers should be designed with a steeper cone angle than used for free-flowing mineral salts.

Dehydration to the Anhydrous State Is Not a Single-Step Process

Thermogravimetric analysis of calcium chloride dihydrate under constant heating at 10 °C/min in nitrogen shows stepwise water loss rather than a single dehydration event. The first water of crystallization is released at moderate temperature, and full conversion to anhydrous CaCl₂ requires sustained heating above 200 °C under vacuum or a dry purge. Continuous vacuum dryers producing anhydrous calcium chloride from the dihydrate operate at jacket temperatures below 200 °C to avoid hydrolysis and chloride loss; the dihydrate is therefore selected when lower dissolution exotherm and lower dusting are more important than maximizing CaCl₂ mass per kilogram. The following table summarizes material-balance differences across the three common hydration states.

Formulation logic therefore follows mass balance: to deliver 100 kg anhydrous CaCl₂, the operator must charge 132.5 kg of dihydrate or 197.4 kg of hexahydrate. This correction is applied in oilfield brine mixing, concrete batching, and food processing where ionic concentration is controlled by conductometric verification.

In cheese and brewing operations, the dihydrate is used as E509 firming agent and calcium ion source, and the hydration water reduces gravimetric drift during batching. Cheese brine is typically prepared at 20–23% CaCl₂, then dosed into milk at 0.02–0.05% by volume; rennet coagulation response is evaluated by a Formagraph or rotational viscometer before full production. Brewing water treatment adds the dihydrate to increase calcium ion concentration without sodium, with addition calculated by ion mass balance and verified by conductivity. In pharmaceutical electrolyte concentrates, the dihydrate is selected over anhydrous grade because the lower heat of solution reduces localized temperature rise during reconstitution. The material is incompatible with sulfate-containing concentrates unless calcium sulfate precipitation is controlled by solubility limits; it also attacks aluminum and zinc equipment in humid environments.