Will Calcium Chloride Continue to Absorb Water Vapor

Accelerators

P.-C. Aïtcin , in Science and Technology of Concrete Admixtures, 2016

19.4.2 Mode of addition

Calcium chloride is commercially available under three different forms: two solid forms and one liquid. Calcium chloride can be bought in the form of flakes containing 77–80% of calcium chloride or granules containing 94% of calcium chloride. In its liquid form, it contains from 30% to 42% of calcium chloride per liter of solution, usually sold as 30% solution.

From a practical point of view, the ACI Manual of Concrete Practice (Committee 212) recommends the introduction of liquid calcium chloride in the water line in order to avoid direct contact between cement and calcium chloride that could cause a flash set. When flakes or granules of calcium chloride are introduced directly in the mixer, these flakes and granules have not always enough time to dissolve during the mixing of concrete. Undissolved calcium chloride may generate undesirable dark spots on concrete surface.

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Group 17 (H, F, Cl, Br, I) Alkaline Earth Compounds

R.C. Ropp , in Encyclopedia of the Alkaline Earth Compounds, 2013

Calcium Chloride

Calcium chloride, CaCl ₂, is a typical ionic halide and is a solid at room temperature. Its molecular weight is 110.98   g/mol and its melting point is 772   °C. Very few natural minerals occur. The occurrence of a dihydrate (mineral "Sinjarite") and hexahydrate ("Antarcticite") is very rare and is connected mainly with dry lakes and brines. "Chlorocalcite", KCaCl₃, is a related mineral (also very rare).

It has a unique structure as exemplified by MgF₂ except for cell parameters of: a  =   4.166   Å, b  =   6.264   Å, c  =   6.436   Å, α  = β  = γ  =   90°, cell volume   =   167.87   ų, cell occupancy is shown in Table 2.39 and the crystal structure is shown in Fig. 2.15.

TABLE 2.39.

No	Site notation	Atom	Multiplicity	Wyckoff	Site symmetry	x	y	z	Occupancy
1	Cl1	Cl	4	g	..m	0.275	0.325	0	1.0
2	Ca1	Ca	2	a	..2/m	0	0	0	1.0

FIGURE 2.15.

Calcium chloride, being very soluble, can serve as a source of calcium ions in a solution, unlike many other calcium compounds, which are insoluble:

3CaCl₂  +   2K₃PO₄ (aq)   ⇒   Ca₃(PO₄)₂ (solid)   +   6KCl (aq)

Molten CaCl₂ can be electrolyzed to produce Ca metal and chlorine gas:

CaCl₂ (liq)   ⇒   Ca (solid)   +   Cl₂ (gas)

Calcium chloride is one of the most versatile of the basic chemicals. It has been produced commercially for over 100 years. The properties and characteristics of CaCl₂ make it useful in a large number of applications. It has several common applications such as brine for refrigeration plants, ice and dust control on roads, and in concrete. The anhydrous salt is also widely used as a desiccant, where it will absorb so much water that it will eventually dissolve in its own crystal lattice water (water of hydration). It can be produced directly from limestone, but large amounts are also produced as a by-product of the "Solvay Process" (which is a process to produce soda ash from brine). Because of its hygroscopic nature, the anhydrous form must be kept in tightly sealed containers.

Because the anhydrous salt is strongly hygroscopic, air or other gases can be channeled through a column of calcium chloride to remove moisture. In particular, calcium chloride is usually used to pack "drying tubes" to exclude atmospheric moisture from a reaction setup while allowing gases to escape. It cannot, however, be used to dry alkaline gases such as ammonia because it will form addition products. It is used to dry kelp, which is then used to produce soda ash. It can also be added to liquids to remove suspended or dissolved water.

The dissolving process is highly exothermic and rapidly produces temperatures of around 60   °C (140   F). In this capacity, it is known as a drying agent or desiccant. It is converted to a brine as it absorbs the water or water vapor from the substance to be dried:

CaCl₂  +   2H₂O   ⇒   CaCl₂·2H₂O

Aided by the intense heat evolved during its dissolution, calcium chloride is also used as an ice-melting compound. Unlike the more common NaCl (rock salt or halite), it is relatively harmless to plants and soil. However, recent observations in Washington state suggest it may be particularly harmful on roadside evergreen trees It is also more effective at lower temperatures as a "deicer" than sodium chloride.

Used for its hygroscopic property, it can be applied to keep a liquid layer on the surface of the roadway, which holds dust down. It is used in concrete mixes to help speed up the initial setting, but chloride ion leads to corrosion of steel "rebar", so it is not be used in reinforced concrete. Anhydrous calcium chloride, used for this purpose, can provide a measure of the moisture present in concrete.

Aqueous calcium chloride (in solution with water) lowers the freezing point as low as −52   °C (−62   °F), making it ideal for filling agricultural implement tires as a liquid ballast, aiding traction in cold climates.

Calcium chloride is also commonly used as an additive in swimming pool water as it increases the "calcium hardness" value for the water. Low calcium hardness values in pool water cause pool water to be corrosive on equipment, pumps and metal fittings.

Other industrial applications include use as an additive in plastics, as a drainage aid for wastewater treatment, as an additive in fire extinguishers, as an additive in control scaffolding in blast furnaces, and as a thinner in "fabric softeners". North American consumption in 2002 was 1,687,000   tons (3.7 billion pounds). A Dow chemical company manufacturing facility in Michigan produces about 35% of the total U.S. production of calcium chloride.

As an ingredient, it is listed as a permitted food additive in the European Union for use as a sequestrant and "firming agent" with the number E509, and considered as generally recognized as safe (GRAS) by the U.S. Food and Drug Administration (FDA). The average intake of calcium chloride as food additives has been estimated to be 160–345   mg/day for individuals. Ingestion of concentrated or pure calcium chloride products may cause gastrointestinal irritation or ulceration. The anhydrous form has been approved by the FDA as a packaging aid to ensure dryness.

Solutions of pure calcium chloride above 69% cannot be prepared by boiling a solution at 760   mm pressure because a eutectic causes the dihydrate to separate as a solid phase and the solution remains constant boiling. Four hydrates have been identified in the open literature. Their properties are shown in the following Table 2.40.

TABLE 2.40.

Property	CaCl2·6H2O	CaCl2·4H2O	CaCl2·2H2O	CaCl2·H2O	CaCl2
Composition (% CaCl2)	50.66	60.63	75.49	86.03	100
Molecular weight	219.09	183.05	147.02	129	110.99
Melting point (°C)	29.9	45.3	176	187	773
Melting point (F)	85.8	113.5	349	369	1424
Boiling point2 (°C)	—	—	174	183	1935
Boiling point (F)	—	—	345	361	3515
Density at 25   °C (77   F), g/cm3	1.71	1.83	1.85	2.24	2.16
Heat of fusion   (cal/g)	50	39	21	32	61.5
Heat of fusion   (Btu/lb)	90	70	38	58	110.6
Heat of solution3 in H2O (cal/g) (to infinite dilution)   (Btu/lb)	17.2	−14.2	−72.8	−96.8	−176.2
	31.0	−25.6	−131.1	−174.3	−317.2
Heat of formation3 at 25   °C (77   F), kcal/mol	−623.3	−480.3	−335.58	−265.49	−190.10
Heat capacity at 25   °C (77   F), cal/g   °C or Btu/lb   F	0.34	0.32	0.28	0.20	0.16

Its solubility in water is shown in Table 2.41.

TABLE 2.41.

g/100   ml of water
Substance	Formula	0   °C	10   °C	15   °C	20   °C	30   °C	50   °C	70   °C	90   °C
Ca chloride	CaCl2	59.5	64.7	74.5	100	128	137	147	159

Calcium chloride is commonly used as an "electrolyte" and has an extremely salty taste, as found in sports drinks and other beverages such as Nestle bottled water. It can also be used as a preservative to maintain firmness in canned vegetables or in higher concentrations in pickles to give a salty taste while not increasing the food's sodium content. It is even found in snack foods, including Cadbury chocolate bars.

It has been used to make a substitute "caviar" from vegetable or fruit juices or added to processed milk to restore the natural balance between calcium and protein for the purposes of making cheese such as Brie and Stilton. Calcium chloride's exothermic properties are exploited in many "self-heating" food products where it is activated (mixed) with water to start the heating process, providing a nonexplosive, dry fuel that is easily activated.

In brewing beer, calcium chloride is sometimes used to correct mineral deficiencies in the brewing water. It affects flavor and chemical reactions during the brewing process, and it can also affect yeast function during fermentation.

Calcium chloride can be injected as intravenous therapy for the treatment of "hypocalcemia" (low serum calcium). It can be used for insect bites or stings (such as Black Widow spider bites), sensitivity reactions, particularly when characterized by "urticaria" (hives). It has also been used in the treatment of:

•

Magnesium intoxication.

•

As an aid in the management of the acute symptoms in lead colic.

•

In cardiac resuscitation, particularly after open heart surgery.

•

It can help to protect the myocardium from dangerously high levels of serum potassium in "hypercalcemia".

•

Calcium chloride can be used to quickly treat "calcium channel blocker" toxicity, from the side effects of drugs such as Diltiazem, helping avoid potential heart attacks.

•

The aqueous form of calcium chloride is used in genetic transformation of cells by increasing the cell membrane permeability, inducing competence for DNA uptake (allowing DNA fragments to enter the cell more readily) (Table 2.42).

TABLE 2.42.

CAS number	1004-82-4 (anhydrate) 22691-02-7 (monohydrate) 10035-04-8 (dihydrate) 25094-02-4 (tetrahydrate) 7774-34-7 (hexahydrate)
Molecular formula	CaCl2
Molecular weight	110.98   g/mol (anhydrate) 128.993   g/mol (monohydrate) 147.014   g/mol (dihydrate) 183.045   g/mol (tetrahydrate) 219.089   g/mol (hexahydrate)
Density	2.15   g/cm3 (anhydrate) 1.835   g/cm3 (dihydrate) 1.832   g/cm3 (tetrahydrate) 1.71   g/cm3 (hexahydrate)
Melting point	772   °C (anhydrate) 260   °C (monohydrate) 176   °C (dehydrate) 45.5   °C (tetrahydrate) 30   °C (hexahydrate)
Boiling point	1965   °C (anhydrate)
Solubility in water	74.5   g/100   ml (20   °C) (soluble in alcohol)
Acidity	pK a  =   8–9 (anhydrate) 6.5–8.5 (hexahydrate)
Crystal structure	Orthorhombic (deformed rutile)
Space group	Pnnm, #58
Coordination chemistry	Octahedral-6-coordinate
Specific free energy of formation	ΔG  =   −750.2   kJ/mol
Standard heat of formation	Δf H  =   −798.4   kJ/mol

Physical constants of CaCl₂

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Corrosion and Degradation of Engineering Materials

P. Lambert , ... C. Atkins , in Shreir's Corrosion, 2010

3.30.3.4 Admixed Chlorides and the Rate of Hydration

Calcium chloride falls into the category of accelerating admixtures and, being a low-priced industrial by-product, was extensively used. In 1977, however, due to the increased risk of corrosion of reinforcement steel, the use of chloride-based accelerators was prohibited in all reinforced or prestressed concrete in the United Kingdom.

Accelerators increase the initial rate of chemical reaction between the cement and the water so that the concrete stiffens, hardens, and develops strength more rapidly. This is a benefit because it allows the earlier striking (i.e., removal) of formwork. They have a negligible effect on workability and 28-day strengths are rarely affected. The addition of small amounts (<1   mass%) of calcium chloride sometimes retards the set; however, this effect is very variable. Larger amounts produce an acceleration, and amounts over 3% have been known to cause a flash set. Sodium chloride produces a less pronounced change in the rate of hydration and the effect is more erratic, sometimes accelerating, sometimes retarding.

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Corrosion inhibitors

Johannes Fink , in Petroleum Engineer's Guide to Oil Field Chemicals and Fluids (Third Edition), 2021

Phosphonates

Calcium chloride and calcium nitrate brines are used in establishing and maintaining oil and gas wells [ 130]. For example, calcium chloride brines are used in drilling muds to cool and to lubricate well bits, and to remove cuttings from the hole. The brines help to maintain the consistency of the drilling muds and add density to them, to better enable the muds to overcome formation pressures and thereby the amounts of oil, gas, and water at the specific locations. Such brines also inhibit clay and shale hydration, and add needed weight to the muds.

Brines are additionally used as completion fluids just before the producing formation is reached, to flush the hole clean of solids so that the casing can be cemented in place. Thus, substantially solid-free brines, calcium chloride, and calcium nitrate brines are ideal as completion fluids [130].

Once a well casing is cemented in place, a smaller diameter tubing is inserted in the casing, which makes the flow of oil or gas more efficient and can be replaced if plugs develop. Tubing is used with packer fluid that keeps the well fluids away from the casing to minimize corrosion. Calcium chloride and calcium nitrate brines are used in the packing injected into the annular space between the tubing and the casing in order to maintain pressure levels.

These brines can also be used as workover fluids, by flushing wells free of solids before they are repaired, or before reworking a well that has been idle.

A principal drawback of such brines is that the brines tend to be highly corrosive to downhole equipment surfaces, causing pitting and erosion, often with the result that the equipment must be repaired or replaced at frequent intervals [130].

These problems can be overcome by the addition of chemicals that are inhibiting the corrosion of metal surfaces in petroleum well equipment when using such brines.

The inhibitor includes respective amounts of a phosphonate or salts, and gluconic acid or salts. Particularly preferred phosphates are amine polyphosphonates.

Specific examples of tertiary amine phosphonates include ammonium phosphonate, diethylenetriamine (DETA) phosphonate or diethylenetriamine penta(methylene phosphonic acid), monoethanolamine phosphonate, and 2-(aminoethoxy)ethanol phosphonate. Nonamino phosphonates are 1-hydroxyethylidene-1,1-diphosphoric acid, phosphonobutane-1,2,4-tricarboxylic acid, and 2-hydroxyphosphonocarboxylic acid [130]. Some of these compounds are shown in Figure 6.11.

Figure 6.11. Corrosion inhibitors.

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C

Stanley A. Greene , in International Resources Guide to Hazardous Chemicals, 2003

Calcium chloride

Aldrich Chemical

Alfa Rio (Brazil)

Asahi Glass Chemicals Div. (Japan)

Ashland Chemical

Atofina (France)

Brunner Mond (UK)

Caffaro (Italy)

Celtic Chemicals (UK)

Central Glass (Japan)

Chemettal (Germany)

Dow Chemical

D3 Chemie (France)

DuPont

Elf Atochem North America

General Chemical

GFS Chemicals

Harcros Chemicals

Honeywell Performance P & C

ICI Group (UK)

Kemira Chemicals (Finland)

LaRoche

LII Europe (Germany)

Merck (Germany)

OxyChem

Polifin (South Africa)

Prince Agri

Serva (Germany)

Showa Denko (Japan)

Solvay Group (Belgium)

TETRA Technologies

Tokuyama Group (Japan)

Tomita (Japan)

Unisource India (India)

Vulcan Performance Chemicals

William Blythe (UK)

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Agricultural and Related Biotechnologies

C. Sitbon , G. Paliyath , in Comprehensive Biotechnology (Second Edition), 2011

4.28.5 Effects of Pre- and Postharvest Treatments on Firmness

Calcium chloride (CaCl ₂) has been widely used as a preservative and firmness-increasing agent in the fruit and vegetable industry for whole, as well as for fresh-cut, produce. Excessive ripening of the fruit can be reduced by the application of calcium, which binds and cross-links with free carboxyl groups of polygalacturonic acid in pectins, enhancing firmness of the fruits. Prestorage calcium treatment of apples has also been shown to reduce the incidence of physiological disorders, softening rates, and decay caused by fungi. Calcium infiltration significantly increased sensory hardness and overall acceptability of apples. Similarly, papaya fruit treated with CaCl₂ solution at 2% retained cell wall integrity because of influx of calcium that could have helped form calcium pectate in the cell wall, thus prolonging the shelf life.

Softening of apple is prevented or delayed by 1-MCP, the effects of treatment often closely associated with ethylene production. The components of texture that are affected by 1-MCP have not been adequately investigated but tissue toughness of apple and pear is greater in 1-MCP-treated fruit than in untreated fruit. Firmness retention can also be excellent in fruit kept at high temperatures (20–24   °C) after treatment. The effects of 1-MCP treatment are often closely associated with inhibition of ethylene production. Interestingly, work with melon has shown that many of the cell-wall-related genes are regulated independently of ethylene, in addition to those that are regulated by ethylene [20]. Lower activities of β-galactosidase, α-arabinofuranosidase, and β-xylosidase were associated with delayed softening of 1-MCP-treated kiwifruit. Decreased softening in 1-MCP-treated bananas is associated with lower expression of an ethylene-induced expansin (MaExp1) gene, and lower activities of pectin methylesterase (PME), polygalacturonase (PG), endo-β-1,4-glucanase (EGase), and pectate lyase activities. Delayed softening of peaches was associated with delayed increases in soluble pectin concentrations. Activities of exo-PG and EGase were lower in 1-MCP-treated plums than in untreated fruit, but treatment did not affect activities of endo-PG and pectin esterase (PE). Effects of 1-MCP on softening of pears were associated with decreased β-galactosidase activity and differential effects on expression of its genes [19], lower glycosidase activities, and transcript accumulation of genes for PG1 and PG2. Delayed softening of 1-MCP-treated avocado fruit is reflected in similar patterns of delayed solubilization and degradation of polyuronides.

Studies have demonstrated that AVG-treated fruit can better maintain fruit firmness through the storage period compared to untreated fruit and remain firmer during storage. AVG-treated Gala and Jonagold apples stored under CA maintained better fruit firmness. Cox's Orange Pippin apples treated with AVG preharvest and stored in static CA conditions (1.2   kPa   O₂, <1   kPa   CO₂, 3.5   °C) with or without ethylene removal were firmer than untreated apples. Flesh firmness of Pink Lady apple was generally higher when stored in CA and treated with ReTain®. AVG-treated Gala apples stored for 8 months in CA with ethylene removal had greater fruit firmness and lower flesh breakdown compared to untreated and 1-MCP-treated apples [6]. A combination of preharvest AVG and postharvest 1-MCP treatments on McIntosh and Cortland apples was most effective in maintaining fruit firmness. AVG-treated white flesh peach rapidly lost their firmness within the first 2 days of ripening, despite the significant suppression of ethylene production, AVG treatment at harvest thus may come too late to reduce the rapid softening which occurs just after harvest.

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12th International Symposium on Process Systems Engineering and 25th European Symposium on Computer Aided Process Engineering

Yi Liu , Haiyan Qu , in Computer Aided Chemical Engineering, 2015

2.1 Reactive crystallization

The solid calcium chloride was purchased from Sigma-Aldrich and the 85 % wt ortho-phosphoric acid was purchased from Merck. CaClH ₂PO₄·H₂O crystals can be produced through the reactive crystallization described in Eq. (1) (Fox, 1939) as follows:

(1) ${\mathrm{CaCl}}_2+{\mathrm{H}}_3{\mathrm{P}\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}={\mathrm{CaClH}}_2{\mathrm{P}\mathrm{O}}_4·{\mathrm{H}}_2\mathrm{O}+\mathrm{H}\mathrm{C}\mathrm{l}\uparrow$

The calcium chloride was firstly dissolved in distilled water and then mixed with diluted phosphoric acid in 1:1 ratio of molal quantities. The feed speeds of calcium chloride solution and phosphoric acid were controlled by two peristaltic pumps (Ole Dich Instrumentmakers, Denmark), respectively, assuring to mix calcium chloride and phosphoric acid in equimolecular amounts. An IKA C-MAG HS7 magnetic stirrer (IKA, Denmark) was applied to stir and to heat the mixture up during crystallization. The temperature of the mixture solution was monitored with a thermometer. After the crystallization process, the crystals were separated by vacuum filtration from the mother liquor through a glass fiber filter paper (ADVANTEC GC-50). The whole procedure was carried out in a fume hood.

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Special Cements

John Bensted , in Lea's Chemistry of Cement and Concrete (Fourth Edition), 1998

14.5.3 ALINITE CEMENT

The incorporation of calcium chloride in the raw material mixture for Portland clinker production by utilising molten salt technology, has enabled the temperature of clinker formation to be reduced by 400–500°C. This clinker contains alinite, a structural leitmotif of alite (tricalcium silicate) incorporating chloride ions. ^{132 , 164 , 165} The quantitative content by weight of the mineral phases present in alinite clinker varies within the following limits: alinite 60–80 per cent, beute (β-dicalcium silicate) 10–30 per cent, calcium chloroaluminate (Ca₆AlO₇Cl) 5–10 per cent, dicalcium ferrite 2–10 per cent. Weak Ca—Cl bonds are developed which result in alinite clinker being softer than alite and requiring less energy for grinding. Gypsum addition is reported ¹⁶⁶ to intensify strength development rather than principally functioning as a regulator of set.

Alinite has also been produced by clinkering steel plant wastes such as fly ash from an in-house power generating plant, limestone fines, mill scale and magnesite dust with calcium Chloride as a sintering aid at 1150°C. ¹⁶⁷ The optimum calcium chloride addition to the raw mix was found to be 7–8 per cent by weight. These cements have been found to be relatively insensitive to the various impurities in the raw mix and can tolerate higher levels of MgO than Portland cements. This low-temperature clinkering route offers scope for the conversion of industrial wastes into hydraulically setting cements. Alinite cement is compatible with Portland cement and additions of 20 per cent by weight of fly ash can be satisfactorily accommodated. Alinite is stable in impure systems with different elements, but is unstable in the pure system CaO—SiO₂—Al₂O₃—CaCl₂. Typical alinite clinker contains alinite (65 per cent), belite (20 per cent), mayenite (C₁₁A₇·CaCl₂; 10 per cent) and C₄AF (5 per cent). ¹⁶⁸ Alinite was ascribed the formulation Ca₂₁Mg[Si_0.75Al_0.25O₄]₈O₄Cl₂. The presence of magnesia appears to be essential for alinite formation. ¹⁶⁹ Jasmundite [Ca₂₂(SiO₄)₈O₄S₂], which has S²⁻ instead of Cl⁻ ions in the crystal lattice, is poorly hydraulic. ¹⁷⁰ Later work showed alinite not to have a fixed composition and to be best represented as Ca₁₀Mg_1-(x/2□ _x/2[(SiO₄)_3+x (AlO₄)_1-x O₂Cl] where 0.35 < x < 0.45 and □ refers to a lattice vacancy. ¹⁷¹ Alinite cement has high early strength properties. The main hydration products of alinite cements are calcium silicate hydrates that appear to incorporate the chloride, which are morphologically different from the poorly crystalline calcium silicate hydrate (C-S-H) found during normal Portland cement hydration below ∼ 100°C. The presence of labile chloride could in theory encourage corrosion of steel reinforcement, but current experimental evidence on this aspect is conflicting. As well as C-S-H, hydration produces calcium hydroxide and an AFm phase C₃A·CaY₂·IOH₂O (Y = Cl⁻, OH⁻, ½CO₃ ²⁻), but microprobe analysis could not positively identify Cl⁻ ions incorporated in the C-S-H gel. Hydration of alinites with different Si/Al ratios has shown no significant differences in hydraulic activity, because of the high degree to which small amounts of free chloride accelerate hydration. Bromide-alinite, in which bromide ions replace chloride, has been made. However, its hydraulic activity has not yet been reported. ¹⁷²

Calcium silicate sulfate chloride [Ca₄(SiO₄)(SO₄)Cl₂], a derivative of alinite having an orthorhombic structure, ¹⁷³ is formed at only ca. 600–800°C. It has appreciable hydraulic activity, ¹⁷⁴ greater than that of belite. Compressive strengths of 25 MPa at 28 days have been found. ¹⁷⁴

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Cement Admixtures

Rodney M. Edmeades , Peter C. Hewlett , in Lea's Chemistry of Cement and Concrete (Fourth Edition), 1998

15.6.4 NON-CHLORIDE ACCELERATORS

Although not as effective as calcium chloride, several chemicals have been proposed and become commercially available for use as accelerators, calcium formate, de-acidised calcium formate, sodium aluminate and triethanolamine being those most commonly mentioned.

As previously indicated, calcium salts appear to have a superior activity compared with most other metal salts, but they commonly suffer from a low solubility in water. Calcium formate acts in a manner similar to calcium chloride, but high dosages are required and its solubility is considerably less (approximately 17 g/100 g compared with 75 g/100 g at 20°C).

Two or more chemicals are often used in combination to increase the concentration and improve the performance of liquid admixtures, which are preferred to powders for accurate dosing of concrete. Calcium formate sodium nitrite blends have been used extensively, with the added advantage of anti-corrosion properties claimed for their use. ⁷⁵ Measurement of the electrode potential of steel embedded in concrete at constant applied current density is one method of monitoring the passivity of the steel. Figure 15.57 compares the protection given by a chloride-free accelerator with the breakdown caused by calcium chloride. ⁷⁶

Fig. 15.57. Variation of electrode potential with time during corrosion test

(source: Ref. 76).

A third minor ingredient, triethanolamine, has also been incorporated in some proprietary admixtures to shorten initial setting times. Triethanolamine will combine with aluminium and so provide dissolution of the C₃A in advance of aluminohydrate formation. It is an accelerator of C₃A hydration at low dosage (0.1–0.5 per cent by weight of cement) but at higher dosages causes retardation of C₃S hydration. Figure 15.58 shows the effect of triethanolamine on the main (C₃S) peak during cement hydration at a dosage of 0.4 per cent. ⁷⁶ The proportion of triethanolamine in compound chloride-free accelerators must therefore be limited, in order for the dosage to be kept below 0.5 per cent at normal levels of addition of the admixture to concrete.

Fig. 15.58. Effect of triethanolamine on Portland cement hydration by conduction calorimetry

(source: Ref. 76).

In recent years, calcium nitrite has been manufactured commercially and become available as a proprietary accelerator. This chemical has a reasonably high solubility (∼50 g/100 g) and is claimed to be almost as effective as calcium chloride. The dosage rate is of the order of 0.3–2.3 per cent (as solids by weight of cement). The toxicity of inorganic nitrites, in contrast to most other chemicals used as cement accelerators, should be noted. Calcium nitrite has also been promoted as a corrosion inhibitor for reinforced concrete. ⁷⁷ Other non-chloride accelerators have been based on calcium nitrate or sodium thiocyanate. The latter is effective but relatively expensive.

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Outline of Biological Chemical Principles: Components, Pathways and Controls

R.J.P. Williams , J.J.R. Fraústo da Silva , in The Chemistry of Evolution, 2006

4.15.6 PREVENTING INHIBITION: REJECTION

Now, in addition to sodium and chloride, calcium had to be removed from all cells since at a concentration > 10⁻³ M it forms precipitates or incorrect structures with many inorganic and organic anions, so that it would have damaged biological metabolism. (Rejection of poisons of all kinds is necessary, but energy wasteful for survival and growth of cells.) In effect, we observe very low free calcium in all prokaryote, < 10⁻⁵ M, and eukaryote cells, < 10⁻⁷ M. The outward pumping of calcium, sodium and chloride (and even manganese) together with the uptake of potassium and phosphate are inevitable requirements that had to be established in "cells" effectively before or as they were coded.

Other elements such as Ni^{2   +}, Co^{2   +}, Cu^{2   +} (Cu⁺) and Zn^{2   +} would also be poisonous if their free ion concentrations exceeded about 10⁻¹⁰ M since they could then compete with Mg^{2   +}, Mn^{2   +} and Fe^{2   +} in essential roles (see Chapter 2). inspection of cells throughout all of the history of life shows that this free level is not exceeded (see Figure 4.3). At the same time, all of these elements are now essential in cells, so that there has to be controlled uptake and rejection to match the synthesis of the functional partners of the ions. The need is met by a set of pumps and transcription factors, which have suitable selectivity and high binding constants for the monitoring of the ion concentrations and then the synthesis of their organic molecule partners – often proteins and coenzymes. In this way, all metal ions are built into an informed total activity with non-metal chemistry. (See how the inverse of the universal free ion concentrations in cells is related to the Irving/Williams stability order, Section 2.17 and Figure 4.3.) The very low concentrations of free ions requires carriers to equilibrate them in the cell and other proteins to store and buffer their activity, e.g. for Ca, Fe, Cu, Zn, Ni, Co and Mo, and this equilibration often extends to their enzymes.

We must not miss in this description that the pumping of gradients in and out of cells of inorganic elements requires a considerable amount of energy (ATP). Together with the concentrated synthetic organic chemicals in a cell and the oxidised external environment this energy is then a considerable contributor to energy storage. These gradients became extremely useful later in uptake and signalling (see Chapters 6–9 Chapter 6 Chapter 7 Chapter 8 Chapter 9 ). We shall note again and again the progression in evolution from recognition and rejection of a poison, e.g. Na⁺, Ca^{2   +}, Mn^{2   +}, Cu^{2   +}(Cu⁺), and Cl⁻, to its later functional value, often of its gradients. In conclusion, we stress that the control of concentrations of about 12 metal ions is an essential requirement of all organisms and is a thermodynamic feature different in different chemotypes. The concentration of attention on the DNA and genetics in modern biochemistry is hiding fundamental features of life limited yet permitted by the environment. There is a "fitness" of life in the environment.

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