Reaction product of fusion between 1000°C and 2000°C of aluminium oxide and calcium oxide based raw materials with at least CaO+Al2O3+Fe2O3 > 85%, in which aluminium oxide and calcium oxide in varying amounts are combined in various proportions into a multiphase crystalline matrix.

Water solubility

Description of key information

According to the solubility studies, the major elements found at the maximum of solubility were 1054 mg/l Al and 856 mg/l Ca, for 100 g of the tested substance, ie approximatively 1%. This is the highest solubility compared to the solubility of commercially available gradings.

Additional information

The defined substance is commercially available in granulates form for non hydraulic properties and use. The substance is commercially provided with different specific grading until a diameter of 25mm.

Some of the mineralogical components reacts with water, as it can be noticed through maximum solubility measurements in water , provided in the dossier. Calcium and Aluminium are the only significantly measured elements in solution. Moreover the observed maximum solubility is very similar to pure CaAl₂O₄. CaAl₂O₄is indeed one the more reactive and richer in aluminium present component in the substance

The test on solubility has been perfromed on the finest commercially grade (0 to 1 mm), which has been ground under 100µm.

Adduitional study comparinig conductimetry of different granulometry shows that the solubility of the finest commercially available grade (0 to 1 mm) is significantly lower that the finally ground under 100 µm sample of the substance (about 5 times)

According to the solubility studies, the major elements found at the maximum of solubility were 1054 mg/l Al and 856 mg/l Ca, for 100 g of the tested substance, ie approximatively 1%

 

Theoritically, it could be considered that basic reactivity in solution of CaAl₂O₄is a pessimistic approach , considering the substance as containing 100% of CaAl₂O₄. Reactivity in water of CaAl₂O₄is strongly dependent on grading (total available specific surface of the solid) . Basic available data on kinetics are for ground solid under 100 microns.

The reaction of CaAl₂O₄with water is in two steps:

-         CaAl₂O₄partially dissolves in water to reach a transient maximum solubility value . Meanwhile , hydrates precipitates so that the apparent equilibrium is only a quasi-stationary equilibrium between dissolution and hydration.

-         Precipitation rate of hydrates increases and CaAl₂O₄will be completely transformed through the following reaction into hydrates

 

CaAl₂O₄+ 4 H₂O - > 1/3 Ca₃Al₂O₆,6 H₂O + 2/3 Al(OH)₃

 

The pH of water will be then basic (above 9) in absence of any dilution or reaction with an other component. The Aluminium is only under Al(OH)^4-form in water.

 

Ca₃Al₂O₆,6 H₂O is a slightly soluble hydrate with a higher solubility than Al(OH)₃. Data are available in scientific literature . Total aluminium concentration is 2.474 mmol/l under Al(OH)^4-form. (ref 1)

 

If diluted in water , Ca₃Al₂O₆,6 H₂O dissolves and provides calcium ion in solution with precipitation of Al(OH)₃due to pH decrease.

With contact of dissolved CO₂in water, Ca₃Al₂O₆,6 H₂O dissolves and precipitation of CaCO₃and Al(OH)₃occurs.

 

Al(OH)₃will be then in any case , the main hydrate present in short and long term.

 

We can then identify 3 periods (for grounded substances under 100 microns):

-         one at short term, when a maximum total Aluminium content is reached (within a few minutes or hours according to the commercial products

-         one at mid-term, when hydration is completed (within a few hours or days) according to the commercial products. The Aluminium content in water will be very similar to Ca₃Al₂O₆,6 H₂O - Al(OH)₃association.

-         one long term, according to dilution and contact with any acid (as atmospheric CO₂) to reach the pH 7 value . The total Aluminium content in water will be then very similar to Al(OH)₃value.

 

The attached graph illustrates data from literature with the use of PHREEQC software (ref 1)

 

The Al(OH)₃solubility will be high for high or low pH , and low in the range of pH (6 to 8) of usual natural environment.

Solubilities of pure Al(OH)₃, (Ca₃Al₂O₆,6 H₂O) , association Al(OH)₃ - (Ca₃Al₂O₆,6 H₂O) , association Al(OH)₃ - CaCO₃are plotted.

The calculated solubility of association Al(OH)₃ - (Ca₃Al₂O₆,6 H₂O) with decreased pH is also plotted.

 

It could be noticed that long term solubility will be at the same level than Al(OH)₃.

 

In this analysis, with a pessimistic approach , the following analogy can be then considered for the finest particles , (corresponding to a maximum 10 to 15% of the weight of commercial products) :

-         at short term, if we consider the substance as a 100% CaAl₂O₄component :

o  1g of ground substance if completely dissolved will provide 341.1mg of total Aluminium (34.1%)

o  Above 3g of ground substance, the solubility will be the same with a maximum 1g of total Aluminium.

o  1g of ground substance = 0.341g of soluble Aluminium from Al(OH)₃ until 3g of substance in dosage-impact through water at similar pH

-         At mid term, if we consider the substance as a 100% CaAl₂O₄component

o  1g of ground substance if completely dissolved will provide 341.1mg of total Aluminium (34.1%)

o  Above 126 mg of ground substance, the solubility will be the same with a maximum 43mg of total Aluminium

o  100mg of ground substance = 0.341 mg of soluble Aluminium from Al(OH)₃ until 126mg of substance in dosage-impact through water at similar pH

-         At long term, if we consider the substance as a 100% CaAl₂O₄component:

o  1µg of ground substance if completely dissolved will provide 0.341µg of total Aluminium (34.1%) until

Al(OH)₃ saturation.

As a conclusion, in theory, short term, mid term and long term contact should be considered with different ratio in Al(OH)₃analogy.

For inhalation study : knowing that maximum CaAl₂O₄content is 70% and maximum percentage under 100 microns (soluble fraction ) is 15% , the ratio between substance and Aluminium equivalent Al(OH)₃ is 0.7 x 0.15 x 0.341 = 0,036. or 1g of Aluminium equivalent Al(OH)₃ = 28 g of substance.

For oral, dermal and ecotoxicological studies : knowing that maximum CaAl₂O₄content is 70% , the ratio between substance and Aluminium equivalent Al(OH)₃ is 0.7 x 0.341 = 0,2387. or 1g of Aluminium equivalent Al(OH)₃ = 4,2 g of substance.

 

With contact of dissolved CO₂ in water, Ca₃Al₂O₆,6 H₂O dissolves and precipitation of CaCO₃occurs.The precipitate CaCO₃is not considered to be toxic, so the toxicity and ecotoxicity investigations will be performed on Al(III) compounds.

NB: Correspondence between Cement Chemistry Notation and Chemical Notation

CA = CaAl₂O₄

C3AH6 =Ca₃Al₂O₆,6 H₂O

AH3 = Al(OH)₃

 

Ref 1: “Investigation of the CaO-Al₂O₃-SiO₂-H₂O system atby thermodynamic calculations”, D.DAMIDOT-F.P.GLASSER, Cement and Concrete Research , vol 25(1) pp22-28 (1995)