Determining the Amount of Reacted Slag in Blended Cement Using EDTA Method


1 Procedure of EDTA

A typical chemical reaction of a slag could be like the following [NIST, D.P Bentz],

C7.88S7.39M3A + 2.6CH + bH → 7.39C1.42SHmA0.046 + 0.66M4.6AHd

C=CaO, S=SiO2, M=MgO, A=Al2 O3, CH=Ca(OH)2, H=H2O.

The method EDTA (Ethylenediaminetetraacetic acid) has been described by Erntroy [Erntry, 1987]. The following is a short description of the method.

  • 1. 93.0g of disodium EDTA 2H2O is dissolved in a mixture of 250 ml triethanolamine and 500 ml water. The solution is transferred to an volumetric flask;
  • 2. 173 ml of diethylamine added and the mixture made up to 1000 ml with water;
  • 3. For the extraction, 50 ml of the above solution is pipetted into a beaker and diluted to approximately 800 ml with water;
  • 4. The solution is brought to a temperature of 20.0±2 ◦C and 0.5 g of the dried and powdered sample paste, weighed to the nearest 0.0001 g, sprinkled over its surface;
  • 5. The solution is stirred for 120 ± 5 min while maintaining the stated temperature and is then filtered under vacuum through a 90 mm diameter Whatman GF/C filter which had been previously washed with 100 ml of distilled water, dried and weighed;
  • 6. The residue is then washed 5 times with 10 ml lots of distilled water, fried at 105 ◦ C for 1 hour and weighed to the nearest 0.0001 g.
  • 7. Calculation of reaction degree of slag.

2 Calculation of reaction degree of slag

Take Msl g slag and MCH g Ca(OH)2 and MH g H2O as reactant. When the test age reaches, say 3 days, take two pieces of paste, dry and weigh
them, then,

  • 1. one is ground to powder for EDTA test;
  • 2. the other piece undergoes a ignition loss test (LOI), from the LOI, the slag mass fraction in the dried paste powder can be determined, the ignition loss of CH should be taken into account, for Ca(OH)2 is decomposed to CaO and H2O.

The LOI fraction is: fLOI, thus, 1 − fLOI is the mass fraction of CaO+slag, so the original slag content in the powder can be determined.

2.1 Correction of hydrotalcite

h= Mass of dried hydrotalcite formed from 1 g of MgO in the slag glass.

The value of h could be 2.35 g based on the assumption in the paper “Degrees of reaction of the slag in some blends with Portland cement”, there the hydrotalcite is considered as M5ACH7.

Therefore, the residue of slag should minus the residue formed from MgO. The content of MgO can be determined by the content of slag of the tested powder.

2.2 Correction of slag

A small amount of slag may be solved in the reagent EDTA solution.

It is better do a pure unreacted slag test using EDTA, to determine the fraction of slag dissolution.

If the dissolution of slag in EDTA is quite small, then there is no need to take account for correction for solved unreacted slag in EDTA reagent, assuming no unreacted slag is solved in the EDTA solution.

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