EFFECT OF ETHYLENEDIAMINETETRAACETIC ACID SODIUM SALTS TO THE PROCESS OF GYPSUM CURING

The concentration dependence of the inhibitory effect of mono-, di-, tri- and tetrasodium salts of ethylenediaminetetraacetic acid (EDTA) on the gypsum curing process has been studied. The activity of these salts has been compared with sodium trisubstituted citrate, which is widely used as a curing retarder in commercially available gypsum formulations. It is shown that the disodium salt of EDTA has the greatest retadant activity of the studied salts. The optimal concentration range of working solutions of EDTA disodium salt is 0.025–0.05 M (0.19–0.37 %) without loss of mechanical strength of castings, but the inhibitory effect of EDTA disodium salt is 3–4 times weaker than sodium citrate. The range of molar concentrations of aqueous solutions of EDTA sodium salts from 0.1 M to 0.0031 M was studied. An analysis of the results shows that sodium salts of EDTA have an inhibitory effect during the curing of gypsum, but their effect varies quite a lot. The inhibition effect for both the beginning and the end of gypsum curing is most pronounced in disodium salt (Na2–EDTA), mono- and trisodium salts exhibit a slightly weaker inhibitory effect, and in tetrasodium salt the inhibition effect is minimal. A comparison of the action of EDTA salts with sodium trisubstituted citrate shows their lower relative activity, however, their study was interesting from the point of view of clarifying the mechanism of inhibitory action. A comparison of the stability constants of the calcium complex with EDTA (3.88·1010) and the calcium citrate complex (1.48·103) shows that the action of EDTA sodium salts is weaker than sodium citrate, despite the fact that the stability constant of the complex with EDTA is 7 orders of magnitude higher than in the case of the citrate complex. Obviously, this is due to the fact that the mechanism of complexation and binding of free calcium in the sealing solution is not the main one. It becomes clear that the processes of sorption of modifiers (EDTA or citrate and their both sodium and calcium salts) on the surface of growing germ crystals play a more significant and decisive role in the process of gypsum curing. A qualitative study of the strength properties of the obtained gypsum castings samples showed that in the studied range of concentrations of EDTA sodium salts, the strength of castings does not decrease to a concentration of 0.025 M, whereas for the two highest concentrations of sealing acid solutions (0.05 and 0.1 M), a slight decrease in the mechanical strength of the castings was observed.