Structure of Na3[Al(L-Lactate)3]2·6H2O Crystallized from Lohtragon AL 250—A Commercial Cement Adjuvant

Abstract

Aqueous aluminum(III) solutions at medium pH are used for industrial applications such as cement adjuvants. The necessary high Al3+ concentrations are achieved by using complexing ligands, such as lactic acid, to prevent precipitation. Here, we grew crystals from commercially available neutralized aluminum lactate solutions to derive an understanding of the underlying aluminum lactate chemistry. Using X-ray diffraction in combination with multinuclear NMR spectroscopy and quantum mechanical calculations, we found that the crystals have a stoichiometry of Na3[Al(l-lactate)3]2·6H2O and consist of negatively charged [Al(l-lactate)3]23– bis-clusters. The remarkable stability of these clusters is achieved by linking both Al(l-lactate)3 subunits via three very short symmetrical hydrogen bonds between the α-hydroxy groups with half of them being deprotonated. The trends for both the 27Al chemical shift and the spin–spin relaxation for the Lohtragon-type solutions as a function of pH suggest the bis-cluster to be...