Research focus

Calcium sulfate binders
In German language, binder materials based on calcium sulfate are referred to as “Gips”. The correct chemical name for gypsum is calcium sulfate dihydrate, which is not the binder but the hydration product of anhydrite (CaSO4) and calcium sulfate hemihydrate (α-, β-CaSO4 · ½H2O) binders. In nature, gypsum can be found as rock in numerous large deposits and is mined all over the world. In addition, gypsum can be obtained as chemical waste or by-product from industry. Since mid eighties, anhydrite from flue gas desulphurization (FGD anhydrite) plants is obtained in a high amount. So called synthetic anhydrite is produced as a by-product from industry during synthesis of different chemical compounds, e.g. the synthesis of hydrofluoric acid from fluorspar and sulphuric acid.
Ready-made building material systems are produced as a combination of CaSO4-binder, aggregates and chemical admixtures. Therefore it is of interest to understand the interaction between the different additives in the binder system.
At the chair for construction chemicals, we investigate the working mechanism of polycarboxylate superplasticizers in calcium sulphate systems. Superplasticizers already work efficiently at very low dosages of approx. 0,05 - 0,2% bwob (by weight of binder) and can improve the fluidity of the binder at constant w/b (water to binder) ratio. They allow reducing the w/b ratio by up to 30% while retaining the same fluidity. This implies a reduced pore volume and therefore an increased compressive strength and durability. We investigate the importance of the structural conformation and molecular size of polycarboxylate-type (variation of the side and main chain length and the anionic charge) for their dispersing action on CaSO4-phases. Fig. 1 shows AFM (Atomic Force Microscopy) micrographs of polycarboxylate molecules with long side chains and low anionic charge on a graphite surface.


Fig. 1:   AFM-micrographs of PC 45 PC 1,5.

In addition, research is carried out on the effect of inorganic salts on the properties of calcium sulphate systems. The sulfate activation through K2SO4 causes formation of double salts such as syngenite (K2SO4 · CaSO4 · H2O) which can accelerate the hydration of anhydrite. However, the presence of soluble sulfate in the binder can perturb the flowability of certain superplasticizers (Fig. 2). 
 
Fig. 2: Effect of the K2SO4-dosage on the adsorption of various PCs on FGD anhydrite.

This effect is based upon competitive adsorption between the sulfate anions and the polycarboxylate molecules on the surface of the binder.
Furthermore, work is being carried out to investigate the influence of retarders such as sodium polyphosphate (Fig. 3) on the hydration of alpha calcium sulfate hemihydrate. The effectiveness of these solidification regulating additives is based on the high affinity of polyphosphate to calcium ions. Thus, the latter are not available for further hydration.


Fig. 3: Chemical structure of linear polyphosphate.