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The specific crystallographic form of inorganic crystalline materials not only determines the physical and chemical properties of the individual crystals but can also affect the bulk properties of a material. One such bulk material that is comprised of a crystal matrix is plasterboard. Inthis material, individual calcium sulphate dihydrate (CSD) crystals are interlocked to form a coherent structure. It has been shown that by adding certain chemical molecules to the standard formulation, that macro-scale properties can be altered. In this thesis, the option to modify the crystal interactions by tailoring the habit and morphology of this mineral was investigated. In the control system, the nucleation and growth of this mineral was observed to be a staged process. Firstly, monocrystals ofCSD expressing (010) > {120}>{111} faces were harvested. At a relatively high supersaturation, (100) contact twins were formed, and the twinning process effected the stabilisation of a new cohort of faces; {II O} which replaced the {120} forms. As the supersaturation fell over a time period (24 hours), the mode of twinning altered; contact twin variants were observed, and the incidence of penetration twinning facilitated by heterogeneous nucleation was significant. Three different types of additives were investigated to see what effect they might have on CSD crystallisation; these being; (i) low molecular weight carboxylic acids, (ii) low molecular weight phosphates and phosphonates and (iii) molecules that contain hydroxyl groups. Low molecular weight carboxylic acid, phosphate and phosphonate additives were found to affect the twinning process of CSD in a concentration dependent manner. At additive loadings the contact twinning seen in the control system was inhibited whereas higher additive concentrations inhibited penetration twinning; observations which suggest that the processes mediating the two types of twinning were subtly different. Additive-specific changes in the morphology of the crystals were also noted suggesting that molecular recognition is a key issue in the habit modification of this mineral. Of particular interest however was the continued dominance of the water stabilised (010) and {120} faces under all conditions which raises some interesting questions about the competing effects of hydrogen bonding and cation-mediated additive docking as processes directing additive interactions with crystal surfaces. In the case of the polyhydroxyl additives, contact twinning was also inhibited but penetration twinning persisted even when the additive was in excess (relative to the calcium concentration). As polyhydroxyls have low calcium stability constants, this suggests that the ability of these molecules to inhibit penetration twinning of CSD is related to their calcium binding affinity which will affect both super saturation and adsorption phenomena.

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