Calcium Sulphate Hemihydrate
Definition
CaSO4 . 1/2 H2O; produced by heating gypsum, rehydrates to gypsum on setting; the basis of plaster of Paris and most interior plasters.
- Formula
- CaSO4 . 1/2 H2O
- Made by
- Heating gypsum (dehydration)
- Sets by
- Rehydrating back to gypsum
- Common product
- Plaster of Paris, interior plasters
Common questions
Why is calcium sulphate hemihydrate useful for making tool or footwear impression casts?+
It sets by rehydrating into interlocking gypsum crystals, which reproduces fine surface detail from a mould with good dimensional stability once cured, which is why plaster of Paris and related dental-stone formulations are used for casting footwear and tool-mark impressions at scenes.
How can forensic analysis distinguish an interior wall plaster from patched or repaired sections?+
Differences in crystal habit, purity, and additive content between an original hemihydrate-based plaster and a later repair batch can be detected by microscopy or XRD, helping establish whether a wall area was disturbed or repaired after an original construction date.
Related terms
- ASTM C150
- US standard specifying Portland-cement composition limits and performance requirements by type (I through V); referenced in forensic mineral-phase characterisation.
- Clinker
- The nodular intermediate product of Portland-cement manufacture, consisting of C3S, C2S, C3A and C4AF mineral phases before grinding.
- ENFSI ENG-FG1
- The European Network of Forensic Science Institutes' guidelines for forensic geological evidence, published in 2018 by the Expert Working Group on Forensic...
- Ettringite
- A calcium sulfoaluminate hydrate that forms early in cement hydration and can re-form later from delayed sulfate attack; its presence or alteration...
- FTIR (Fourier Transform Infrared Spectroscopy)
- A vibrational spectroscopy technique used in toner analysis to identify the polymer backbone and functional group composition of toner binder material; ATR...
- Portland Cement
- A hydraulic binder produced by firing calcium and silica sources at 1450°C; the four clinker phases (C3S, C2S, C3A, C4AF) hydrate on...
- Portlandite
- Calcium hydroxide (Ca(OH)2) crystallised during cement hydration; a bright birefringent phase visible in thin section that dissolves progressively as concrete carbonates, providing...
- Pozzolan
- A silica- or alumina-rich material (fly ash, silica fume, volcanic ash) that reacts with portlandite to form additional C-S-H gel; used as...
- Rietveld Refinement
- A full-pattern fitting method for XRD quantification that models the entire diffractogram using crystal structure data for each phase, adjusting phase abundances...
- SEM-EDS
- Scanning electron microscopy with energy-dispersive X-ray spectroscopy; provides elemental composition of each paint layer at 1-5 µm spatial resolution.
- XRD (X-Ray Powder Diffraction)
- Analytical technique identifying crystalline phases by their unique d-spacing pattern; primary method for phase-composition quantification in building materials.