Necessity of Adding Gypsum (CaSO4) in Cement Manufacture Portland is obtained from grinding clinkers and usually sets and hardens immediately after the addition of water. In order to slow down the setting time of cement, gypsum @ 3% to 4% is added during the process of clinker grinding.
- If the quantity of gypsum(CaSO4) is more, the cement becomes very slow setting.
- It also hardens slowly which results in delay the removal of formwork,
- Gypsum combines with tricalcium aluminate and prevents flash setting,
- But, if more gypsum is added with cement, the excess amount after combining with tricalcium aluminate remains free in cement.
It expands and makes the cement unsound,
Read Also:
: Necessity of Adding Gypsum (CaSO4) in Cement Manufacture
Retarders – Calcium sulphate, usually as gypsum, is universally added to ground cement to control the otherwise rapid ‘flash set’. Many other compounds have a retarding effect and these have been put on a systematic basis by Forsen, according to their effect on the solubility of alumina.
Following his categorization, retarders may be divided into four sets depending on their actions as a function of concentration. Typical examples from each group as (i) CaSO 4 ∙2H 2 O, (ii) CaCl 2, (iii) Na 2 CO 3, (iv) Na 3 PO 4, Type (iv) retarders may hold up setting and hardening indefinitely if used in sufficient quantity, but they are not all harmful and some, such as the calcium lignosulphonates, are used as water-reducing agents.
Read full chapter URL: https://www.sciencedirect.com/science/article/pii/B9781855733480500148
Contents
What is the chemical name of CaSO4?
| Names | |
|---|---|
| Other names Plaster of Paris Drierite Gypsum | |
| Identifiers | |
| CAS Number |
|
| 3D model ( JSmol ) |
Interactive image |
| ChEBI |
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| ChEMBL |
ChEMBL2106140 |
| ChemSpider |
22905 |
| DrugBank |
DB15533 |
| ECHA InfoCard | 100.029.000 |
| EC Number |
231-900-3 |
| E number | E516 (acidity regulators,,) |
| Gmelin Reference | 7487 |
| KEGG |
|
| PubChem CID |
(dihydrate): 24928 |
| RTECS number |
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| UNII |
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| CompTox Dashboard ( EPA ) |
DTXSID9029699 |
| show InChI | |
| show SMILES | |
| Properties | |
| Chemical formula | CaSO 4 |
| Molar mass | 136.14 g/mol (anhydrous) 145.15 g/mol (hemihydrate) 172.172 g/mol (dihydrate) |
| Appearance | white solid |
| Odor | odorless |
| Density | 2.96 g/cm 3 (anhydrous) 2.32 g/cm 3 (dihydrate) |
| Melting point | 1,460 °C (2,660 °F; 1,730 K) (anhydrous) |
| Solubility in water | 0.26 g/100ml at 25 °C (dihydrate) |
| Solubility product ( K sp ) | 4.93 × 10 −5 mol 2 L −2 (anhydrous) 3.14 × 10 −5 (dihydrate) |
| Solubility in glycerol | slightly soluble (dihydrate) |
| Acidity (p K a ) | 10.4 (anhydrous) 7.3 (dihydrate) |
| Magnetic susceptibility (χ) | -49.7·10 −6 cm 3 /mol |
| Structure | |
| Crystal structure | orthorhombic |
| Thermochemistry | |
| Std molar entropy ( S ⦵ 298 ) | 107 J·mol −1 ·K −1 |
| Std enthalpy of formation (Δ f H ⦵ 298 ) | -1433 kJ/mol |
| Hazards | |
| NFPA 704 (fire diamond) | 1 0 0 |
| Flash point | Non-flammable |
| NIOSH (US health exposure limits): | |
| PEL (Permissible) | TWA 15 mg/m 3 (total) TWA 5 mg/m 3 (resp) |
| REL (Recommended) | TWA 10 mg/m 3 (total) TWA 5 mg/m 3 (resp) |
| IDLH (Immediate danger) | N.D. |
| Safety data sheet (SDS) | ICSC 1589 |
| Related compounds | |
| Other cations | Magnesium sulfate Strontium sulfate Barium sulfate |
| Related desiccants | Calcium chloride Magnesium sulfate |
| Related compounds | Plaster of Paris Gypsum |
| Except where otherwise noted, data are given for materials in their standard state (at 25 °C, 100 kPa). verify ( what is ?) Infobox references |
Calcium sulfate (or calcium sulphate ) is the inorganic compound with the formula CaSO 4 and related hydrates, In the form of γ- anhydrite (the anhydrous form), it is used as a desiccant, One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum,
Does CaSO4 affect the phase conversion of calcium aluminate cement?
Abstract – Conversion from metastable phase to stable phase in calcium aluminate cement (CAC) causes volumetric instability, resulting in loss of compressive strength. This study investigated the effect of CaSO 4 on the phase conversion of calcium aluminate cement.
- The specimens with different dosage of CaSO 4 were exposed to 60 °C to trigger the phase conversion of CAC after curing at 20 °C for 7 days.
- Analysis with X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, and scanning electron microscopy were conducted.
- The incorporation of CaSO 4 inhibited the formation of unstable CAH 10 by direct precipitation of ettringite.
As a result, the reduction of strength of CAC specimens due to the increased pore volume by conversion was inhibited by ettringite and monosulfate formation.
What is CaSO4 anhydrite and hemihydrate?
Hydration states and crystallographic structures – The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals:
- CaSO 4 ( anhydrite ): anhydrous state. The structure is related to that of zirconium orthosilicate (zircon): Ca 2+ is 8-coordinate, SO 2− 4 is tetrahedral, O is 3-coordinate.
- CaSO 4 ·2H 2 O ( gypsum and selenite (mineral) ): dihydrate.
- CaSO 4 · 1 / 2 H 2 O ( bassanite ): hemihydrate, also known as plaster of Paris, Specific hemihydrates are sometimes distinguished: α-hemihydrate and β-hemihydrate.
What are the three levels of hydration of CaSO4?
The compound exists in three levels of hydration corresponding to different crystallographic structures and to different minerals in nature: CaSO 4 ( anhydrite ): anhydrous state. CaSO 4 · 2 H 2 O ( gypsum and selenite (mineral) ): dihydrate. CaSO 4 · 1⁄2 H 2 O ( bassanite ): hemihydrate, also known as plaster of Paris.
