Insight into Metastable Lifetime of α‐Calcium Sulfate Hemihydrate in CaCl₂ Solution

Abstract

Alpha calcium sulfate hemihydrate (α‐HH), an important kind of cementitious material, is a metastable phase in CaSO4–H2O system. In CaCl2 solution, thermodynamic metastable window for α‐HH preparation from calcium sulfate dihydrate is constructed as a function of CaCl2 concentration (0–4.0m) and temperature (60°C–130°C), and then the metastability of α‐HH is characterized by the metastable lifetime (MLT). As the concentration of CaCl2 is raised from 1.5 to 4.0m and temperature from 80°C to 110°C, MLT of α‐HH decreases gradually from 2920 to 60 min. The reducing effect of CaCl2 on MLT arises from its contribution to lowering the total interfacial Gibbs free energy required for the subsequent phase transition of α‐HH to anhydrite, the thermodynamic stable phase. This work establishes a method to evaluate the phase metastability and provides a potential approach to optimize the reaction time in metastable material synthesis according to the MLT value for purity control.

Nucleation Pathway of Calcium Sulfate Hemihydrate (Bassanite) from Solution: Implications for Calcium Sulfates on Mars

“…However, bassanite precipitation induction times were measured as a function of supersaturation and temperature, from which the interfacial free energy (~9 mJ/m 2 ) 12 of bassanite nucleated in concentrated electrolyte solutions could be extracted. Other studies focused on establishing the temperature and salinity regions where bassanite is the main phase formed from direct precipitation experiments 7,13,14 . Interestingly, the temperature for the primary precipitation of bassanite can be significantly lowered by increasing the ionic strength of the reacting solutions, in particular by using highly concentrated electrolyte solutions 7,13,14 .…”

“…Other studies focused on establishing the temperature and salinity regions where bassanite is the main phase formed from direct precipitation experiments 7,13,14 . Interestingly, the temperature for the primary precipitation of bassanite can be significantly lowered by increasing the ionic strength of the reacting solutions, in particular by using highly concentrated electrolyte solutions 7,13,14 . For example, bassanite is formed already at 80°C in the presence of 4.3 M NaCl 7 , while a temperature of around 50°C is sufficient to produce pure bassanite from solutions containing >6 M CaCl 2 and small amounts of Na 2 SO 4 13 .…”

Inhibition of calcium sulphate hemihydrate crystallization under simulated conditions of phosphoric acid evaporation

“…The estimated value matches the expected value (~10 mJ/m −2 ) [ 61 ] for a sparingly soluble salt with a baseline of 11.9 ± 0.4 mJ/m 2 . Other reported values for bassanite in aqueous solutions are estimated at 9.9 [ 62 ] and 11 mJ/m 2[ 63 ] from induction time measurements as well ~6 mJ/m 2[ 64 ] determined from sessile drop contact angle measurements.…”

“…The estimated value matches the expected value ($10 mJ/m À2 ) [61] for a sparingly soluble salt with a baseline of 11.9 ± 0.4 mJ/m 2 . Other reported values for bassanite in aqueous solutions are estimated at 9.9 [62] and 11 mJ/m 2 [63] from induction time measurements as well $6 mJ/m 2 [64] determined from sessile drop contact angle measurements. Much like with the turbidity changes, PEI was the only additive that showed a significant change in the interfacial surface energy with an increased value of 14.3 ± 0.5 mJ/m 2 .…”

Preparation of High Percentage <i>α</i>-Calcium Sulfate Hemihydrate via a Hydrothermal Method

“…The metastable life time (MLT) period, in which a metastable phase maintains its form, of α-HH is affected by some factors, such as temperature, liquid medium, etc. [3] [7]. Due to α-HH's better paste workability and higher mechanical strength of the hardened material, it exhibits a notably advantageous performance over β-HH in a wide range of fields, such as modern construction industry, molding, binder system, orthopedic and other medical applications [2] [6].…”