Calcium sulfate

Summary

Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 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. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water. Hydration states and crystallographic structures The compound exists in three levels of hydration corresponding to different crystallographic structures and to minerals:

CaSO4 (anhydrite): anhydrous state. The structure is related to that of zirconium orthosilicate (zircon): Ca2+ is 8-coordinate, SO42- is tetrahedral, O is 3-coordinate.
CaSO4·2H2O (gypsum and selenite (mineral)): dihydrate.
CaSO4·1/2H2O (bassanite): hemihydrate, also known as plaster of Paris. Specific hemihydrates are sometimes distinguished: α-hemihydrate

Official source

https://en.wikipedia.org/wiki/Calcium_sulfate

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Self-levelling flooring compounds (SLC) are applied in thin layers (less than 1cm) to level floors before further finishing cover such as parquet or plastic cover. They are composed of a ternary binder system based on Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate to provide fast set. This mixed binder is modified by additives in order to obtain specific properties such as flow properties or defoaming. In total, more than 10 components make up the formulations of SLCs. SLCs are one of the most complex systems in the field of cementitious materials. It is known that ettringite is the main hydration product of mixed binder systems but the hydration development at early ages is little known. The main objective of this work is thus to clarify the mechanisms of the development of the hydration at early ages of ternary binder systems modified by additives. Two formulations were studied: a system in which the mixed binder system is dominated by PC and one in which it is dominated by CAC. Both systems exhibit the formation of ettringite, but the formation of this phase is mainly due to the hydration of monocalcium aluminate CA from the CAC. Thus, the PC formulation is not robust. Parameters such as the minor phases of the PC and the type of the calcium sulfate have a major impact on the hydration development. As CA is a minor constituent of this mixed binder system, the amount of ettringite formed is low. Thus, the PC dominated formulation reacts relatively slowly. The CAC dominated formulations react faster because the CA content is higher. The porosity is lower and internal and external parameters such as the minor phases of the PC and evaporation affect much less the development of hydration. As the CAC dominated system forms more ettringite, the chemical shrinkage is higher but the autogenous expansion measured is lower than that observed for the PC dominated formulations. Calcite is also added in the formulation in large amount as a filler to fill the porosity. The formation of monocarboaluminate from the reaction of calcite is also observed.

EPFL2007

Hydration and durability of fast hardening binders incorporating supplementary cementitious materials

Sarra El Housseini

The use of supplementary cementitious materials (SCMs) is widely considered the most promis-ing approach to reduce CO2 emissions relative to cement production. SCMs are not commonly present in binders used for special applications. On the other hand, due to their low reactivity at early age, cements incorporating SCMs such as Limestone calcined clay cement (LC3) exhibit low early age strength when compared to PC. This could restrict their field of applications and therefore solutions have to be found.This thesis focused on two main axes: The first one is to lower the CO2 footprint of ternary blends composed of Portland cement (PC), calcium aluminate cement (CAC) and calcium sulfate (Cs) using calcined clay as clinker substitute. The hydration of simple systems composed of PC with an increas-ing amount of CAC and Cs with and without calcined clay from 3 hours up to 90 days of hydration helped to understand and identify the contribution of each component. The results showed that in-creasing the CAC and Cs content in a PC system with and without calcined clay led to an increase of the ettringite content and a decrease of the portlandite content due to the portlandite consumption to form ettringite. The C3S hydration was not delayed in presence of CAC and Cs. However, the pres-ence of CAC, Cs and calcined clay led to slightly lower degree of hydration especially at later ages. The hydration and strength of a more realistic system "a fixing mortar formulation" with and without calcined clay was investigated. Overall, results showed that calcined clay can replace Portland cement by up to 20% with acceptable strength results. The second aspect on which this thesis focused was the influence of incorporating CAC and Cs as limestone substitute in LC3. The effect of the C/A molar ratio of the used CAC and its content were the main parameters investigated. The incorporation of 10% (CAC, Cs) improved the strength com-pared to LC3 at all ages. The massive ettringite formation during the first hours of hydration explained the early age strength improvement. Results showed that the C3S hydration was hindered when the C/A ratio of the CAC is equal to 1.7. The calculation of saturation indices indicated that a super satu-ration with respect to portlandite is likely needed to trigger the C3S hydration. The high late age strength in LC3-CAC blends was explained by a high volume occupied by hydrates in comparison with LC3 blends. In terms of durability, the resistance to chloride ion penetration was investigated through the mini-migration method. LC3-CAC blends showed a better chloride resistance than LC3, which is mainly explained by the porosity refinement compared to PC, and the decrease of the total porosity in comparison with LC3. On the other hand, the natural carbonation study showed that LC3-CAC blends have a higher carbonation rate when compared to PC. Contrary to PC, accelerated car-bonation of LC3 and LC3-CAC blends resulted in a decrease of the strength. However, the strength of carbonated LC3-CAC specimens is still at least similar to the non-carbonated PC specimen.

EPFL2022

Cement hydrates and their chemically bound water content are sensitive to changes in relative humidity (RH) and temperature. This may cause specific solid volume changes affecting dimensional properties of hydrated cement paste such as shrinkage, swelling and expansion, and therefore impact the performance and the transport properties of cementitious materials. The present thesis studies the impact of drying conditions on the structure and thermodynamic properties of crystalline cement hydrates in different hydration states (i.e. varying molar water contents). The cement hydrates studied include the most important AFm and AFt phases present in different types of cements. A novel multi method approach, including XRD, TGA, DSC, sorption balance measurements, sorption calorimetry and the hydrate pair ¿ humidity buffer method, was used to derive physico-chemical boundary conditions and the thermodynamic properties of the studied hydrated phases. The stability and hydration states of AFm phases depend on the anion content and the exposure conditions. Some phases such as monosulfoaluminate and hydroxy-AFm are very sensitive to different relative humidities. On the other hand monocarboaluminate and strätlingite present very good volume stability with varying RH. Ettringite shows a strong hysteresis during desorption/adsorption, and the thermodynamic properties associated with its decomposition and reformation were determined. The experimental results were included into a thermodynamic model capable of predicting the response of cementitious systems considering the ¿true¿ crystal water content on the final phase assemblage in two different scenarios: i) during hydration, even at conditions where the amount of water added was insufficient for complete hydration, and ii) during drying and re-wetting processes in already hydrated systems. Our work hence opens the possibility to model the response of various cement materials exposed to different climatic conditions and to engineer cementitious systems with respect to minimizing volume changes in the course of drying, which may positively impact properties like drying shrinkage and total porosity.

EPFL2015

Hydration and durability of fast hardening binders incorporating supplementary cementitious materials

Sarra El Housseini

EPFL2022

EPFL2007

EPFL2015