Hemihydrate cement hydration

In normal portland cement, hydration leads to the formation of ettringite. In many instances, the formation of ettringite leads to expansion. To prevent the formation of ettringite, a gypsum-based material with 75% hemihydrate, 20% portland cement, 5% silica fume, and a superplasticizer was fabricated. The pastes were cured in water for 1 to 10 minutes and subjected to DTA. In Fig. 10, thermograms show the stepwise dehydration endothermal peaks (shown upwards) at 150° and 200°C as is typical of gypsum. There was no indication of ettringite or monosulfate, which normally are identified by peaks at 125-130° and 190-195°C. 

It is common that commercial calorimeters have internal, automatic calibration. Although this makes a calorimeter user friendly, it is problematic if the user does not know whether the calibrations are accurate. One way to check whether the instrument is working properly and whether the user is performing the measurement in a correct way is to run a validation procedure, i.e. an experiment with a known outcome (proficiency test). A number of such chemical calibration systems are described by Wadso and Goldberg (2001) however, none is similar to cement hydration measurements. It is therefore of interest to establish reference cements - or other similar systems, for example, based on calcium hemihydrate - that can be used to validate the quality of calorimetric cement measurements in a laboratory. 

Likewise, the calcination of gypsum drives away water. Calcinating gypsum (composed of hydrated calcium sulfate) causes that part of the water combined with the calcium sulfate to evaporate, leaving a solid, friable residue usually known as plaster of Paris (composed of calcium sulfate hemihydrate) plaster of Paris is used as a cement and mortar as well as an efficient casting material (see Textbox 35). 

Plaster of Paris has long been used as a casting material, a cement, and a mortar. If mixed with water, plaster of Paris forms a very soft and pliable mixture. After a very short time, lasting only 5-8 minutes, the wet, pliable mixture sets, that is, it hardens into a stable, firm solid. The setting process entails the incorporation of water molecules (a process known as hydration) into the calcium sulfate hemihydrate and the consequent formation and crystallization of hydrated sulfate of calcium. In other words, when water is added to plaster of Paris, the two combine, again forming gypsum, which soon crystallizes into a hard solid mass ... 

One of the first hydrates to be investigated in detail was calcium sulfate hemihydrate (CaS04-l/2 H2O) which LeChatelier (he of the principle ) showed to be what forms when when the form of CaS04 known as plaster of Paris hardens the elongated crystals of the hydrate bind themselves into a cement-like mass. 

These discussions and doubts result first of all from the complexity of reaction of unhydrous cement phases with water. This can be illustrated taking into account a simple, on a first appearance, example of gypsum hemihydrate hydration. This reaction served as a basis of Le Chatelier s crystallization theory. The uncompUcated, at first sight, reaction ... 

The complexity of any individual binder reaction with water is well illustrated on the example of gypsum hemihydrate hydratioa The hydration process in the case of multiphase material as Portland cement is, must be much more complex. The reactions of individual cement phases with water occur simultaneously and interfere the presence of minor components, first of all alkalis and sulphates, modifies further the composition of the liquid phase. Therefore the rate of hydration of basic cement phases is strongly affected. 

At ambient or moderately elevated temperature calcium sulfate hemihydrate or anhydrite react with liquid water and convert to calcium sulfate dihydrate. Anhydrite-in may also react readily with water vapor, but only to hemihydrate, which subsequently may convert to dihydrate, if allowed to hydrate with liquid water. The hydration of hemihydrate and anhydrite is a through-solution reaction, associated— at appropriate water/cement ratios— with setting and hardening. The setting and hardening process is accompanied by an overall macroscopic expansion of the paste caused by the manner of the growth of dihydrate crystals. The extent of this expansion may be controlled by suitable additives. 

Type M expansive cement is a combination of ordinary Portland cement, calcium aluminate cement, and additional gypsum or calcium hemihydrate, typically in the ratio 66 20 14 (Mikhailov, 1960 Budnikov and Kravchenko, 1968). It may be produced by mixing these constituents in a plant. Alternatively, an expansive additive consisting of high-alumina cement, gypsum or henuhydrate, and hydrated lime may be added to the fresh concrete mix during mixing. By this approach the extent of expansion may be controlled by the amount of additive added to the mix. 

Observation of the hydration process indicated the following. In the first few minutes of the dissolution of free lime, the hydration of anhydrite and hemihydrate and the formation of calcium aluminate hydrate and monosulfate hydrate occur. Ettringite is formed within one hour, monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours, with the maximum heat ofhydration of C3S at about 10 hours. Measurements of the non-evaporable water indicate that the amount of combined water is 60-80% of the theoretically determined total amount of combined water at complete hydration. (The total amount of combined water was estimated to be 36%.) The amount of ettringite in the paste is estimated to be about 18-25% for the period one hourto seven days. The monosulfate content increases from about 10% at six hours to about 15-25%inone day. The alite in Reg Set cement paste is approximately 65-70% hydrated in one day and 8 0-95% hydrated in seven days. It is suggested that fluoride is tied up as A1(0H)2F. The possibility of fluoride-substituted ettringite and the formation of halo-aluminate hydrates of the form ( A CaXj wHjO is conceivable. 

Reg Set cement mortar with a citric acid retarder has greater strength than Reg Set cement regulated with calcium sulfate hemihydrate. The increased strength in Reg Set cement paste or mortar made with the addition of citric acid is usually a result of an increased degree of hydration, increased amount of ettringite, increased volume concentration of small pores, and lower total porosity. 

Calorimetry eurves forthe two Jet Seteements deseribed above are shown in Fig. 21. There are four main peaks in the heat evolution curves. The first peak appears irmnediately and is due to the following dissolution of free lime, hydration of anhydrite and hemihydrate, and the formation of C-A-H and monosulfate hydrate. The seeond peak is attributed to the formation of ettringite, the third to the formation of monosulfate hydrate, and the fourth peak to the formation of C-S-H. The overlap of the second and third peaks (cement B) and the larger third peak are attributed to active eonversion of ettringite to monosulfate hydrate. The broader fourth peak (eement B) occurred later indicating a less active formation of C-S-H gel than for cement A. 

Differential scanning calorimetry (DSC) is used to study the heat effects of changing the temperature. This is a very common technique in many fields in the cement field typical applications are, e.g. quantifying gypsum and hemihydrate in anhydrous cement (Dunn et al. 1987) or determining the free (freezable) water via low-temperature DSC, assessing freezing processes (Kaufmann 2004) or hydration kinetics (Ridi et al. 2003). 

Figure 2.8 Comparison between measured and calculated heats of hydration of a calcium aluminate cement blended with hemihydrate (with a ratio of calcium alumi-nate to hemihydrate of 80 20 by mass). The sample was hydrated at 20°C using a water-solid ratio of 0.40. Heat flow was calculated from quantitative XRD analyses by using the dissolution enthalpies of the anhydrous phases and the precipitation enthalpies of the hydrate phases. CA calcium aluminate Ett ettringite HH hemihydrate Ms monosulfate. (From Bizzozero, J., Hydration and dimensional stability of calcium aluminate cement based systems , PhD Thesis no. 6336, Ecole Polytechnique Fed rale de Lausanne, Switzerland, 2014.)...

Dry mix mortars often exhibit a quite complex mix composition, especially if they are rapid setting and/or rapid hardening. In the latter case, they generally contain binary or ternary binders based on calcium aluminate or calcium sulfoaluminate cements in blends with calcium sulfate without and with portland cement. Isothermal calorimetry is an efficient method to use for optimising mix designs of such mortars with respect to the hydration kinetics. As only small cement mortar or paste samples are used, the influence of the binder composition as well as of different combinations of accelerators, retarders, water reducers, plasticisers, etc. can quickly be tested. Two examples of how the amount of calcium sulfate addition is able to influence hydration kinetics are shown for blends of calcium aluminate cement with hemihydrate (Figure 2.22) and ternary binders based on port-land cement, calcium sulfoaluminate cement and anhydrite (Figure 2.23). 

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