Effect of calcium sulphate bearing materials on physical and mechanical properties

Previous research has shown the importance of cement paste microstructures on its physical and mechanical properties (Thomas & Jennings, 2006). In this section, the effect of calcium sulphate types and dosage on the initial, final setting times and early compressive strengths is assessed. Figure 4.13 shows the variations in strength of mortars with hemihydrate and natural gypsum at various concentrations. Each compressive strength value was an average of six collected readings, as detailed in Appendix D.

Figure 4.13: Compressive strength of cement mortar with natural gypsum and hemihydrate at different concentrations hydrated during 1 day

A quadratic relationship between the compressive strength of mortars and the concentration of both calcium sulphate bearing materials was observed. The maximum strength was obtained at a concentration of 4% for both natural gypsum and hemihydrate. Above this concentration, mortars experienced a decrease in strength that was more pronounced in the presence of natural gypsum and progressive when hemihydrate was used. In all cases, mortar prepared with hemihydrate had higher compressive strength than those with natural gypsum. The average difference in strength between the two calcium sulphate bearing materials was approximately 15%.

The strength improvement at low calcium sulphate concentrations, as observed in Figure 4.13, would probably be related to low SO3 prevailing within the cement systems, as shown in Table

3.3, and moderate ettringite formation. The latter phenomenon could be due to the fact that cement systems with small total SO3 contents are expected to have low SO4-2 concentrations at

early hydrations. Conseqently, less ettringite would precipitate, thereby availing more space for C3S hydration. This resulted in mortar strength improvements. On the other hand, the decrease

in strength at higher calcium sulphate concentrations could be attributed to important ettringite formations within the cement system due to higher total SO3 contents. According to Zhang et al.

(2018), the hydration process of C3S is usually limited in cement systems with a higher quantity

of ettringite which covers the unhydrated cement grains. Besides, cement microstructures experience excessive expansion as a result of this large amount of generated ettringite. This increases porosity within the system that significantly reduces its overall strength.

As a matter of fact, the hydration kinetics of cement systems as presented in Figure 4.1 and Figure 4.2 supports this hypothesis. During the acceleration period, the reaction rate of cement systems seemed to decrease with the increase in calcium sulphate concentrations. In addition, Figure 4.4 showed that the reaction rate of cement with natural gypsum was lower when compared to that with hemihydrate. This has also been observed by Quennoz (2011) who noticed that with a significant amount of sulphate ions within the suspension, the specific surface areas of the anhydrous silicate and aluminate compounds decrease. Because of a large amount of ettringite and C-S-H formed, the space available for the reaction of these cement phases also decreases, thereby affecting the reaction rate of the overall system. The phenomenon observed in Figure 4.10, showing a decrease in the amount of portlandite with the increase in calcium sulphate concentrations at the time before the exhaustion of sulphate and lower strengths of natural gypsum compared to hemihydrate, can then be explained. Figure 4.14 shows the initial and the final setting times of cement pastes with different concentrations of calcium sulphate.

Figure 4.14: Final and Initial setting times of cement paste with natural gypsum and hemihydrate at different concentrations

In the presence of natural gypsum, the setting times of cement systems seemed to increase linearly with the increase in concentration. A quadratic relationship between the setting times and the concentration of calcium sulphate was observed when hemihydrate was used. Changing the concentration of calcium sulphate from 2% to 7% induced an increase of 15% in final setting time when natural gypsum was used, while a decrease of about 12% was estimated when hemihydrate was used as set regulator. In the same way, a 19% increase in initial setting time was noticed when natural gypsum was used, while a decrease of around 33% was observed in cement pastes with hemihydrate.

In all cases, the final setting times of systems with hemihydrate were shorter when compared to those with natural gypsum. However, at concentrations below 4%, the initial setting time of cement pastes seemed to be independent of the type and concentration of calcium bearing materials.

Cement setting is a consequence of C-S-H precipitation that strengthens the newly formed microstructure and enables the cement paste to withstand some stresses (Hildago et al., 2008; Bishnoi & Scrivener, 2009; Roussel et al., 2012). The kinetics and reactivity induced by the presence of these calcium sulphate bearing materials elucidate their respective setting time values.

First of all, the formation of C-S-H appeared to be favoured in systems with hemihydrate, as their hydration rate was more improved than those with natural gypsum (see Figure 4.4). Also, the dormant phase of systems with hemihydrate were shorter than those with natural gypsum (see Figure 4.5). This suggests that the onset of the acceleration period would occur earlier, justifying the shorter initial setting times obtained in systems with hemihydrate than those with natural gypsum. These observations are in agreement with those of Nelson et al. (1990) and Pedrajas et

al. (2014) who noticed that cements with long induction or dormant phases experienced higher

initial setting time values that improved their pumpability.

Secondly, the short final setting times in systems with hemihydrate could be due to the amount and rate of ettringite formation during hydration. Actually, the precipitation of these hydrate products allows the formation of a network structure that is able to enclose a large amount of free water necessary for the progression of hydration reactions (Zhang et al., 2018). Xu et al. (2012) have also observed disparities in setting times of mortars with different calcium sulphate bearing materials used as set regulators, attributing these differences to the solubility of the set regulators and the formation of ettringite.

4.6

Conclusion

The influence of the type and concentration of natural gypsum and hemihydrate used as set regulators in OPC was investigated. The kinetics and reactivity induced by these calcium sulphate bearing materials were studied and correlated to the mechanical and physical properties of corresponding cement systems.

The hydration kinetics of all prepared cement systems described all four hydration stages pertaining to that of normal OPCs. In particular, cements with hemihydrate presented shorter durations of dormant periods than those with natural gypsum. The reaction rates of cement systems with hemihydrate were faster than those with natural gypsum and generally tended to

decrease with the increase with concentration. This behaviour was attributed to the unavailability of possible reaction spaces within the system due to the early precipitation of important hydrate products.

The consumption rate of calcium sulphate was higher in cement systems with hemihydrate than those with natural gypsum, with an averaged difference of 19%. Generally, this rate seemed to increase with the increase in calcium sulphate concentration. Consequently, cement with hemihydrate experienced higher degrees of hydration, especially at times corresponding to the exhaustion of sulphate ions within the system. After the depletion of sulphate, all cements had almost equal degrees of hydration.

In these events, hemihydrate systems had more ettringite and portlandite formed than that of natural gypsum. However, the amount of ettringite increased with the increase in calcium sulphate concentration to a certain extent (4%), and thereafter remained constant regardless of the dosage within the system. Conversely, the amount of portlandite decreased with the increase in calcium sulphate and also remained unchanged after 4%. These phenomena were attributed to the availability of SO4-2 ions and their interactions with Ca+2 ions which were conditioned by the

dosage of calcium sulphate within the system.

The evaluation of the enthalpy formation of these early hydrate products attested the formation of AFm phases in the cement systems. However, while this hydrate phase prevailed in systems with natural gypsum, it was not depicted in those with hemihydrate after 30 minutes of hydration as observed with the scanning electon microscopy. Additionally, the estimated energy formation of ettringite phases revealed that those generated from hemihydrate were less stable than those from natural gypsum. This implied that the phenomenon observed during the deceleration period was mostly related to the remaining hydration space on the C3A compound after the depletion of

sulphate within the system. It was also established that mechanical and physical performances of cement systems were dictated by the kinetics observed at microstructural level.

Generally, cements with hemihydrate had higher compressive strengths when compared to those with natural gypsum, with a maximum strength for both calcium sulphate bearing materials achieved at 4% concentration. This early strength development was attributed to the C3S

hydration, whose hydration space depends on the amount of ettringite formed.

In the same way, the amount of ettringite increased with the increase in calcium sulphate concentration to a certain extent (4%), and thereafter remained constant regardless of the dosage within the system. Conversely, the amount of portlandite decreased with the increase in calcium

sulphate and also remained unchanged after 4%. An increase of 15% in the final setting time for natural gypsum systems in comparison to a 12% decrease in those with hemihydrate were observed when the dosage of calcium sulfate was varied from 2% to 7%. A 19% increase and 33% decrease were noted, respectively, in the initial setting times of natural gypsum and hemihydrate systems.

This behaviour was in connection with the kinetics of the cement systems in terms of their hydration rates and dormant phase durations. The formation of a network by the generated ettringite and the solubility of the set regulators used was also suspected to be responsible for these cement physical properties.

Chapter 5 Rheokinetics of cement paste with mix proportion