Effect of cement characteristic, limestone and superplasticiser on the

superplasticiser on the rheokinetics of cement paste

Considering the above, it is clear how the rate of hydration kinetics is very important to the rigidification process of cement paste.Firstly, the difference in rheological behaviour of the cements could be depicted by the established rheokinetic coefficients such as self- acceleration constant, characteristic time, relative rate of hydration and real time. Secondly, these disparities were attributed to both physical and chemical properties of cements.

Previous studies conducted separately by Michaux et al. (1989) and Vikan (2005) concluded that during early hydration, C3A has a stronger influence on the rheological properties of the cement paste, while other cement compounds such as C3S have a higher impact at later stages. In fact, C3A has a higher reactivity as compared to the other cement compounds. For this reason, the rheological behaviour of cement paste is assumed to be affected mostly by the hydration of C3A cement phase in the early stage.

Effect of superplasticiser type on the hydration

The interactions between cement and superplasticiser are known to influence the flow properties of the cement paste. From the results obtained in this work, generally speaking, the addition of SP in the mixtures somewhat retards the hydration reaction, regardless of the dosage and SP type used. According to Puertas et al. (2005), Zhang and Kong (2015) and Lange and Plank (2016), SP is adsorbed on C3A particles, coating its surfaces, and by steric hindrance effect repulses adjacent particles. Trapped water between particles therefore improves the workability. This repulsion ability of SP depends on the type of superplasticiser used and on a few other cement properties such as alkali content in the clinker, cement fineness and calcium sulphate content.

In the case of SP1, the retardation effect is illustrated in Figure 4.6, showing that SP1 prolongs the hydration process of the four cements. C1 and C2 presented the highest characteristic time ϴ followed by C3 and C4. Cements with high C3A and alkali content resulted in low hydration rate, as for cement C1 (3.95, 0.75) and cement C2 (2.75, 0.41). Cements C3 (1.84, 0.29) and C4 (2.63, 0.33) had low C3A and alkali content which thus resulted in a high hydration rate that affected their hydration time. These results are in

The effect of extending four cements with limestone with addition of super- plasticisers on the hydration reaction of SCC cement

Rihab Elmakki

agreement with a previous study conducted by Bellotto (2013) who found that the rate of hydration increased with increasing C3A content in the absence of SP.

With the addition of SP2, cements behaved completely differently as compared to their rheokinetics behaviour when in the presence of SP1. In this case, the hydration rate was the lowest for C1 and increased through C4, C2 and C3 successively. The presence of sulphate alkali (K2SO4) in the cements was found to be the cause of this rheological behaviour. Table 3.2 shows the concentration of K2SO4 for each of the cements. In fact, according to Vikan et al. (2007), the interaction between cement and SP depends on SO42- concentration in the solution. Arcanite or sulphate alkali (K2SO4) decreases the adsorption of SP on cement compounds during the initial hydration. This causes more SP to remain in the solution, thereby increasing the dispersion between cement particles and consequently the flowability of cement paste. It is of great importance, therefore, to consider the concentration of K2SO4 in a given cement when a polycarboxylate-based SP is used. The higher the K2SO4 content, the longer the hydration reaction. In particular, it was observed that with the addition of SP2 to the cement pastes, cements C1 and C4 exhibited the highest characteristic time while cements C2 and C3 presented the lowest values at all SP2 dosages.

Looking at the effect of SP concentration on the rheological behaviour of cements, results showed that an increase in SP dosage decreased the rate of the hydration reaction, subsequently increasing the time required for the reaction to come to completion, as demonstrated in Figures 4.7 and 4.8. Both the characteristic time and real time of each cement increased with increasing SP dosage. In a separate investigation by El-Gamal et al. (2012), an increase in superplasticiser concentration delayed the rate of cement hydration, potentially leading to retardation of initial and final setting times.

It was also important to assess the effectiveness of the two superplasticisers used with the objective of determining which one had more effect on the rheokinetics of cement paste during hydration. In general, it was found that the use of SP1 further retarded the hydration with high values of characteristic time compared to those obtained when SP2 was used. This effect can be attributed to their difference in alkali, Na2O content (Table 4.6). The effect of the concentration of soluble alkali in the solution has been addressed by Vikan (2005). In addition, Wistuba et al. (2007) reported that Na2O led to a delay in the formation of ettringite in the early stages of hydration, implying a delay of the reaction between the remaining C3A and ettringite to yield more stable hydrates responsible for setting. This could explain the difference in hydration rates observed for the two superplasticiser types. It can also be

The effect of extending four cements with limestone with addition of super- plasticisers on the hydration reaction of SCC cement

Rihab Elmakki

suggested that SP1 would be better suited for applications where more time is required before the hardening of the paste.

Table 4-6: Na2O content for SP1 & SP2 Alkali content

(Na2O equivalent) (%)

SP1 SP2

<2.5 1.0

Effect of limestone on cement hydration

From the results, it can be observed that the addition of limestone in the cement paste with SP resulted in the lowest hydration rate. This was attributed to the interaction between SP, C3A and sulphate that mostly affects the hydration time of cement paste.Similar results have been recorded by Moratti et al. (2012) in their recent investigations, as they confirmed that the use of polycarboxylate as superplasticiser leads to the retardation effect of cement hydration when blended with limestone.

A number of studies conducted by other scholars such as Lerch (1946), Michaux et al. (1989) and Ghorab et al. (2010) indicated that in the presence of sulphate, cement paste with a higher C3A and alkali content exhibited a longer hydration time when compared to those with low C3A content pastes. In particular, Lerch (1946) found that regardless of alkali content in the clinker, cements with higher C3A content presented a low hydration rate due to the dissolution of sulphate and calcium ions which stem from the hydrolysis of various compounds (i.e. gypsum and lime in water). This lime-gypsum solution saturated the solution thereby depressing the solubility of alumina and consequently the hydration of C3A. Lerch (1946) revealed that in the case of cements with both low C3A and alkali content, the speed of their hydration in the presence of gypsum depends strongly on the formation of a crystalline hydrated calcium sulfoferrite which does not affect the hydration of C3A phase. Hence, hydration occurs at a higher rate. This was also confirmed by Ghorab et al. (2010).

The adsorption of SP on the cement particles depends on the competition between C3A and sulphate, as reported by Vikan et al. (2007) who found that for the initial hydration, sulphate is known to prevent the adsorption of SP on C3A, resulting in more SP remaining in the suspension, and thus allowing the repulsion effect between cement particles. Consequently, the hydration of cement paste would take longer in the presence of gypsum (Ghorab et al., 2010).

The effect of extending four cements with limestone with addition of super- plasticisers on the hydration reaction of SCC cement

Rihab Elmakki

It was also noted that when limestone was used with SP, the hydration reaction depended on the C3A content in cements. This can be explained by the fact that a higher amount of C3A leads to a stronger reaction with CaCO3 to form monocarbonate (Celik et al., 2015). This hydrate has been reported in the literature as an agent that indirectly provides a high stabilisation of ettringite around cement particles (Lothenbach et al., 2008; De Schutter, 2011; Matschei et al., 2007). Ettringites stem from the reaction between C3A and sulphate that precipitates on cement compounds and slows the hydration process.Cements with high C3A content (Table 3.2) in the presence of SP and limestone therefore exhibited a low rate of hydration, while those with a low content of C3A exhibited a rapid hydration rate, as presented in Figures 4.7 and 4.8.

The effect of extending four cements with limestone with addition of super- plasticisers on the hydration reaction of SCC cement

Rihab Elmakki