SEARCH WITHIN CONTENT
Citation Information : Architecture, Civil Engineering, Environment. Volume 12, Issue 1, Pages 81-89, DOI: https://doi.org/10.21307/ACEE-2019-007
License : (CC-BY-NC-ND 4.0)
Received Date : 15-July-2018 / Accepted: 30-November-2018 / Published Online: 20-May-2019
The paper presents the results of rheological research, conducted to examine how the properties and composition of cement affects the yield stress and plastic viscosity of Portland limestone cements. Portland limestone cements used in the research were obtained by mixing four Portland cements of various chemical and phase compositions with two limestones of different origin, in an amount of 6, 10, 20 and 30% of cement mass. The yield stress and plastic viscosity were determined using the Viskomat NT rheometer, after 5 and 60 minutes from combining the constituents. Obtained results indicate a significant effect of the C3A content in cement on the rheological properties of mortars, mainly yield stress. The specific surface area of the tested cements has a lesser effect on the rheological properties of mortars with Portland limestone cements.
Portland limestone cement CEM II/A, B-LL is widely used in many European countries, and the limestone is the most frequently used non-clinker constituent of cement, according to CEM Bureau statistics . In Poland, the consumption of limestone as a main constituent of cement is extremely limited. Limestone is mainly used as a secondary constituent of cement and a constituent of composite cements, such as CEM II/A,B-M (V,LL) and CEM II/A,B-M(S,LL). Due to the limited amounts of ground granulated furnace slag and fly ash, limestone is going to be used more widely [2, 3].
Portland limestone cement CEM II / A, B-LL is characterized by a number of properties advantageous in construction practice, such as lower water demand and better workability, and, in combination with other mineral additives, even an increase in the durability of concrete [4, 5, 6]. However, the influence of limestone on the number of properties of mortars and concretes with limestone cements, such as setting time and rheological properties, is ambiguous [4, 7].
The results of the research  indicate a reduction of the yield stress and plastic viscosity with the increase in limestone content in cement, while the results of studies  indicate the opposite relationships. Authors of works [3, 10] showed a decrease in the yield stress and an increase of plastic viscosity with an increase in the limestone content. The results of tests are usually only related to the chemical and physical properties of limestone, whereas the influence of clinker or Portland cement itself is rarely taken into account [3, 8, 9, 11]. Seeing as rheological properties of the mortars and concrete mixes are important for predicting the workability and workability loss in time, this discrepancy should be a subject of further in-depth research.
The purpose of the presented research was to determine how the phase composition and of Portland limestone cement affects the rheological properties (yield stress and plastic viscosity) of CEM II/A, B-LL limestone Portland cements. Initial research in the topic was presented in . In the experimental studies, four Portland cements CEM I were used, which were mixed with two ground limestones of various origin and grain size, in the amount of 6%, 10%, 20% and 30% of the cement mass. Thus obtained mortars have been subjected to rheological tests in a rheometer after 5 and 60 minutes from the moment of adding water to the binder.
Four kinds of commercial Portland cements were used in the research – CEM I 42.5 R NA, CEM I 52.5 R, CEM I 42.5 R “O”, CEM I 42.5 N SR 5/NA. The cements were chosen on the basis of their different phase composition, especially concerning C3A content, and different specific surface area. Cement properties are shown in Tables 1–3.
For the research, two LL limestones were used: limestone B with Blaine’s specific surface 5630 cm2/g, limestone T – with specific surface 4790 cm2/g. The chemical composition of both limestones is shown in Table 4, and the granulometric composition in Fig. 1.
Portland limestone cements were obtained by homogenizing the ingredients. Prepared Portland limestone cement contained 6%, 10%, 20% and 30% of limestone. The compositions of the tested mortars were similar to standard mortar according to EN 196 , however, due to physical limitations of the rheometer, the water-cement ratio of the tested mortars had to be set to 0.55 instead of 0.5, as the consistency of mortars with water-cement ratio 0.5 was too stiff for the rheometer to complete the measurement. Standard sand with grain distribution according to EN 196  was used as an aggregate.
The rheological properties of the mortars were tested in the Viskomat NT rheometer (Fig. 2). Measurements were conducted 5 and 60 min after adding water to the cement and sand. The test consists of measuring the torque on the probe, exerted by the mortar placed in a metal cylinder, which rotates at a changing speed. Due to the fact that numerous studies indicate that the mortars behave as the Bingham’s viscoplastic body, a simplified Bingham model was adopted for calculations of the results of the rheometry tests [14–17].
Where: M – torque, N – rotational speed of probe or measuring cylinder, g – shear resistance, h – plastic flow resistance. Using equation (1), it is possible to calculate shear resistance and plastic flow resistance on the basis of torque measured at different rotational speeds. Shear resistance g corresponds to the yield stress τ0 and the plastic flow resistance h – plastic viscosity ηpl, and are further referred as such.
Exemplary measurement with the method of obtaining yield stress g and plastic viscosity h is shown in Fig. 2.
There were 3 measurements conducted for each configuration, and on this basis, confidence intervals were calculated.
Mortar with cement CEM I 42.5 R “O” was characterized by the highest yield stress. The yield stress for this cement was so high that it was difficult to perform measurements in the rheometer, and in the case of cements with limestone B content above 10% it was not possible to perform the measurement.
After 60 minutes from mixing, the mortar yield stress increased by 30–40%. In the case of CEM I 42.5R “O” cement, the increase of the yield stress made it impossible to perform the test.
The addition of limestone to cement does not change or slightly decreases the increase of the yield stress in time, but there is no clear difference between the influence of limestone T and B.
It can be noticed that limestones T and B have different effects on the rheological properties of mortars. Limestone T does not change or slightly decreases the yield stress g of mortars and does not change plastic viscosity h. Addition of up to 5% of limestone B does not change the yield stress and slightly decreases the plastic viscosity, while the higher content of limestone B in CEM II/A, B-LL clearly increases the yield stress (Fig. 4) and decreases the plastic viscosity (Fig. 5).
The difference in the influence of T and B limestones on rheological properties can be attributed to the differences in their particle distribution, that was shown in Fig. 1. Limestone T is characterized by a particle distribution similar to Portland cement, and thus its impact on rheological properties is small. Limestone B, on the other hand, contains coarse fractions, above 100 µm, which do not occur in limestone T and Portland cement. The content of coarse fraction may increase the yield stress.
The influence of C3A content in the cement and specific cement surface on the yield stress of mortars from CEM II/A, B-LL cements is shown in Fig. 6 and 7.
The studies show a clear correlation between the content of C3A phase in the cement and the yield stress. This is consistent with the current state of knowledge, as the relationship between high content of C3A and high yield stress was indicated by studies [16, 18]. After 60 minutes from adding water to binder in tested mortars, the relationship between the content of C3A and the yield stress is less pronounced.
In the case of CEM II/A, B-LL cement with limestone T, the relationship between the content of C3A and the yield stress is more pronounced than in the case of cement with limestone B. This is due to the fact that the presence of limestone T does not notably affect the yield stress of the mortar. In the case of cements with a high content of limestone B (20%, 30%), the effect of C3A content is less pronounced. This is due to the fact that the high content of limestone B increases the yield stress g of mortars. This effect is the most pronounced for cements based on CEM I 42.5R NA (C3A content 7%), as can be seen in Fig. 3, and for this cement with limestone B content over 20%, the influence of limestone on the yield stress is more pronounced than of the C3A content. Therefore, yield stress of mortars with CEM I 42.5R NA (7% C3A) with 20–30% of limestone B is higher than yield stress of mortars with CEM I 52.5R (9% C3A), as the limestone B increases its yield stress to a lesser degree. The source of the discrepancy might be linked to a particle size distribution of cements, however, it requires further research.
Despite numerous publications, including [14, 16, 18, 19, 20], indicating that the specific surface area of cement can affect the yield stress to an extent comparable to the influence of the content of C3A, in the case of the tested Portland-limestone cements CEM II/A, B-LL no unambiguous correlation was observed, neither after 5 min nor after 60 minutes from mixing of the ingredients. The results of measurements of three of the four cements (CEM I 42.5R NA, CEM I 52.5, CEM I 42.5N SR5/NA) indicated an increase of the yield stress with increasing specific surface area. Very high yield stress of CEM I 42.5R “O” cement, associated with very high C3A content, did not conform to this relationship. This may indicate a greater effect of the C3A content than the specific surface area on the yield stress of mortars in case of the cement tested.
The effect of C3A on plastic viscosity is ambiguous. It can be seen that for Portland cements without limestone, the plastic viscosity decreases with increasing C3A content in the cement. In the case of tested CEM II/A, B-LL cements, this relationship is not met for CEM I 52.5 R (C3A content – 9%) with limestone T and with limestone B, and for CEM I 42.5N SR5/NA (C3A content – 4%) with limestone B. This may this indicate the dominant influence of other material factors on plastic viscosity. It is important to remember the fact that the cements come from different manufacturers and the method of their preparation (type of furnace installation) may affect the reactivity of Portland clinker (cement).
The specific surface area S of the cement is also not a decisive factor (Figure 9). The tested Portland cements show a clear relationship between the plastic viscosity of the mortar and the specific surface area S of cement 5 minutes after mixing – the higher the surface, the lower the viscosity. This conclusion is consistent with the available literature in this regard, including [20, 21, 22].
In the case of tested CEM II/A, B-LL cements, both for limestone T and for limestone B, this relationship is not preserved. Mortars made of Portland cement CEM I 42.5R “O” with limestone B and Portland cement CEM I 52.5 R with limestone T and B are characterized by a high viscosity, despite their high specific surface area.
Combined effect of C3A and specific surface area (calculated as a product of these two values) on the plastic viscosity of the investigated cements is also ambiguous. This indicates the influence of other factors on the plastic viscosity, possibly related to the granulation of limestone. This issue requires further research and analysis.
The paper presents the results of rheological tests of mortars with Portland – limestone cements, obtained by homogenizing four commercial Portland cements with two types of limestone in amount of 6, 10, 20, 30% of cement mass. Tests consisted of measuring yield stress and plastic viscosity after 5 and 60 min from the moment of mixing. Investigated was the effect of properties of cement, namely: type and amount of limestone, C3A content in Portland cement and specific surface of Portland cement, on the rheological properties. Obtained results led to the following conclusions:
1. The phase composition of Portland cements CEM I has a clear effect on the yield stress of the tested cements. Cements with a higher content of C3A were characterized by a higher yield stress both in the case of CEM I cements and in the case of CEM II/A, B-LL cements (Fig. 6).
2. For Portland cements, the plastic viscosity decreases with increasing C3A content in the cement, what is consistent with known literature. In the case of tested CEM II/A, B-LL cements, this relationship is not unequivocal.
3. The specific surface area of the tested CEM II/A, B-LL cements has a secondary effect on the yield stress of the tested mortars in comparison to the effect of C3A content in cement (Fig. 7).
4. A clear relationship can be observed between the plastic viscosity of the mortar and the specific surface of the cement in case of the Portland cements CEM I (Fig. 8) – the higher the specific surface, the lower the plastic viscosity, what is consistent with the literature. However, no unambiguous relationship was observed between the surface area and the plastic viscosity of CEM II / A, B-LL cements.
5. The influence of limestone T and B on the rheological properties of mortar was different, which can be attributed to the difference in the particle distribution of both limestones. Addition of limestone T to cement in the amount of 6–30% of cement mass did not change the yield stress. Addition of limestone B in the amount of 6–30% caused an increase in yield stress – the higher the amount of limestone B, the higher the yield stress (Fig. 4, 5).
The initial version of the paper was presented during the 19th Conference of the Scientific Conference for Civil Engineering PhD Students, 17–18th May 2018 in Wisła under the title “Wpływ składu cementu na właściwości reologiczne zapraw z cementów portlandzkich wapiennych (Influence of cement composition on rheological properties of mortars with portland – limestone cement)”.