Keywords

1 Introduction

The manufacturing of cement contributes significantly to overall CO2 emissions at various stages of its production, starting from quarrying to packing. Out of all the processes involved, the CO2 emitted during limestone decomposition and burning of fuel in the kiln accounts to major portion of overall CO2 emission, which are unavoidable. In order to reduce the overall CO2 emissions, cement industry is constantly undertaking huge efforts like improving the processes by adopting state of the art technologies, utilisation of alternative fuels, etc. The present trend in cement industry is to achieve lower clinker factors in cements without compromising on performance. Binary and ternary blended cements are prepared by replacing some portion of clinker with one or more supplementary cementitious material (SCM). Calcined clay or metakaolin is one such SCM that can be used for replacing cement.

Metakaolin is obtained from calcining a clay having high amount of kaolinite (Al2Si2O5[OH]4) mineral. These kaolinitic clays are calcined at temperatures ranging from 500 to 850 °C to make it a pozzolanic material [1,2,3,4]. The temperature up to which it should be calcined is still a question of debate. It is reported that highest pozzolanic reactivity is achieved by calcining at 800 °C [1]. However, beyond 850 °C, if calcined, clay may transform to less reactive crystalline material like cristobalite and mullite [5]. Furthermore, it must be noted that even the duration to which the clay is subjected to calcination can influence the properties of calcined clay.

Calcined clay rich in amorphous alumina and amorphous silica readily reacts with water and Ca(OH)2 liberating enormous amount of heat. The reactivity of calcined clay depends largely on the type of cement and the same is delayed when used with low C3A cement [6]. It has been reported that the pozzolanic reaction is more in clays containing lesser amount of kaolinite replaced at higher levels [7]. Calcined clay contains high alumina and is finer than ordinary Portland cement (OPC). Thus, sulphates are added in extra quantities in calcined clay blended cements to control the hydration reaction. In addition to controlling the aluminate hydration, sulphate can also influence other reactions. Hence, it becomes necessary to investigate the influence of sulphate in cements blended with calcined clay. The current study is mainly focused on isothermal calorimetry and phase assemblage studies during initials ages of hydration.

2 Materials and Methods

The oxide composition of clinker and calcined clay used in making the cements is presented in Table 1. Cement with low C3A and low alkali content was intentionally chosen to better understand the reaction of calcined clay which is rich in aluminates. Precisely, the clinker contains 52.14% C3S, 29.08% C2S, 1.41% C3A and 17.36% C4AF.

Table 1 Chemical composition of clinker and calcined clay
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The calcined clay procured from industry was not calcined properly as some amount of kaolinite was still left in it. Consequently, the calcined clay procured was again calcined at 800 °C in a muffle furnace for 1 h. The X-ray diffractograms of both originally procured calcined clay and the treated calcined clay are shown in Fig. 1 and their phase composition is shown in Table 2. The amorphous content has been increased drastically after calcination indicating the complete conversion of kaolinite to metakaolin (Table 2). The treated calcined clay and the clinker are then subjected to particle size analysis. The particle size distribution results are presented in Table 3.

Fig. 1
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XRD of calcined clay (K-Kaolinite, Q-Quartz, A-Anatase, M-Muscovite and H-Haematite)

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Table 2 Phase composition of calcined clay before and after calcining in muffle furnace
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Table 3 Physical properties of clinker and calcined clay
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Calcium sulphate dihydrate obtained from Merck which is of laboratory grade with 99% purity was used in all the blends. The sulphate content in cements was varied as 2.5 and 5.0%. The proportions of various materials required in the preparation of each blend are shown in Table 4. The blend OPC-2.5 was treated as the control blend to which the other blends were compared.

Table 4 Material proportions for blend preparation
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3 Tests Performed on Cement

3.1 Isothermal Calorimetry on Cement Pastes

Isothermal calorimetry test was conducted to know the effect of sulphate addition on heat evolution in cements at early stage. All the materials required for the test were stored in a closed chamber where temperature was maintained as 27 °C always. The pre-conditioning of materials was done for 24 h. A fixed water to cement ratio of 0.40 was used to make the cement paste. After thorough mixing, the paste was transferred to calorimeter cups immediately. The cups were closed properly and placed inside the isothermal calorimeter (Calmetrix I-cal 8000) for logging. And the logging was carried out for 48 h.

3.2 X-Ray Diffraction on Cement Pastes

The cement paste was made as described in Sect. 3.1. The paste was filled in 2 cm diameter cylindrical tubes and allowed to mature till the day of testing. At 1 day and 3 days, the hardened paste was taken out of the tube and 3 mm thick slices were cut with the help of precision saw. The slices made were scanned in X-ray diffractometer (Bruker D8 Advance Eco). The scan was performed from 5° to 65° (2-theta) with a step size of 0.019° and 0.3 s per step. The scan files were analysed by performing Rietveld refinement by external standard method. High purity rutile was used as an external standard. The hydration products like ettringite and portlandite were quantified at 1 day and 3 days.

4 Results and Discussions

4.1 Hydration Studies

The rate of heat evolution curve for all the blends is shown in Fig. 2. The rate of heat evolution at induction period and at silicate peak and the slope of the curves at acceleration period are calculated and are presented in Table 5. The rate of heat evolved at induction period and at silicate peak is more for CC-2.5 compared to OPC-2.5 (Table 5). On the other hand, the rate of heat evolution at silicate peak is less for CC-5.0 compared to control blend. A clear aluminate peak is not observed in OPC-2.5 as it is a low C3A cement. However, in CC-2.5 and CC-5.0 the aluminate peaks occur at 9 h and 32 h, respectively.

Fig. 2
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Rate of heat evolution in blends OPC-2.5, CC-2.5 and CC-5.0

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Table 5 Salient features from calorimetric curves
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Among all the blends, the rate of heat evolution is more in CC-2.5 at induction period which indicates early onset of acceleration period. It is also evident from the slope of acceleration curve (Table 5) that the reaction occurs at a faster rate during the acceleration period in CC-2.5 blend indicating the early activity of aluminates. The higher rate of heat evolution in CC-2.5 blend during acceleration period is the combined effect of OPC hydration and pozzolanic reaction. The same is not observed in CC-5.0 blend because it has lower clinker content and due to significant delay in the hydration of aluminates. It is also seen that the reaction of aluminates is delayed significantly at higher dosage of sulphates. The aluminate hydration appears as a shoulder peak in CC-2.5 around 9 h and the same is observed as a broad hump in CC-5.0 with its peak occurring around 32 h (see Fig. 2).

The cumulative heat released curve for all the blends is shown in Fig. 3. It is clearly observed from the curves (see Fig. 3) that at the end of 48 h, calcined clay blends CC-2.5 and CC-5.0 are releasing more heat than the control blend OPC-2.5. At all times, the cumulative heat energy released is more in CC-2.5 compared to the other two blends. At initial stages, till 5 h the heat energy released is same for both OPC-2.5 and CC-5.0 but in OPC-2.5 the heat released increases gradually thereafter. However, after 30 h, the heat energy value increases rapidly because of delayed aluminate hydration in CC-5.0 blend. Also, the energy gap between OPC 2.5 and CC-5.0 goes on reduces. Finally, at 38 h, CC-5.0 curve crosses OPC-2.5 curve around 32 h indicating the delayed activity of aluminates. Overall, after 48 h, both calcined clay blends release more heat than the control one showing that the effect of reduction in clinker is well compensated by the calcined clay.

Fig. 3
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Cumulative heat energy released in OPC-2.5, CC-2.5 and CC-5.0 blends

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4.2 Phase Assemblage Studies

The phase assemblage studies are carried out on hardened cement paste specimens to know the effect of sulphate content on the formation of hydration products. The quantity of ettringite and portlandite formed at the end of 1 day and 3 days is presented in Table 6.

Table 6 Ettringite and portlandite formed in g per 100 g cement paste after 1 and 3 days
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The quantity of portlandite is increased in OPC-2.5 from 1 day to 3 days due to hydration of silicates. Likewise, in CC-2.5 there is an increase in portlandite content from 1 day to 3 days. However, in CC-5.0 blend, a reduction in portlandite content is observed from 1 day to 3 days. It is reported that in properly sulphated cement, in the presence of gypsum, the dissolution of Ca2+ ions increase [8]. Also, it has been reported that C3S hydration is increased when both C3A and sulphates are present and the same is decreased when aluminium ions are present [9]. In CC-5.0 blend, the portlandite content is more at 1 day compared to CC-2.5 might be due to enhanced silicate reaction at higher sulphate content.

In low C3A clinker, the reaction of calcined clay is slow [6] and hence there is an increase in portlandite content from 1 day to 3 days in CC-2.5 blend. However, the same trend is not observed in CC-5.0 blend. At higher sulphate dosages, i.e. in CC-5.0 blend, the rate of depletion of CaOH2 is faster than its rate of formation. Thus, a reduction is observed from 1 day to 3 days in portlandite content which may be due to more pozzolanic reaction.

In control blend OPC-2.5 the sulphate to alumina ratio is high and thus the quantity of ettringite is increased from 1 day to 3 days (Table 6). However, a reduction in ettringite content is observed from 1 day to 3 days in CC-2.5 blend. In blend CC-2.5, the reactive alumina content is more, and the sulphate content is same as in OPC-2.5 leading to reduction in sulphate to alumina ratio which favours the AFm transformations. Furthermore, in CC-5.0 where there is ample amount of sulphates, ettringite quantity is increased from 1 day to 3 days. It is reported that the alumina uptake can take place in C-S-H resulting in C-(A)-S-H formation in calcined clay rich in kaolinite content [10]. However, at higher dosage of gypsum, where sulphate to alumina ratio is high ettringite formation is favoured instead of C-(A)-S-H.

5 Conclusions

The following conclusions were drawn based on hydration and phase assemblage studies conducted on calcined clay blends.

  • Though, the aluminate peak is significantly delayed in Portland cement—calcined clay systems, overall heat is more than the control OPC blend at 48 h.

  • The initial hydration of silicates may be affected due to higher sulphate contents.

  • At higher dosage of sulphates, ettringite is formed at the expense of C-(A)-S-H.