Hydration and phase assemblage of ternary cements with calcined clay and limestone

Construction and Building Materials 222 (2019) 64–72

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Hydration and phase assemblage of ternary cements with calcined clay and limestone Sreejith Krishnan ⇑, Arun C. Emmanuel, Shashank Bishnoi Department of Civil Engineering, Indian Institute of Technology, Delhi, India

h i g h l i g h t s 3


The individual impact of calcined clay, limestone and gypsum in LC has been studied. 3


Additional ettringite is formed when gypsum content is increased in LC . 3


The limestone reactivity reduces when gypsum is increased in LC .

a r t i c l e

i n f o

Article history: Received 14 August 2018 Received in revised form 12 June 2019 Accepted 14 June 2019

Keywords: Carboaluminates Ettringite Calcined clay Limestone Gypsum optimisation Stratlingite

a b s t r a c t This study aims to understand the impact of calcined clay, limestone and gypsum on the hydration mechanisms and phase development in limestone calcined clay cement (LC3). Crushed quartz was used as an inert filler to isolate the individual impact of calcined clay and limestone in LC3 systems. The hydration and the microstructure development in these systems were studied using X-ray diffraction (XRD), isothermal calorimetry, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP) as well compressive strength experiments. The presence of calcined clay helped in achieving a well-refined microstructure in LC3 within 7 days of hydration. A reduction in the degree of hydration of clinker phases was observed due the presence of calcined clay. The AFt/AFm phase assemblage was seen to depend on the blend composition. Additional ettringite was seen to form when the amount of gypsum was increased in the LC3 system. However, the increased gypsum content also resulted in the reduction in reactivity of limestone. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The global cement production is being mainly driven by the significant demand for concrete by the construction industry. Presently, around 3.6 billion tonnes of cement is produced across the world, and it is expected that the annual cement production will reach approximately 5 billion tonnes by the year 2030 [1]. Production of portland cement is inherently accompanied by emissions of carbon dioxide, which is the primary greenhouse gas. Therefore, finding sustainable alternatives to ordinary portland cement (OPC) is the most critical challenge faced by the cement industry as well as cement researchers. Blended cements that use the synergetic effect between multiple supplementary cementitious materials (SCMs) and OPC are being explored nowadays as potential sustainable alternatives [2–5]. In addition to the pozzolanic reaction of the SCMs with the calcium hydroxide, various secondary hydration reactions which enhance the mechanical ⇑ Corresponding author. E-mail addresses: [email protected], (S. Krishnan), [email protected] (S. Bishnoi). https://doi.org/10.1016/j.conbuildmat.2019.06.123 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

[email protected]

and durability properties even at low clinker factors, occur in these blended cements. Formation of carboaluminate phases (calcium hemicarboaluminate and calcium monocarboaluminate) by the reaction of CO2 3 ions in the presence of reactive aluminates is an example of these secondary reactions in blended cements [6,7]. Presence of these phases have been observed in portland limestone cement [8,9], ternary cements with fly ash and limestone [5] as well as ternary cements with calcined clay and limestone [2,10], modifying the final phase assemblage of the hydrated system. Availability of suitable SCMs (of required physical and chemical properties) is an important consideration while optimising the composition of blended cements. Due to the widespread availability of suitable clays [11–13], blended cements that incorporate calcined clay appears to be one of the promising alternatives for producing more sustainable cements. The main reaction products of cement hydration are C-S-H gel (an amorphous or semi-crystalline product with varying composition), which is the main binder, and calcium hydroxide (CH). Additionally, ettringite (C6A$3H32) is formed by the reaction of gypsum with C3A, which subsequently gets converted into monosulphate

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

3C$H2 þ C3 A þ 26H ! C6 A$3 H32

ð1Þ

C6 A$3 H32 þ 2C3 A þ 4H ! 3C4 A$H12

ð2Þ





3C4 A$H12 þ 2C C þ18H ! C6 A$3 H32 þ 2C4 A C H11 



C3 A þ C C þ11H ! C4 A C H11

ð3Þ ð4Þ

It has been well established by now that the calcite, if present in the portland cement systems, can react to form additional hydration products known as carboaluminates [8,17]. Calcium hemicarboaluminate 



(C4AC 0.5H12)

and

calcium

monocarboaluminate

(C4AC H11) are the two most commonly observed carbonate AFm phases in the Portland cement. Other carboaluminate phases such has calcium tricarboaluminate also exist but have not been commonly reported in the hydrated cement systems [18]. Thermodynamic modelling approach has been used to predict the final phase assemblage in various cementitious systems by the minimisation of the Gibbs energy [19–21]. These calculations show that hemicarboaluminate is a metastable phase, with monocarboaluminate being the final stable phase in limestone blended systems. It has also been reported that the phase stability of various carboaluminates depends on the amount of CO2, SO3 and Al2O3 present in the system [22,23]. The main source of reactive alumina for these reactions is the aluminate phase (C3A) in the clinker. The ferrite phase (C4AF) can undergo a similar set of reactions like that of C3A, forming ettringite as well carboaluminates, but at lower rates than C3A [24–26]. Availability of a fast reacting source of aluminosilicate such as metakaolin (AS2) can ensure sufficient availability of reactive alumina for these secondary reactions to take place. In this study, the hydration and microstructure development of a ternary cement that incorporate calcined clay, limestone and quartz as SCMs was investigated. The most important blend studied had a composition of 50% clinker, 30% calcined clay, 15% limestone and 5% gypsum. Significant focus on this particular composition of ternary cement with, also called limestone calcined clay cement or LC3, has recently been reported in the literature [2,27–31]. This study tries to understand the impact of calcined clay, limestone and gypsum in the LC3 system. The phase assemblage and microstructure development has been studied using various characterisation techniques such as quantitative X-ray diffraction (QXRD), thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), isothermal calorimetry and scanning electron microscopy (SEM). 2. Experimental procedure 2.1. Materials Suitable raw materials were identified, and the blends were prepared by individually grinding clinker, calcined clay and limestone in a laboratory ball mill, followed by blending the ground

raw materials in the required proportions. Crushed quartz was used as an inert filler for replacing calcined clay and limestone. The particle size distributions of the crushed materials (Fig. 1) was measured using laser diffraction (Malvern Mastersizer 3000). The D50 values of the crushed cement, limestone, calcined clay and quartz were 15.9 mm, 12.5 mm, 18 mm and 10.6 mm respectively. No specific fineness levels were targeted during the grinding process. Therefore, all the raw materials were ground for a fixed time by fixing the number of rotations in the ball mill. The ball mill was operated at 34 rotations per minute, with a raw material to ball ratio of 1:5. The quantitative phase estimation of the clinker used in the study was done with Rietveld analysis (48.1% C3S, 29.4% C2S, 4.34% C3A and 17.67% C4AF). Calcined clay was produced by the static calcination of kaolinitic clay (estimated kaolinite content of approximately 60%) at 800 °C in a muffle furnace with a soaking time of a half hour. A clinker-grade limestone (approximately 80% calcite content) procured from a cement plant in Gujarat was used in this study. The blends have been designed to isolate the influence of individual component on the hydration, strength and microstructure development. Blend C-K-L used in this study has similar composition as the industrially produced LC3 cement [30,31]. It is seen that AFt/AFm phases play an important role in the microstructural development in LC3, with the final phase assemblage depending upon the amount alumina, sulphate and carbonate present in the system. Therefore two additional blends were prepared by adding 5% and 10% gypsum additionally to the blend C-K-L to understand the effect of additional sulphate content on the hydration of LC3 systems. It was ensured that the relative proportion of the clinker, calcined clay and limestone was kept the same. The details of the blend used in the study are shown in Table 1, and the chemical composition of the raw materials are shown in Table 2. Isothermal conduction calorimetry was carried out on all blends at 27 °C using isothermal calorimeter (Calmetrix I-Cal 8000). Fixed paste content of 72.5 g was used to observe the rate of heat evolution for all the blends. Cylindrical paste specimens (3 cm diameter) were cast for XRD analysis. The specimens were removed from the mould after 24 h and continuously cured under lime saturated water till the time of testing. XRD analysis (using Bruker D8 Advance Eco diffractometer) was done on paste slices (3 mm thick) with voltage 40 mV and current 25 mA with an approximate scan time of 18 min (step size 0.019°/s). A fixed divergence slit (0.6 mm) was used. Quantitative Rietveld refinement was used for estimation of the hydrated phases formed using the external standard method [32], using rutile (around 97% crystallinity) as

100

OPC Calcined clay Quartz

Volume Passing (%)

(C4A$H12) on the exhaustion of available gypsum. It is becoming accepted that it is, in fact, the ability of the hydration products to occupy more space that is critical to the development of mechanical properties than the type of hydration products formed [14]. A low-density phase such as ettringite (density of 1.77 g/cc) [15], whose contribution towards the development of microstructure and mechanical properties was not well understood previously, can be an important contributor in the development of strength. The ettringite formed in OPC gradually transforms to monosulphate with time. When CO2 3 ions are available, the monosulphate reacts to form carboaluminate and ettringite (Eqs. (1)–(3)), thereby ‘‘stabilising” the ettringite [16].

80

Limestone

60

40

20

0 0.01

0.1

1

10

100

1000

Particle size (µm) Fig. 1. Particle size distribution of the crushed raw materials.

10000

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

Table 1 The composition of the blends used in the study. Blend name

Clinker (C) (%)

Calcined clay (K)(%)

Limestone (L) (%)

Quartz (Q) (%)

Gypsum (G) (%)

OPC OPC-Q C-K-L C-K-Q C-Q-L C-K-L(+5G) C-K-L(+10G)

95 52.25 50 50 50 47.62 45.45

– – 30 30 – 28.57 27.27

– – 15 – 15 14.28 13.63

– 45 – 15 30 – –

5 2.75 5 5 5 9.53 13.65

Table 2 Major oxide composition of the raw materials used in the study. Oxide (%)

Clinker

Calcined clay

Limestone

Gypsum

CaO SiO2 Al2O3 Fe2O3 MgO Na2O K2O SO3 LOI

65.10 20.97 4.80 4.31 2.47 0.29 0.24 0.75 0.56

0.06 54.96 39.75 4.16 0.02 0.18 0.17 0.1 0.24

44.24 11.02 2.53 1.55 1.96 0.5 0.28 – 36.96

32.62 2.77 0.62 0.36 1.20 0.06 0.037 38.75 23.02

the external standard. The slices were then immersed in isopropanol to stop hydration and stored in a vacuum desiccator for SEM-EDX and MIP analysis (Pascal 140 and Pascal 440). A contact angle of 140° was assumed for the MIP analysis. SEM – EDX analysis was carried out for two blends OPC and C-K-L to determine the composition of the calcium silicate hydrate gel formed at 28 days. The samples were impregnated under vacuum in epoxy and polished using diamond spray. Additional details concerning the sample preparation and test protocols can be found elsewhere [33]. The compressive strength of the blends was determined by casting cubes of size 7.06 cm * 7.06 cm * 7.06 cm according to Indian Standards [34]. Fixed water to cement ratio of 0.45 was used for all the mixes.

3. Results and discussions 3.1. Influence of calcined clay and limestone 3.1.1. Phase assemblage- AFt/AFm and C-A-S-H The impact of the combination of calcined clay and limestone in C-K-L as well as their individual impact on the phase development and the final assemblage is shown in Fig. 2. In the C-K-L system, a distinct peak of hemicarboaluminate phase was observed after 24 h of hydration in the diffractogram (2H = 10.8°). Minor monocarboaluminate (2H = 11.7°) peak was observed at 28 as well as 90 days in this system. However, the complete conversion of hemicarboaluminate to monocarboaluminate was not observed. In the 

absence of calcined clay (C-Q-L), C4AC H11 is the major phase seen at the end of 90 days. AFm phases such as hydrated gehlenite (stratlingite or C2ASH8) are precipitated instead of carboaluminates in the absence of carbonate sources in the blend C-K-Q. Although, given the large quantities of alumina available in the system, it was expected that the ettringite would completely transform to monosulphate in the absence of limestone, ettringite peaks were still visible after 90 days in the blend C-K-Q. Similar observations were made in a recent study by Zajac et al. [35]. The complete transformation of the hemicarboaluminate to monocarboaluminate in the blend C-Q-L is interesting when compared to blend C-K-L where only a partial conversion was observed. This suggests that a higher amount of calcite can react

Fig. 2. X-ray diffractogram of hydrated cement pastes at age. E – Ettringite, Hc – Hemicarboaluminate, AFmss – A solid solution of OH-, SO3- and CO3-AFm phases F – Ferrite, P – Portlandite, S – Stratlingite, Ms – Monosulphate, Mc – Monocarboaluminate.

in C-Q-L. It has been reported that the amount of alumina available for the formation of the carboaluminate phases [22,35] controls the reactivity of calcite. The results of this study suggest that there could be other factors, in addition to the aluminate content, that affects the kinetics of calcite dissolution (like the availability of portlandite) in low clinker factor cements such as the C-K-L blend studied here. There are only traces of portlandite observed in blends C-K-L and C-K-Q after 24 h. This is as expected due to the presence of fast reacting calcined clay. Minor peaks of hemi and monocarboaluminate were observed in blends OPC and OPC-Q which could be due to the carbonation of the sample during the XRD analysis. SEM-EDS analysis (Fig. 3) was used to characterise the C-A-S-H gel formed since it cannot be characterised by X-ray diffraction due

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

0.7

C-K-L

OPC

OPC

OPC-Q

C-K-L

C-K-Q

C-Q-L

1

Degree of hydration of belite

Al/Ca (Atomic Ratio)

0.6 0.5 0.4 0.3 0.2 0.1

0.8

0.6

0.4

0.2

0

0 0

0.2

0.4

0.6

0.8

Days

Si/Ca (Atomic Ratio) Fig. 3. SEM-EDS analysis of the inner C-A-S-H gel present in 28 days hydrated C-K-L and OPC paste. The average composition of C1.61A0.115SHx and C1.76A0.047SHx was observed for C-K-L and OPC respectively at 28 days.

Fig. 4. Degree of hydration of belite in hydrated paste samples estimated through QXRD (error ± 0.02).

OPC

OPC-Q

C-K-L

C-K-Q

C-Q-L

to its amorphous nature. The presence of alumina from calcined clay modifies the structure and morphology of calcium silicate hydrate, with alumina being incorporated into the gel. The alumina uptake in C-S-H gel is dependent on various parameters such as the grade of calcined clay [36], available alkali content [37,38], the Ca/Si ratio of the C-S-H gel [39] amongst others. The estimated average C-A-S-H composition in C-K-L at the end of 28 days was C1.61A0.115SHx implying a higher alumina uptake compared to OPC. An average composition of C1.67A0.27SHx was reported by Antoni et al. [2] for a hydrated LC3 system after 300 days. There appears to be a practical limit for the amount of alumina that can be taken up in C-A-S-H gel. A higher Ca/Si ratio with minimum alumina uptake was observed in hydrated ordinary portland cement, with an average gel composition of C1.76A0.047SHx, which is in agreement with the reported values for neat cements [14,40]. Data points were found to be clustered in neat portland cement suggesting more or less uniform composition of the gel formed. However, this was not the case in C-K-L systems where the data points were found to be distributed over a wide range. Therefore, the composition of the C-A-S-H gel formed in C-K-L cannot be defined accurately. Wilson et al. [41] have observed a significant intermixing of the hydration products in LC3 systems, which might explain the scattering of the observed data points.

Degree of hydration of alite

1

0.9

0.8

0.7

0.6 0.1

1

10

100

Days Fig. 5. Degree of hydration of alite in hydrated paste samples estimated through QXRD (error ± 0.02).

development is expected to occur during the early ages, as compared to ordinary Portland cement. The addition of calcined clay results in the formation of a wellrefined pore structure [13] in cementitious systems. Avet et al. [42] suggest that the slowing down of the clinker hydration is due to

50 1 Day 2 Days 7 Days 28 Days

40

Total porosity (%)

3.1.2. Impact on alite and belite hydration The degree of hydration of the alite and belite phases in the blends studied up to 90 days is shown in Figs. 4 and 5. The degree of hydration was estimated by quantitative rietveld analysis. A lower degree of hydration of alite and belite were observed at later ages in blends that incorporate calcined clay when compared with other blends. The final degree of hydration of alite was nearly 15% less in the blends C-K-L as well as C-K-Q compared to the other blends. The hydration of belite was seen to be affected when calcined clay is present. In both blends C-K-L as well as C-K-Q, the degree of hydration of belite was found to be approximately around 35% and 45% respectively after 90 days. In the absence of calcined clay, the degree of hydration of belite was more than 80%. There appears to be a greater reduction in the hydration of belite when limestone was present with calcined clay than when quartz was present along with calcined clay. There is no significant hydration of alite and belite after 7 days in blends C-K-L and C-K-Q which implies that a larger fraction of the 28-day strength

30

20

10

0 0.001

0.01

0.1

1

10

100

Pore diameter(µm) Fig. 6. Cumulative intrusion curve for the blend C-K-L.

1000

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72 600

200 1 Day

OPC

500

Energy / total paste content (J/g)

7 Days

Critical pore entry diameter

28 Days

400

dV/log(dr)

OPC-Q

C-K-L

C-K-Q

C-Q-L

2 Days

300

200

100

0 0.001

0.01

0.1

1

10

100

160

120

80

40

0

1000

0

Pore Diameter (µm)

8

16

24

32

40

48

56

64

72

80

Time (Hours)

Fig. 7. Differential intrusion curve for blend C-K-L.

Fig. 9. Total energy released up to 72 h for the blends studied at 27 °C.

lack of large pores. From the mercury intrusion porosimetry results (Figs. 6 and 7), it is seen that the limiting critical pore entry diameter in C-K-L is attained by seven days beyond which there is no significant reduction in critical pore size. The degree of hydration of clinker also does not appear to change much after seven days.

depletion of the sulphate content (the onset of peak II) occurs around 9.5 h from the initial addition of water. The broadened and delayed aluminate hydration peak was observed in blend CQ-L. 3.1.4. Development of compressive strength Fig. 10 depicts the development of compressive strength in the blends studied at various ages. The major strength development in LC3 systems happens until 28 days. The presence of calcined clay plays a critical role in improving the early age strength in the blends that contain calcined clay. There is a minimal strength gain from 28 to 90 days in both clay containing blends C-K-L and C-K-Q. The reason for this trend becomes clearer on the closer inspection of the hydration trends of C3S and C2S. As discussed in Section 3.1.2, a reduction in the long term degrees of hydration of C3S and C2S have been observed in the presence of calcined clay due to the significant pore refinement that occurs at early ages. Another point of interest is that the hydration reactions behind the strength development in the blend C-K-L and C-K-Q are different. The measured strengths are higher for C-K-L until 7 days, but at 28 days both systems showed similar strengths. The improvement from 7 to 28 days in C-K-Q corresponded with the formation of lowdensity stratlingite (C2ASH8) phase which is different from the phases that form in C-K-L systems (i.e. stabilisation of ettringite and formation of carboaluminate). Although the precise mechanism of stratlingite formation in blended cement systems is still

5

1 Day OPC

3 Days

7 Days

28 Days

56 Days

90 Days

60

OPC-Q C-K-L

4

C-K-Q

Compressive Strength (MPa)

Rate of heat evolution/total paste content (mW/g)

3.1.3. Reaction kinetics during early hydration The early hydration of the blends was monitored using isothermal calorimetry (Fig. 8). Characteristic isothermal calorimetry of systems that incorporate limestone and calcined clay shows two peaks [43]. The first peak (marked as I) is the silicate hydration peak and the second peak (marked as II) is the aluminate hydration peak [2,44]. The presence of SCMs accelerates the hydration of clinker phases due to the filler effect [45], which provides additional surface area for the C-S-H gel to nucleate. The earlier occurrence of peak I is observed in blends C-K-L, C-K-Q as well as C-Q-L with higher measurements for C-K-L and C-K-Q. The earlier occurrence of peak I in the blend C-Q-L when compared to OPC-Q is interesting, as both quartz and limestone as mostly expected to remain inert during the early ages of hydration, with similar trends in the total heat released for both the blends till 72 h (Fig. 9). It has been shown that limestone contributes additional Ca2+ ions to the hydrating cement systems, that accelerate the hydration of clinker phases and shortens the induction period [46]. A distinct aluminate hydration peak is observed for all the blends except OPC. In blends C-K-L, C-K-Q and OPC-Q, the

C-Q-L

I

3

II I

2

II 1

50

40

30

20

10 0 0

4

8

12

16

20

24

28

32

36

40

44

48

Time (h)

0 C-K-L

Fig. 8. Rate of heat evolution until 48 h for the blends studied at 27 °C. I – Silicate hydration peak, II – Aluminate hydration peak.

OPC

OPC-Q

C-K-Q

C-Q-L

Fig. 10. Compressive strength development of the blends studied.

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

not very clear, it is possible that the availability of additional silica leads to its formation. 3.2. Influence of gypsum 3.2.1. Phase assemblage The development of phase assemblage in C-K-L system in the presence of additional gypsum is shown in Fig. 11. Higher gypsum additions were used so that the effects of sulphate additions become amplified and measurable using the experimental techniques used. The initial SO3 content available in the C-K-L system modifies the final phase AFt/AFm assemblage. Hemicarboaluminate was observed in the blend C-K-L after 24 h of hydration, which was partially converted to monocarboaluminate by the end of 90 days. No carboaluminate formation was observed in CK-L (+5G) and C-K-L (+10G) after first 24 h. After three days of hydration, peaks corresponding to hemicarboaluminate formation were observed in C-K-L (+5G). Only traces of hemicarboaluminate were observed in C-K-L (+10G) even after 90 days. The C3A content available from the clinker is usually sufficient for the complete consumption of gypsum present (ranging from 3 to 5%) in OPC (Eq. (5)).

C3 Aþ 3C$H2 þ 26H ! C6 A$3 H32 270g

516g

468g

1254g

ð5Þ

From a simple calculation, it can be seen that the 5 g of gypsum requires around 2.61 g of C3A for complete reaction. There is sufficient C3A content in the blend C-K-L to consume nearly 83% of the gypsum available. As there is additional alumina available in the calcined clay, as well as portlandite available during the early ages of hydration, the additional gypsum can form ettringite. This is confirmed by quantification of the ettringite present all the three systems (Fig. 13). The other potential source of alumina in this sys-

tem is the ferrite phase. As the rate of reaction of ferrite is slow, it is reasonable to assume that the source of the additional alumina required for the production of ettringite is the calcined clay. The amount of ettringite present in the hydrated cement systems remains more or less similar after 7 days in all the three blends. The formation of ettringite in systems that contain portlandite, gypsum and aluminosilicates have been reported [47–49]. The reaction of calcined clay with portlandite and gypsum is expected to proceed as shown in Eq. (6)

AS2 þ CH þ C$H2 þ H ! C6 A$3 H32 þ C  A  S  Hx

ð6Þ

Zajac et al. [22] observed that the reaction of calcite starts only after complete depletion of the gypsum in portland cements containing limestone. It was also observed that the dissolution rate of calcite depended upon the alumina available for the formation of hemicarboaluminate and monocarboaluminate. The investigations by Zajac et al. [22] were done on portland cement blended with limestone with higher clinker factors. In such systems, calcium hydroxide is available even at later ages since there is no significant pozzolanic reaction [23]. The present results are partially in agreement with these observations. Delayed formation of carboaluminates was observed in both C-K-L(+5G) and C-K-L(+10G). However, the intensities of the carboaluminate peaks were found not to increase even after the complete consumption of gypsum, especially in C-K-L(+10G) even when additional alumina (from calcined clay) was available for the formation of carboaluminate phases. In C-K-L systems, the pozzolanic reaction is expected to proceed in the as shown in Eq. (7). The Eqs. (6) and (7) are left unbalanced as the alumina uptake in the C-S-H gel cannot be estimated accurately and will vary depending on the systems studied. 



AS2 þ CH þ C C þH ! C  A  S  Hx þ C4 A C H11

ð7Þ

The consumption of portlandite by gypsum to form ettringite would reduce the available portlandite for the pozzolanic reaction. The portlandite available at later age is also limited due to slowing down of clinker hydration at later ages as discussed in the previous sections. The absence of significant carboaluminate peaks even after 90 days of hydration is an indication of the reduced pozzolanic reaction (Eq. (6)) in the systems with additional sulphate content. It was also observed that the amount of calcite consumed (Fig. 12) was lowest in the blend C-K-L(+10G) indicating reduced reactivity of calcite in the presence of additional sulphate content. 3.2.2. Influence of gypsum on the alite and belite hydration Figs. 14 and 15 depict the influence of additional sulphate content on the hydration of alite and belite. The presence of gypsum is

Calcite reacted (g/ 100 g anhydrous cement)

5

4

3

2

1 C-K-L

C-K-L(+5G)

C-K-L(+10G)

10

100

0 0.1

1

Days Fig. 11. X-ray diffractogram of hydrated cement pastes with additional gypsum with E – Ettringite, Hc – Hemicarboaluminate F – Ferrite, P – Portlandite, S – Stratlingite, Ms – Monosulphate, Mc – Monocarboaluminate.

Fig. 12. The amount of calcite reacted blends with additional sulphate content estimated with QXRD (Error ± 1 g).

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S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

C-K-L

C-K-L(+5G)

C-K-L(+10G)

30 25 20 15 10 5 0 0.1

1

10

100

Days Fig. 13. The amount of ettringite present in the blends with additional sulphate content estimated with QXRD (Error ± 1 g).

Degree of hydration of alite

1

0.9

0.85

0.8

C-K-L

C-K-L(+5G)

C-K-L(+10G)

0.7 0.1

1

10

100

Days Fig. 14. Effect of additional sulphate content on the hydration of alite estimated with QXRD (error ± 0.02).

0.7 C-K-L

C-K-L(+5G)

C-K-L(+10G)

0.6 Degree of hydration of belite

3.2.3. Influence of gypsum on the reaction kinetics The rate of heat evolution in the presence of extra gypsum is shown in Fig. 16. Delayed occurrence of the aluminate hydration peak along with peak broadening was observed with the increasing gypsum content. In the blend C-K-L(+10G), the aluminate hydration peak extended for more than 16 h. The impact of an extended aluminate hydration peak on phase assemblage is still not very clear. The intensity as well the time of occurrence of the induction period and the silicate hydration peak was not influenced by the sulphate content. However, it should also be noted that the position, intensity, and duration of aluminate peak is not exclusively dependent on the gypsum content. Other parameters such as the particle size distribution and purity of calcined clay can also affect the aluminate peak which requires further investigations. 3.2.4. Influence of gypsum on compressive strength development Fig. 17 shows the compressive strength development in the cement blends with additional sulphate content. Similar trends in the strength development are seen for all the three blends at different ages. Even though the microstructure is rich in ettringite, it

0.95

0.75

in C-K-L(+5G), and C-K-L(+10G) compared to C-K-L after 90 days. While higher degrees of hydration of alite and belite were observed in the presence of additional gypsum compared to C-K-L during the first 7 days, no significant increase of hydration was observed after 7 days.

Rate of heat evolution/total clinker content (mW/g)

Ettringite (g/100g of anhydrous cement)

35

10

C-K-L

C-K-L (+5G)

C-K-L (+10G)

8

II I

6

4

II II

2

0

0

4

8

12

16

20

24

28

32

36

40

44

48

Time (h) 0.5

Fig. 16. Rate of heat evolution until 48 h for blends with additional gypsum at 27 °C. I – Silicate hydration peak, II – Aluminate hydration peak.

0.4

0.3

1 Day

3 Day

7 Day

28 Day

56 Day

90 Day

50

0.1 0.1

1

10

100

Days

Fig. 15. Effect of additional sulphate content on the hydration of belite estimated with QXRD (error ± 0.02).

known to accelerate the hydration reaction of the cement phases [50,51]. The hydration of alite was found to be accelerated in the presence of additional sulphate content in C-K-L systems. Around 90% of the alite was found to have been hydrated in C-K-L(+10G) blend after 24 h. However, the greater dissolution of alite during early ages was seen to reduce the reactivity of belite at the same time. The degree of hydration for belite was found to be greater

Compressive Strength (MPa)

0.2

40

30

20

10

0 C-K-L

C-K-L(+5G)

C-K-L(+10G)

Fig. 17. Compressive strength development in blends with additional gypsum content.

S. Krishnan et al. / Construction and Building Materials 222 (2019) 64–72

does not appear to influence the strength directly. It is generally expected that the presence of water rich, low-density phase such as ettringite would have a positive influence on the strength development due to reduced porosity and densified microstructure. Additionally, the hydration of alite and belite was found to be higher in C-K-L(+5G) and C-K-L(+10G) which should translate into improved performance. However, in the case of the systems being studied, the consumption of the portlandite in the formation of additional ettringite would prevents the formation of the carboaluminate phases, at least partially compensating for any increase in strength that might occur due to the additional ettringite in the long term. 4. Conclusions The role of calcined clay, limestone and gypsum in the hydration mechanisms as well as strength development of limestone calcined clay cement systems have been investigated in this study. Crushed quartz was used as an inert filler to understand the effects of the individual components: calcined clay and limestone. Final phase assemblages (especially the AFt/AFm phases) were found to depend upon the supplementary cementitious material present. Hemicarboaluminate was the major AFm phase formed in the presence of calcined clay and limestone. When calcined clay was replaced with quartz, the transformation of hemi-carboaluminate into monocarboaluminate was observed. In the absence of carbonate ions, stratlingite was found to be precipitated. A well refined and dense microstructure was observed in the CK-L system due to the fast reaction of calcined clay. However, the well-refined microstructure also resulted in the reduction of long term clinker hydration. This was mirrored in the compressive strength results, where no notable strength gain was observed beyond 28 days. The optimisation of gypsum, therefore, becomes critical as the additional gypsum can combine with portlandite and reactive alumina from calcined clay to form ettringite. This may further reduce the portlandite available for the synergetic reaction with calcined clay and limestone. The compressive strengths of the blends with additional gypsum were similar to the control blend. The formation of additional ettringite was seen to have compensated for the reduced pozzolanic reaction. The gypsum contents used in this study were significantly higher than the typically recommended values for sulphate addition in portland cements. Other considerations especially concerning durability have to be made while fixing the final sulphate content in the CK-L systems. From the point of view of gypsum optimisation, it is sufficient to ensure the availability of adequate sulphate content required to prevent flash setting. Declaration of Competing Interest None. Acknowledgements The authors would like to acknowledge the support of Swiss Agency for Development and Cooperation for supporting the Low Carbon Cement project in India. The MIP and SEM studies were carried out at Laboratory of Construction Materials (LMC) at EPFL, Switzerland. The authors would like to acknowledge the support of Prof. Karen Scrivener (Head, LMC) for her support. The authors would also like to acknowledge the support of Dr. Geetika Mishra in conducting the SEM analysis.

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