Use of X-ray diffraction to quantify amorphous supplementary cementitious materials in anhydrous and hydrated blended cements

Cement and Concrete Research 64 (2014) 89–98

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Use of X-ray diffraction to quantify amorphous supplementary cementitious materials in anhydrous and hydrated blended cements R. Snellings a,⁎, A. Salze a, K.L. Scrivener a a

Laboratory of Construction Materials, Institute of Materials, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland

a r t i c l e

i n f o

Article history: Received 11 April 2014 Accepted 26 June 2014 Available online xxxx Keywords: X-ray diffraction (B) Blended cement (D) Amorphous material (B) Cement manufacture (E) Kinetics (A)

a b s t r a c t The content of individual amorphous supplementary cementitious materials (SCMs) in anhydrous and hydrated blended cements was quantified by the PONKCS [1] X-ray diffraction (XRD) method. The analytical precision and accuracy of the method were assessed through comparison to a series of mixes of known phase composition and of increasing complexity. A 2σ precision smaller than 2–3 wt.% and an accuracy better than 2 wt.% were achieved for SCMs in mixes with quartz, anhydrous Portland cement, and hydrated Portland cement. The extent of reaction of SCMs in hydrating binders measured by XRD was 1) internally consistent as confirmed through the standard addition method and 2) showed a linear correlation to the cumulative heat release as measured independently by isothermal conduction calorimetry. The advantages, limitations and applicability of the method are discussed with reference to existing methods that measure the degree of reaction of SCMs in blended cements. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In present-day society the cement industry fulfils a pivotal role in the recycling of waste streams into construction materials. In particular, the partial replacement of Portland clinker by supplementary cementitious materials (SCMs) offers the opportunity to reuse large volumes of industrial by-products, notably blast furnace slag and fly ash. As the reactivity of SCMs is lower than that of clinker, cement replacement levels are mainly limited by the early-age strength gain. Further decreases in clinker contents will depend on an improved control and understanding of the SCM reactivity in blended cements. Key to unravelling the parameters governing SCM reactivity is the accurate measurement of the degree of reaction of SCMs in hydrating blended cements. To this purpose, a range of different methods has been applied with mixed results. All reported methods are subject to more or less serious limitations [2]. Recent studies deemed selective dissolution approaches [3,4] unreliable, showing large, unrealistic discrepancies with other methods and problems with interlaboratory reproducibility [5,6]. Application of image analysis to electron microscopy images and element maps has shown promising results for blast furnace slag [6], and to some extent for fly ash [7,8]. However, the proposed approaches are time-demanding and cannot resolve the reaction of particles smaller ⁎ Corresponding author at: MXG 234, LMC IMX STI EPFL, Station 12, CH-1015 Ecublens, Switzerland. Tel.: +41 21 693 72 52; fax: +41 21 693 58 00. E-mail addresses: ruben.snellings@epfl.ch (R. Snellings), karen.scrivener@epfl.ch (K.L. Scrivener).

http://dx.doi.org/10.1016/j.cemconres.2014.06.011 0008-8846/© 2014 Elsevier Ltd. All rights reserved.

than about 2 μm, rendering the method unsuitable for fine materials such as silica fume and metakaolin. A more generally applicable method makes use of 29Si and 27Al solid-state NMR spectroscopy. The method has been successfully applied to binders containing blast furnace slag [6,9], fly ash [10], and silica fume [11–13]. However, widespread use will remain limited because access to solid-state NMR equipment is restricted. 29Si NMR data acquisition is time-consuming (1–2 days per measurement) and measurements are restricted to systems with low levels of paramagnetic ions. It can thus be concluded that the direct measurement of the degree of reaction of amorphous SCMs remains a difficult task, and that a generally applicable, reliable and accessible quantification method is still needed. In this respect, the present contribution explores the potential of a novel X-ray diffraction (XRD) based methodology. Over recent years, XRD has become one of the most prominent analytical techniques in the characterisation of Portland cement based systems [14–20]. The main advantages of XRD are the ease and speed of measurement and its accuracy compared to traditional quantitative phase analysis methods such as Bogue calculations and optical microscopy [21,22]. Over the last decade, the success of XRD has been rooted largely in advances in detector performance [23], and in the development of powerful and user-friendly analysis software. Simultaneously, quantification approaches shifted from single peak to full XRD profile fitting methods such as the Rietveld method [24,25] and profile summation methods [26]. Its capability to accommodate adjustment of crystal structure parameters has favoured strongly the use of the Rietveld method as many cementitious phases display extensive chemical

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variability. However, phase quantification by the Rietveld method depends on two preconditions: 1) the quantified phase displays threedimensional structural periodicity i.e. is crystalline, and 2) the crystal structure of the phase has been resolved. Nowadays the large majority of crystalline phases encountered in cements have resolved crystal structures and can be quantified. Despite these advances, the quantification of individual amorphous phases such as SCMs remains problematic. Common approaches making use of internal and external standards can determine the total amount of unknown and amorphous phases [27,28], but cannot distinguish between the individual contributions of different amorphous phases [27,28]. In this respect, profile summation methods offer the advantage of relying only on a calibrated XRD profile of each individual phase, which is also possible for amorphous phases. The advantages of both Rietveld and profile summation methods were combined recently in the Partial Or No Known Crystal Structure (PONKCS) method [1]. The method is devised to be easily implementable in common Rietveld refinement software for the simultaneous quantification of phases calculated from crystal structure data and phases from calibrated XRD profiles. In the field of cement, the PONKCS method has been applied recently on alkali-activated metakaolin to determine the extent of metakaolin reaction [29], and in the quantification of C–S–H in hydrating alite pastes [30]. The quantification results were reported to compare well to results from independent quantification methods such as mass balance calculations, calorimetry and electron microscopy. This paper describes the application of the PONKCS method to the study of blended cements by XRD. The PONKCS method was used in combination with the external standard method [31,32] and a determination of the bound water to obtain a quantification of the degree of reaction of several SCMs. The repeatability and accuracy of the quantification results were assessed for anhydrous and hydrated blended cements containing one or more amorphous components. The performance of the quantification method is evaluated for increasingly complex systems going from simple binary synthetic mixes over anhydrous blended cements to hydrated samples of portlandite–SCM model mixes and blended cements. Both repeatability and accuracy errors are reported. Potential sources of bias are identified and recommendations are made accordingly.

2. Materials and methods 2.1. Materials, mix design, and sample preparation The method was tested on a series of mixes of increasing complexity. The materials used in the preparation of the mixes were: ground highpurity quartz (K13, Bernasconi), blast furnace slag (Dunkerque, Holcim), metakaolin (Burgess), limestone (Durcal 5/15, Omya), grey Portland cement (CEMI 52.5N, Heidelberg), and white Portland cement (Aalborg cement) hydrated for 7-years. The chemical composition of the anhydrous samples measured by X-ray fluorescence (XRF) Table 1 Chemical composition (wt.%) and mass absorption coefficients (MAC) of the grey Portland cement (OPC), white Portland cement (WC), metakaolin (MK), blast furnace slag (Slag), quartz, and limestone samples. Oxides/wt.%

OPC

WC

Slag

MK

Quartz

Limestone

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O TiO2 LOI MAC (cm2/g)

19.27 5.65 3.63 63.65 1.62 3.16 0.15 1.24 0.29 0.75 99.35

24.68 2.11 0.43 68.67 0.58 1.82 0.17 0.06 0.05 0.97 96.91

36.61 12.21 0.85 41.59 7.18 0.63 0.18 0.28 0.35 – 73.63

50.62 46.91 0.38 0.02 0.09 0.08 0.28 0.18 1.29 0 35.90

97.91 1.00 0.05 0.02 – – – 0.77 0.03 0.15 36.72

0.04 0.06 0.05 56.53 0.10 – 0.04 0.04 0.03 43.09 74.72

spectrometry is shown in Table 1 along with the sample mass absorption coefficients (MAC) calculated from the international tables of crystallography for CuKa radiation [33]. The Loss On Ignition (LOI) was considered as H2O, except for limestone where CO2 was used in the calculation. An overview of the experimental sample matrix is given in Table 2. Simple binary mixes of quartz and slag were prepared at 10, 30, and 50 wt.% of slag content. The samples were mixed thoroughly by wetgrinding in isopropanol in a McCrone micronizing mill for 10 min. Resulting particle sizes (d50) were in the range of 3–5 μm. The 30 wt.% slag sample was prepared twice to assess errors involved in sample mixing and preparation. A second sample series consisted of ternary mixes of quartz, slag and metakaolin in 2:1:1 and 1:1:2 weight proportions. The 2:1:1 mix was weighed and mixed twice. Similar synthetic samples were prepared of anhydrous grey cement and slag and/or metakaolin to simulate the performance of the method in the analysis of anhydrous blended cements. Binary mixes were prepared of cement and slag at 20, 40, and 60 wt.% slag levels, and of cement and metakaolin at 20, 30, and 40 wt.% metakaolin. One ternary mix of anhydrous cement and two SCMs was tested at a 2:1:1 cement:slag:metakaolin weight proportion to assess to what extent the two different amorphous phases can be quantified in a mix with a complex multiphase material such as Portland cement. An additional level of complexity was introduced in mixes of white cement hydrated for 7-years and fixed quantities of slag or metakaolin. This system was selected as a way to evaluate the accuracy of the quantification results for actual hydrated blended cements. To stop the hydration of the white cement paste, the hardened paste was ground to below 50 μm and immersed in isopropanol for 15 min. Afterwards the isopropanol was evaporated in an oven at 40 °C and the powder was stored in a desiccator over silica gel. The solvent exchange and drying Table 2 Sample matrix giving the composition of the preblended simulated mixes, and the hydrated cement mixes. AS stands for low (L) or high (H) alkali sulfate additions (detailed in text). Simulated mixes Sample

Slag

S10-Q90 S30_Q70 (2×) S50_Q50 S25_M25_Q50 (2×) S25_M50_Q25 S20_C80 S40_C60 S60_C40 M20_C80 M30_C70 M40_C60 S25_M25_C50 S10_H90 S30_H70 S50_H50 M10_H90 M20_H80 M30_H70 M40_H60

10 30 50 25 25 20 40 60

MK

Qtz

25 50

90 70 50 50 25

OPC

80 60 40 80 70 60 50

20 30 40 25

25 10 30 50

HWC

90 70 50 90 80 70 60

10 20 30 40

Hydrated cements Samples

CH

MK

P-M P-M-LAS P-M-HAS P-M-LAS-Q P-M-LAS-Cc20 P-M-LAS-Cc6 CEM-MK CEM-MK-Q CEM-MK-Cc6 CEM-Slag

50 50 50 40 40 40

50 50 50 40 40 40 30 30 30 60

AS

Cc

L H L L L

20 (coarse) 20 (fine)

Qtz

OPC

Slag

20

15 (fine)

15 55

70 55 40

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(d50 of 6.3 μm) and quartz fillers replaced 15 wt.% of the Portland cement in two additional metakaolin blended cements. 1.5 wt.% gypsum was added to the metakaolin blended cements to balance the supply of aluminate by metakaolin [34]. Pastes were prepared at a water to binder ratio of 0.4 and sealed-cured until analysis at 3, 7 and 28 days of hydration. All pastes were cured in a 20 °C temperature controlled room. Slices of the hydrating pastes were cut at the set times of hydration and hydration stopped by solvent exchange in isopropanol for 7 days, finely ground and measured by XRD. To check the accuracy of the quantification for hydrating systems the method of standard additions was used. 20 and 40 wt.% metakaolin were mixed into hydration-stopped, dried powders of reacted model systems. The accuracy of the quantification can then be assessed from the intercept of the calibration curve between measured and added metakaolin. 2.2. XRD data collection Fig. 1. XRD patterns of grey Portland cement (OPC), hydrated white cement (HWC), blast furnace slag (Slag), and metakaolin (MK). The measured (black) and fitted, calculated (grey) XRD patterns are superimposed. The decomposition of HWC, slag and MK is depicted in more detail to show the respective contributions of the amorphous phase and the (accessory) crystalline phases.

procedure was designed to minimize the decomposition of the hydrate phases. Binary mixes of the hydrated cement powder and slag were prepared at 10, 30, and 50 wt.% of slag incorporation and at 10, 20, 30, and 40 wt.% metakaolin incorporation. Mixing of the hydrated cement containing samples was carried out by hand to preserve the hydrate assemblage. The measurement of the degree of SCM reaction was tested in two different reacting systems. First, pastes of portlandite (chemical grade), metakaolin and water with variable additional alkali and sulfate (added as KOH and K2SO4) were prepared to simulate and analyse the pozzolanic reaction. The portlandite to metakaolin weight ratio was fixed to 1:1, while the water to binder ratio was kept at 1.2. The main variables were the K2O/MK molar ratio of 0, 0.18, and 0.29 and the SO3/MK molar ratio of 0, 0.16, and 0.27 for the samples called P-M, P-M-LAS, and P-M-HAS, respectively. The effect of adding fine fillers on the SCM degree of reaction was studied in mixes of portlandite: metakaolin:filler in a 2:2:1 weight proportion. Finely ground quartz (d50 of 13.7 μm), and two limestone samples of different fineness (d50 of 20.0 and 6.3 μm) were added to the P-M-LAS system, keeping all other compositional ratios fixed. The samples were cast into plastic containers and sealed cured until analysis at 1, 4, and 7 days of hydration. A second series of hydrated samples were blended cements consisting of grey Portland cement with 60 wt.% slag, or 30 wt.% metakaolin. Limestone

XRD data were collected using CuKα radiation on a PANalytical X'Pert Pro diffractometer operated at 40 mA and 45 kV. Measurements were made in flat-plate Bragg–Brentano θ–2θ geometry, incident beam and receiving Soller slits of 0.04 rad were used and the incident beam divergence slit was fixed at 0.25°. Air scattering was reduced using a beam knife and a receiving antiscatter slit of 1° was positioned in the diffracted beam path. An X'Celerator linear position-sensitive Xray detector with a length of 2.122 °2θ was used for data acquisition. All scans were measured over an angular range of 8 to 70 °2θ with a 0.017 °2θ step size and accumulated time per step of 59.6 s, resulting in a total measurement time of about 30 min per scan. All measurements were carried out in triplicate to calculate repeatability errors. Powder samples were prepared using the back-loading technique to minimize preferred orientation effects and were repacked between repeated measurements. During measurement the samples were spun around the vertical goniometer axis to improve particle statistics. 2.3. XRD data analysis Rietveld quantitative phase analysis was carried out using TopasAcademic v4.1 software. The refinement strategy and structure models for crystalline phases were as reported in [35]. The external standard method was used to quantify the total amount of amorphous and unidentified phases [31,32]. The external standard method calculates the absolute weight fraction of a phase k by comparison of the refined phase scale factor Sk to the scale factor Ss of an external rutile standard (Kronos 2300 TiO2), measured under identical conditions. The calculation takes into account the (ZMV) phase constants and corrects for the

Table 3 Phase composition of the grey Portland cement (OPC), hydrated white Portland cement (HWC), metakaolin (MK), and blast furnace slag (Slag) as determined by XRD. Phase

C3S C2S C3A cubic C3A orthorhombic Ferrite Periclase Anhydrite Arcanite Portlandite Ettringite Calcite Quartz Mullite Anatase Amorphous/C–S–H

OPC

HWC

Slag

MK

g/100 g anhydrous

g/100 g anhydrous

g/100 g anhydrous

g/100 g anhydrous

63.6 8.9 1.3 5.2 14.2 0.4 4.0 2.3

2.4

25.1 4.4 0.7 0.6

–

92.6

98.7

5.9 1.5 92.6

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differences in mass absorption coefficients of the sample μ m and the standard μms (124.6 cm2/g) according to: wk ¼

ðZMV Þk sk μ   ws  m ðZMV Þs ss μ ms

ð1Þ

where Z represents the number of formula units in the unit cell, M is the unit cell mass, and V the volume of phase k or standard s. The crystallinity of the rutile standard ws was calibrated to be 96.4 wt.% using the external standard approach and the certified NIST SRM 676a α-Al2O3. The (ZMV)k phase constants are readily calculated for crystalline phases from the refined crystal structure. However, for amorphous or nano-crystalline phases α without known crystal structure, the phase constant (ZMV)α needs to be calibrated separately. The phase constant can be determined by the internal standard method as in [1,36], or can be obtained from a rearrangement of Eq. (1) using the external standard method if the sample chemical composition and the weight fraction of the phase of interest in the sample are known. Therefore, the calibration sample needs to contain only the phase of interest, or the phase of interest and a series of accurately quantifiable crystalline phases. Amorphous or nano-crystalline phases generally present a diffuse scattering contribution or ‘hump’ which can be fitted using a so-called ‘peaks phase’, consisting of a set of one or more peaks and a background function. It was found that an appropriate choice of diffractometer settings and control on the background function parameters delivered the most robust quantification strategy. Beam overflow at low angles should be avoided and a background function with as few coefficients as possible should be used. Here, a first order Chebyshev polynomial was combined with a 1/2θ term to fit the background signal in both calibrations and analyses. The diffuse scattering signals of slag and C–S–H (humps) were fitted using characteristic sets of pseudo Voigt peaks, the metakaolin peak was modelled using one asymmetric split pseudo Voigt peak. The calibration of the phase constant (ZMV)α was derived by comparing the ‘peaks phase’ scale factor of the fitted phase contribution in a reference sample of known composition to the external standard measured under identical conditions. Knowing wα and μm, the (ZMV)α phase constant can be calculated by setting Sα equal to 1 in the calibration sample. The obtained phase constant can then be used in Eq. (1) to quantify the weight fraction of phase α in samples of unknown composition. The measured and calculated XRD patterns of the SCMs (metakaolin and slag), the hydrated white cement (HWC) and the anhydrous grey Portland cement (OPC) are shown Fig. 1. The calculated patterns were decomposed to show the contribution of the main amorphous or nanocrystalline phases present. Using the external standard method, first the concentrations of the crystalline phases were determined and the amorphous weight

Fig. 2. Plot of the measured vs. weighed values (wt.%) of slag and metakaolin (MK) in their respective preselected mixes with quartz. Error bars represent the 2σ repeatability interval. The full grey line shows the 1:1 relationship, dashed lines depict a ±2 wt.% interval. Sample designations in bold indicate which amorphous component is plotted.

Fig. 3. XRD pattern decomposition of a sample composed of 50 wt.% metakaolin, 25 wt.% blast furnace slag, and 25 wt.% quartz.

fraction was calculated by difference. The resulting phase composition is given in Table 3. The results for the hydrated white cement were recalculated to an anhydrous basis using the bound water. The content of amorphous material was then adopted as phase fraction wα in the calculation of the calibrated phase constant by Eq. (1). It should be noted that the phase constant value has no direct physical meaning as the constituent Z, M and V values are unknown. In the Rietveld quantitative phase analyses the calibrated peak phases were introduced as separately quantifiable phases. Except for the phase scale factor, all profile parameters were kept fixed during refinement. As in the calibration measurements, the background signal

Fig. 4. XRD pattern decomposition of the simulated blended cements composed of: a) 40 wt.% OPC, and 60 wt.% blast furnace slag, b) 50 wt.% OPC, 25 wt.% blast furnace slag, and 25 wt.% metakaolin (MK). The difference curve is shown at the bottom of the graphs.

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was fitted using a first order Chebyshev polynomial and a 1/2θ term. The profiles of slag and metakaolin were adopted from the calibration. The C–S–H profile was based on the model obtained from the HWC decomposition. As slight changes in C–S–H peak widths were observed between different hydrating systems, a single lorentzian peak broadening parameter for all peaks was refined for the most advanced age of hydration. This reparametrized C–S–H model was then used for all previous ages. To calculate the mass absorption coefficients of the hydrated pastes the water content of the paste was included as the bound water content determined by the weight loss of samples heated to 600 °C for 1 h in a laboratory furnace for solvent exchanged and dried samples. Similarly, the calculation of the SCM degree of reaction needs to be corrected for the dilution of the solid phases by the bound water content of the paste. 2.4. Isothermal conduction calorimetry Isothermal conduction calorimetry was used as an independent way of following the degree of reaction over time. The heat released by the pozzolanic reaction in the different portlandite–metakaolin model systems was measured using a TAM Air calorimeter (TA Instruments). 15 g of paste was introduced into the calorimeter glass flasks and the heat flow was measured at a constant temperature of 20 °C up to 7 days of hydration. The heat flow curves were integrated to obtain the cumulative heat release. 3. Results 3.1. Anhydrous blends

Fig. 6. XRD pattern decomposition of the simulated hydrated blended cements composed of: a) 70 wt.% HWC, and 30 wt.% blast furnace slag, b) 70 wt.% HWC, and 30 wt.% blast furnace slag. The difference curve is shown at the bottom of the graphs.

In the following, the quantification results are discussed in terms of absolute repeatability errors (95% or 2σ confidence interval assuming a normal distribution of errors) and absolute bias (the difference between weighed and measured contents) reported in wt.%. The XRD methodology was first applied to the quantification of the slag and metakaolin content in simple binary mixes with quartz. The results are shown graphically in Fig. 2. In the slag–quartz mixes the repeatability (2σ) on slag quantification was always smaller than 0.5 wt.%. The 2σ repeatability on separately prepared samples of identical mix was somewhat higher, i.e. 0.9 wt.%. This means that sample preparation and homogenization are more limiting the precision than the XRD data collection and analysis procedure for the simple binary mixes. The absolute accuracy was always better than 0.8 wt.% and the average deviation from the as weighed composition was 0.5 wt.%. Next the method was applied to the quartz–slag–metakaolin mixes containing two amorphous phases. A measured and calculated XRD

pattern as well as the corresponding pattern decomposition is shown in Fig. 3. The measured XRD pattern shows a single broad diffuse scattering maximum. Using the calibrated peak profiles of the metakaolin and slag components, good fits to the measured patterns were obtained. The results are shown in Fig. 2. For both amorphous phases, the average 2σ repeatability was 0.36 wt.% and not higher than 0.72 wt.% for triplicate measurements of the same sample. Separately prepared samples of identical mix composition showed a 2σ repeatability of 0.3 wt.%. The measured vs. weighed bias was 1.1 wt.% on average (max. 2.1 wt.%). The presence of multiple amorphous phases thus slightly reduced the accuracy, however the results remain acceptable. In a next step the precision and accuracy of the method were evaluated for mixes of slag, metakaolin and grey Portland cement (OPC) as a complex multiphase crystalline material. First binary mixes of OPC and

Fig. 5. Plot of the measured vs. weighed values (wt.%) of slag and metakaolin (MK) in the simulated blended cements. Error bars represent the 2σ repeatability interval. The full grey line shows the 1:1 relationship, dashed lines depict a ±2 wt.% interval. Sample designations in bold indicate which amorphous component is plotted.

Fig. 7. Plot of the measured vs. weighed values (wt.%) of slag and metakaolin (MK) in the simulated hydrated blended cements. Error bars represent the 2σ repeatability interval. The full grey line shows the 1:1 relationship, dashed lines depict a ±2 wt.% interval.

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either slag or metakaolin were measured. Fig. 4a shows the pattern decomposition of a blend of 40 wt.% OPC and 60 wt.% blast furnace slag. The OPC and the blast furnace slag profiles are strongly overlapped and incorrect fitting of the blast furnace contribution would lead to significant deviations in the OPC quantification. Nevertheless, the quantification results for the binary blended cements (Fig. 5) were very repeatable with a 2σ repeatability smaller than 0.6 wt.% for the slag blends and less than 1.2 wt.% for the metakaolin blended cements. The accuracy was slightly less good compared to the quartz mixes, 0.8 wt.% on average for the slag blends (max. 2.0 wt.%), and 0.5 wt.% on average for the metakaolin blends (max. 1.1 wt.%). The results show that excellent analytical accuracy and precision on the quantification of SCMs in anhydrous blended cements can be achieved if the SCM profile is appropriately calibrated. The advantage of the PONKCS method over methods that indirectly quantify the amorphous content is highlighted by the analysis of the XRD pattern of a ternary blended cement composed of grey cement, metakaolin, and slag. The XRD pattern in Fig. 4b shows a relatively low and indistinctive amorphous hump. Nevertheless, the pattern decomposition enables both SCMs to be quantified separately with a high level of precision (2σ of 0.8 wt.% on average) and good accuracy (bias of 1.2–1.6 wt.%). Overall it can be concluded that for anhydrous mixes the method enables the quantification of amorphous SCMs with a precision better than 1 wt.% (2σ) and an accuracy better than 2 wt.%.

3.2. Hydrated systems Hydrated blended cements mostly contain several amorphous or nanocrystalline phases. Apart from the usually amorphous SCM, also the C–S–H phase (and potentially also C–A–H phases) cannot be quantified directly using Rietveld quantitative phase analysis. In addition, if the scattering contribution of the C–S–H phase is incorrectly accounted for, the deviations between the measured and calculated patterns may be partially compensated by other phases, strongly decreasing the accuracy of the analysis. As a first step in quantifying the SCM content in hydrated systems, mixes of hydrated white cement (HWC), hydration-stopped and thus containing a constant C–S–H phase, and fixed quantities of slag and metakaolin were tested. Samples of representative XRD patterns are shown in Fig. 6. The calculated pattern decomposition demonstrates the extensive peak overlap between the C–S–H and the slag contribution in particular. The quantification results are compared to the as weighed composition in Fig. 7. The results show that, even in the case of extensive overlap, a very good correlation was found between measured and weighed SCM contents. Absolute deviations were 0.33 wt.% on average (max. 0.6 wt.%) for the metakaolin–HWC mixes, and 0.7 wt.% on average (max. 1.0 wt.%) for the slag–HWC mixes. The analytical precision decreased however. In the slag–HWC samples an average 2σ repeatability of 2.5 wt.% (max. 3.3 wt.%) was found, in the

Table 4 Initial content (corrected for the bound water content) and unreacted content of the main reactive phases, i.e. portlandite, cement and SCMs (metakaolin and blast furnace slag) and the resulting extent of reaction. Pozzolanic reaction model systems Sample

P-M

P-M-LAS

P-M-HAS

Age (days)

1 4 7 1 4 7 1 4 7

Initial content (wt.%), bound water corrected

Unreacted content (wt.%)

Extent of reaction (%)

Portlandite

Metakaolin

Portlandite

Metakaolin

Portlandite

Metakaolin

46.4 38.1 36.9 36.5 34.1 33.2 34.9 32.5 32.2

43.0 35.3 34.2 33.8 31.6 30.7 32.4 30.1 29.8

40.7 23.5 12.3 12.9 8.5 7.9 11.3 8.3 7.5

40.1 26.9 19.3 22.5 19.0 17.6 22.3 18.7 17.4

12 38 67 65 75 76 68 74 77

7 24 44 33 40 43 31 38 42

Model systems with fillers Age (days) P-M-LAS-Q

P-M-LAS-Cc20

P-M-LAS-Cc6

Portlandite

Metakaolin

Portlandite

Metakaolin

Portlandite

Metakaolin

29.6 27.7 27.0 29.6 27.5 27.3 29.6 27.5 27.3

27.4 25.7 25.0 27.4 25.5 25.3 27.4 25.5 25.3

18.3 2.7 0.6 13.9 1.1 0.5 13.4 0.8 0.4

24.1 13.7 10.4 21.2 13.6 12.5 20.8 13.0 12.2

38 90 98 53 96 98 55 97 99

12 46 58 22 46 51 24 49 52

Age (days)

Cement

SCM

Cement

SCM

Cement

SCM

3 7 28 3 7 28 3 7 28 3 7 28

62.3 58.9 56.0 53.0 50.1 48.4 51.9 49.7 48.0 35.5 34.8 33.3

24.7 23.4 22.2 21.0 19.9 19.2 20.6 19.7 19.1 52.6 51.5 49.4

16.2 13.9 8.3 12.3 10.8 5.3 12.9 11.6 6.1 6.7 2.7 1.5

24.0 21.7 17.5 20.1 18.0 14.6 19.0 17.3 14.5 46.7 39.6 31.8

74 76 85 77 79 89 75 77 87 81 92 96

3 7 21 4 10 24 8 12 24 11 23 36

1 4 7 1 4 7 1 4 7

Blended cements

CEM-MK

CEM-MK-Q

CEM-MK-Cc6

CEM-Slag

R. Snellings et al. / Cement and Concrete Research 64 (2014) 89–98

Fig. 8. Hydration assemblage of the P-M, P-M-LAS, P-M-HAS, and the MK blended cement at 7 days of hydration. Ms, Hc, and Mc identify with monosulfoaluminate hydrate, hemicarboaluminate hydrate, and monocarboaluminate hydrate.

metakaolin–HWC samples a better of precision of 1.5 wt.% on average (max. 2.7 wt.%) was obtained. The decrease in precision was most noted for the slag–HWC samples and was most likely due to the important degree of peak overlap between both. The quantification results of the C–S–H phase in the mixes showed very similar levels of accuracy and precision as the SCMs. It can thus be concluded that also in blends containing C–S–H an accuracy better than 2 wt.% can be expected, however the measurement precision is lowered, advocating the use of repeats.

95

The application of the method to the determination of the extent of reaction of SCMs was first tested on simplified systems containing metakaolin and portlandite with additional alkalis and sulfate to simulate the reaction environment of blended Portland cements. The metakaolin content of the reacting system was determined at 1, 4, and 7 days and normalized to the initial content (bound water corrected) in order to calculate the extent of reaction as presented in Table 4. The effect of the various parameters (alkalis, sulfate, and presence of fillers) on the degree of reaction is the subject of another study which will be reported on later. Due to these variations there were some small differences in the hydrate assemblages formed (Fig. 8) but these are not discussed in detail here. The focus here is just the degree of reaction of the metakaolin component. An example of the XRD profile decomposition is shown in Fig. 9 for the P-M-LAS system with quartz and fine limestone filler at 1 day of hydration. The accuracy of the quantification results was cross-checked in two ways. First, standard additions of 20 and 40 wt.% metakaolin were made to selected reacted P-M-LAS model systems. The total metakaolin content was measured and plotted as a function of the metakaolin addition in Fig. 10. The extrapolated intercept with the ordinate gives an estimation of the metakaolin content in the original sample. These metakaolin contents compared well to the directly measured content in the samples with differences of 0.9 wt.% on average (max 1.9 wt.%). Secondly, the extent of the pozzolanic reaction measured by XRD was compared to the overall cumulative heat released by the exothermal hydration reactions. Normalized by the initial content of metakaolin in Fig. 11a, the cumulative heat release followed the same trends that were found by quantitative XRD. Quantitatively, the cumulative heat release correlated well with the extent of reaction measured by XRD in Fig. 11b. Nevertheless, the relationship between the extent of metakaolin reaction and the

Fig. 9. XRD pattern decomposition of the hydrated binders: a) P-M-LAS with quartz filler hydrated for 1 day, b) P-M-LAS with limestone (fine) filler hydrated for 1 day, c) blended cement with 30 wt.% metakaolin (MK) and 15 wt.% limestone hydrated for 3 days, d) blended cement with 60 wt.% slag hydrated for 3 days. Ms, and Hc stand for monosulfoaluminate hydrate and hemicarboaluminate hydrate, respectively.

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Fig. 10. Measured vs. weighed, added metakaolin mass fractions. The correspondence between the intercept of the trend line and the measured fraction in the original sample without additions gives an indication of the level of accuracy of the quantification method. Cc and Q refer to respectively the limestone and quartz fillers added to the reacting samples.

heat release also changes between systems due to changes in the hydrate assemblage. Finally, the extent of reaction of SCMs was measured for hydrated blended cements. The SCM reaction was followed in blended cements containing either slag (60 wt.%) or metakaolin (30 wt.%) with and without fillers. Measured and calculated XRD patterns of metakaolin and slag blended cements hydrated for 3 days are shown in Fig. 9. The pattern decomposition shows the contributions of the various raw materials and hydration products to the calculated XRD profile. The calculated degrees of reaction of the OPC and the SCM are given in Table 4. The results demonstrate that the degree of cement hydration is enhanced by increasing levels of cement replacement (filler effect). 4. Discussion The present paper demonstrates that the PONKCS method enables the quantification of amorphous phases in complex materials such as anhydrous and hydrated Portland cements with a level of precision (2σ) of 2–3 wt.% and an accuracy better than 3 wt.%. Table 5 summarizes the results on analytical precision and accuracy for the different systems. The performance of the PONKCS method for amorphous phases is therefore deemed comparable to that of the Rietveld method for crystalline phases in cement systems [18,22,37]. It should be noted that the estimated experimental errors on the SCM degree of reaction

scale with the initial SCM content of the blend. The smaller the SCM fraction in the blended cement the larger the error on the degree of reaction. Moreover, the analytical accuracy will also decrease at high degrees of reaction because of the relatively high detection limit of amorphous phases. The levels of accuracy and precision established by the present study indicate that the method is only practical for blended cements with replacement levels higher than 10%. Below 10% replacement, the accuracy of the method will be poor, due to the estimated 2–3 wt.% error on the SCM quantification. Moreover, detection limits are relatively high for amorphous phases and it is doubtful whether SCM residues below 3–5 wt.% can be reliably quantifiable. The presented quantification method assumes that the peak phase profiles and phase constants of the calibrated phases are representative for the corresponding phases in the unknown sample. In the case of blended cements, aberrations from this assumption may occur for both SCMs and the C–S–H phase and reduce the accuracy of the method. The decomposition method of the XRD patterns assumes a congruent dissolution of the SCM component. At high pH this is a realistic assumption for homogeneous SCMs such as slags and metakaolin [38]. For heterogeneous materials such as fly ashes, this condition may not hold and the unreacted residue may change in composition due to differences in reactivity of the various components. This may affect the peak phase profile and the phase constant of the SCM and thus bias the quantification results. An added bias may originate from variations in the composition of the precipitated C–S–H phase. The reaction of SCMs in blended cements is known to decrease the Ca/Si ratio and, in the case of Al-rich SCMs such as metakaolin, increase the Al uptake of the C–S–H phase [39]. In addition, both the morphology [40] and the XRD profile of the C–S–H [41] were found to vary between OPC and a range of blended cements, especially at later ages. The compositional and microstructural changes encountered may affect the C–S–H peak phase profile and the calibrated phase constant. The C–S–H model in this study was based on the C–S–H phase formed in the white cement hydrated for 7 years. A better match between the C–S–H model and the C–S–H in blended cements may be obtained using peak phase models of synthetic C–S–H of the appropriate composition and microstructure, or a model for C–S–H in fully reacted blended cements. Obtaining a C–S–H model directly from hydrated blended cements is experimentally difficult as it requires separating (and quantifying) the C–S–H phase contribution from other amorphous or nanocrystalline phases present in the hydrated blended cement (AFm phases, unreacted SCMs, …). Even so, XRD studies of synthetic C–S–H of varying composition have not indicated significant changes in the C–S–H XRD profiles with decreasing Ca/Si down to

Fig. 11. Cumulative heat release measured by calorimetry for pozzolanic reaction model systems with and without fillers (a), and plot of the measured amount of reacted metakaolin vs. the cumulative heat released (b).

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Table 5 Summary of the analytical precision (2σ) and accuracy for the different systems. Anhydrous blends Average (max.) wt.%

Binary systems

Ternary systems

Quartz (single phase) + SCM

Anhydrous cement (multiphase) + SCM

Quartz (single phase) + 2 SCMs

Anhydrous cement (multiphase) + 2 SCMs

Precision (2σ) Accuracy

0.3 (0.9) 0.5 (0.8)

0.7 (1.2) 0.7 (2.0)

0.4 (0.7) 1.1 (2.1)

0.8 (1.4) 1.4 (1.6)

Hydrated blends Average (max.)

Precision (2σ) Accuracy a

Weighed mix

Reacted system

Hydrated white cement + SCM

Pozzolanic reaction model system

2.0 (3.3) 0.5 (1.0)

3.0 (4.2) 0.9 (1.9)a

Estimated by the standard addition method.

Ca/Si ratios of 1.0 [42,43]. In addition, incorporation of Al into C–S–H was found to shift only the basal (002) peak and leave other peaks unchanged [43–45]. The present study did not consider low-angle reflections and the C–S–H basal peak was not comprised in the present model. Nevertheless, even if there are few indications that the C–S–H XRD profile changes significantly with composition, the phase constant may well vary and render the PONKCS method less suitable for the quantification of the C–S–H phase. It should be noted however that an inaccuracy in the determination of the C–S–H phase constant will only change the quantification results by a constant factor, and that the quantification of other phases is not affected. 5. Conclusions The XRD based PONKCS method was successfully applied to the quantification of the content and extent of reaction of SCMs in anhydrous and hydrated blended cements. The PONKCS method was implemented into Rietveld refinement software and combined with an external standard approach to enable the quantification of crystalline and individual amorphous phases. The quantification method and approach was described in detail. The analytical accuracy and precision of the method were evaluated through an extensive series of model mixes of increasing complexity, ranging from simple binary mixes of an SCM and a single crystalline phase over ternary mixes of anhydrous grey Portland cement with two separately measurable SCMs to mixes of hydrated cement and SCMs. Overall, the precision (2σ) on the measured SCM content was 2–3 wt.% or less for triplicate analyses. The accuracy of the measurement was better than 2 wt.%, even for the more complex mixes. In hydrating binder systems the results were satisfactory in that 1) quantification results obtained by the standard addition method corresponded to the initial measurement of the SCM content in the original sample, and 2) the cumulative heat release measured by calorimetry scaled linearly with the extent of SCM reaction. The notable advantages of the application of the PONKCS method to the measurement of the SCM content and extent of reaction are 1) the widespread availability of XRD equipment, 2) the rapid data collection (15–30 min), 3) the straightforward and fast data analysis, and 4) the potential general applicability to all SCMs. The method can be implemented fairly easily into existing software packages for Rietveld analysis by a skilled operator. The method is therefore likely to find a more widespread use, both in research and in production control, particularly because there are in principle no specific limitations regarding the SCM type or composition. Acknowledgements RS gratefully acknowledges financial support of the European Community under FP7-Marie Curie IEF grant 298337.

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