Synchrotron Radiation Pair Distribution Function Analysis of Gels in Cements

crystals Article Synchrotron Radiation Pair Distribution Function Analysis of Gels in Cements Ana Cuesta 1ID , Jesus D. Zea-Garcia 2, Diana Londono-Zuluaga 2ID , Angeles G. De la Torre 2, Isabel Santacruz 2, Oriol Vallcorba 1ID and Miguel A. G. Aranda 1,*ID 1ALBA Synchrotron radiation facility, Carrer de la Llum 2-26, 08290 Cerdanyola del Vallès, Spain; [email protected] (A.C.); [email protected] (O.V.) 2Departamento de Química Inorgánica, Cristalografía y Mineralogía, Universidad de Málaga, 29071 Málaga, Spain; [email protected] (J.D.Z.-G.); [email protected] (D.L.-Z.); [email protected] (A.G.D.l.T.); [email protected] (I.S.) *Correspondence: [email protected]; Tel.: +34-93-592-4305 Academic Editor: William Clegg Received: 22 September 2017; Accepted: 17 October 2017; Published: 18 October 2017 Abstract: The analysis of atomic ordering in a nanocrystalline phase with small particle sizes, below ≈ 5 nm, is intrinsically complicated because of the lack of long-range order. Furthermore, the presence of additional crystalline phase(s) may exacerbate the problem, as is the case in cement pastes. Here, we use the synchrotron pair distribution function (PDF) chiefly to characterize the local atomic order of the nanocrystalline phases, gels, in cement pastes. We have used a multi r-range analysis approach, where the ~4–7 nm r-range allows determining the crystalline phase contents; the ~1–2.5 nm r-range is used to characterize the atomic ordering in the nanocrystalline component; and the ~0.2–1.0 nm r-range gives insights about additional amorphous components. Specifically, we have prepared four alite pastes with variable water contents, and the analyses showed that a defective tobermorite, Ca 11 Si 9 O 28 (OH) 2· 8.5H 2 O, gave the best fit. Furthermore, the PDF analyses suggest that the calcium silicate hydrate gel is composed of this tobermorite and amorphous calcium hydroxide. Finally, this approach has been used to study alternative cements. The hydration of monocalcium aluminate and ye’elimite pastes yield aluminum hydroxide gels. PDF analyses show that these gels are constituted of nanocrystalline gibbsite, and the particle size can be as small as 2.5 nm. Keywords: Portland cement; cementitious materials; C-S-H gel; amorphous phases; total scattering 1. Introduction 1.1. Cements Portland cement (PC) is considered the most manufactured product in the world, as it is the main component of the construction industry [ 1 ]. PC is composed of the Portland clinker, the setting regulator (a calcium sulfate source), and in many cases additions and admixtures [ 2 ]. The four main phases of any Portland clinker are: ~65 wt% alite or Ca 3 SiO 5 ; ~15 wt% belite, or Ca 2 SiO 4 ; ~15 wt% ferrite, or Ca4Al2Fe2O10, and ~5 wt% tricalcium aluminate, or Ca3Al2O6[2]. The hydration of PC basically consists of two different types of reactions that interact with each other: the silicate hydration reactions, and the aluminate hydration reactions. The reaction kinetics of alite and tricalcium aluminate are quite fast; comparatively, the kinetics of belite and ferrite hydration reactions are much slower [ 2 , 3 ]. Hereafter, and for PC hydration, we restrict the introduction and discussion to the hydration of pure alite, although we are aware that this is a simplification. The main hydration reaction in PC is that produced by alite. It consists of the dissolution of Ca 3 SiO 5 , the supersaturation of the medium with the different ionic species, and then, the precipitation Crystals 2017,7, 317; doi:10.3390/cryst7100317 www.mdpi.com/journal/crystals Crystals 2017,7, 317 2 of 17 of a calcium silicate hydrate, C-S-H gel in cement nomenclature, and the crystallization of portlandite, c-Ca(OH) 2 , which can be followed by powder diffraction [ 4 ]. This set of actions can be schematically represented by reaction (1), but the different processes (may) have different kinetics and be locally inhomogeneous. Therefore, reaction (1) is a simplification for alite hydration. Ca3SiO5+ 5.2H2O→crystalline: 1.2Ca(OH)2+gel: (CaO)1.8(SiO2)(H2O)4.0 (1) The literature includes descriptions of slightly different hydration reactions for alite, which lead to calcium silicate hydrate gels with slightly different densities and compositions [ 5 – 8 ]. Here, we have chosen the Ca-to-Si average ratio of 1.8, because it is compatible with many previous reports, which gave values ranging from 1.7–2.0. The calcium-silicate-hydrate (C-S-H) gel is the main hydrated phase in PC pastes, and it is the main constituent responsible for the strength and durability of the resulting mortars and concretes. However, there is still controversy about the atomic structure of the C-S-H gel, and consequently, many recent works have reviewed and addressed this issue [9–11]. There are many studies showing that the C-S-H gel contains water within large pores and a calcium silicate hydrate of nanocrystalline nature. However, due to the low degree of crystallinity of this phase (and the presence of other crystalline phases such as portlandite, unreacted alite, calcium carbonate from partial carbonation, and several other phases in real cements), there is still no consensus on whether this nanocrystalline phase is close to tobermorite-14Å, Ca 5 Si 6 O 16 (OH) 2· 7H 2 O, ρ= 2.19 g/cm3 ; tobermorite-11Å, Ca 4 Si 6 O 15 (OH) 2· 5H 2 O, ρ = 2.40 g/cm 3 ; or jennite, Ca 9 Si 6 O 18 (OH) 6· 8H 2 O, ρ= 2.31 g/cm3 minerals/structures [ 12 ]. This is key to understanding the density of the C-S-H gel that governs the space-filling nature of the cement paste. Several experimental and theoretical techniques have proposed that tobermorite better justifies the properties of this calcium silicate hydrate [13–15]. If this emerging picture is correct, then it opens another conundrum: how can nanocrystalline tobermorite, which has a Ca/Si ratio of 0.67 or 0.83, explain the observed Ca/Si ratio in gels, which commonly range from 1.7 to 1.9 in pastes without additions? Defective tobermorite could justify a Ca/Si atomic ratio close to 1.2–1.3, but not higher than that. One way to reconcile all of the observations is to suppose that the microstructure of the C-S-H gel, arising from the hydration of alite, is a fine intermixing of tobermorite with amorphous Ca(OH) 2 plus the water in the small and large gel pores [ 6 , 13 , 16 ]. An updated mechanism of growth of the C-S-H gel, and consequently, the evolution and nature of the gel water, have been very recently proposed [17]. Portland cement is not the only cement that is commercially available. Calcium aluminate cements (CAC) [ 18 ] are another kind of cements that is currently used for several special applications. The main component of CAC cements is monocalcium aluminate, or CaAl 2 O 4 . At temperatures higher than 30 ◦ C, CaAl 2 O 4 hydrates according to reaction (2), and after its dissolution, the crystallization of hydrogarnet, Ca 3 Al 2 (OH) 12 , and the precipitation of a nanocrystalline aluminum hydroxide gel take place. 3CaAl2O4+ (12+4n)H2O→crystalline: Ca3Al2(OH)12 +nanocrystalline: 4Al(OH)3(H2O)n(2) Furthermore, another type of cement that is also commercially available for special applications are calcium sulfoaluminate (CSA) cements [ 19 ]. These cements have calcium sulfoaluminate, or ye’elimite Ca 4 Al 6 O 12 SO 4 , as its major constituent. The hydration of ye’elimite depends upon the available sulfate content [ 20 ]. In the presence of enough sulfate, it results in the crystallization of ettringite [Ca 6 Al 2 (OH) 12 (SO 4 ) 3· 26H 2 O or AFt] and the precipitation of a nanocrystalline aluminum hydroxide gel, see reaction (3): Ca4Al6O12SO4+ 2CaSO4·2H2O + (34+4n)H2O→crystalline: Ca6Al2(OH)12(SO4)3·26H2O + nanocrystalline: 4Al(OH)3(H2O)n(3) Crystals 2017,7, 317 3 of 17 1.2. X-ray Pair Distribution Function Analysis In addition, the pair distribution function (PDF), also known as G(r), shows the probability of finding pairs of atoms separated by a distance r.G(r) is experimentally obtained by a Fourier transform of the total scattering powder pattern, according to Equation (4) [21–23]. G(r)=4πr[ρ(r)−ρ0]=2 πZ∞ 0Q[S(Q)−1]sin(Qr)dQ (4) where ρ (r) is the microscopic atomic pair density, ρ0 is the average atomic number density, S(Q) is the total scattering structure function, and Qis the momentum transfer, Q = 4 π sin( θ )/ λ ). In order to obtain PDF patterns of very high quality, the recorded Q-range has to be large. For this reason, the use of short wavelength X-rays and high 2 θ diffracting angles is very important [ 24 ]. Total scattering data analyzed by PDF methodology may distinguish phases that are amorphous, nanocrystalline, or crystalline, and this methodology provides quantitative information concerning phase contents and local bonding environments [21]. In order to shed light and open questions in the chemistry and local atomic structure of C-S-H gels, neutron and synchrotron total scattering data analyzed by PDF methodology have been used, and these studies have been reviewed [ 25 , 26 ]. Here, we distinguish the PDF studies on synthetic C-S-H gels [ 13 , 27 – 29 ] from those carried out for C-S-H gels formed from the hydration of alite [ 29 – 31 ], as the local atomic arrangements could be quite different. These investigations have revealed that the C-S-H gel from alite hydration presents a nanocrystalline nature with atomic ordering up to approximately 40 Å [ 29 , 31 ]. Very recently, it has been reported that C-S-H gel has an atomic arrangement close to that of tobermorite, and the presence of nanosheets of calcium hydroxide was inferred by using a differential PDF approach [13]. Moreover, we have reported a PDF study on the hydration of CaAl 2 O 4 to get insight into the nanocrystalline nature of the resulting aluminum hydroxide gel. It was shown that this phase was nanocrystalline gibbsite, with an average particle size close to 5.5 nm [32]. The overall objective of this research is to quantitatively analyze the atomic structure of the amorphous/nanocrystalline gels in cement pastes by using the synchrotron X-ray pair distribution function. Specifically, the questions addressed here address whether it is possible to confirm that tobermorite fits the scattering signal of C-S-H gel samples produced by the hydration of alite better than jennite. This study also investigated whether different water-to-alite ratios modify the nanocrystalline nature of the resulting C-S-H gels. Finally, this type of research has been extended to the aluminum hydroxide gels produced in the hydration of monocalcium aluminate and ye’elimite samples. 2. Results and Discussion The hydration of three different set of samples, Ca 3 SiO 5 , CaAl 2 O 4 , and Ca 4 Al 6 O 12 SO 4 have been studied here. These cementitious phases were investigated in order to simplify the problem, as the real cements containing these phases yield even more complex pastes. In any case, all of the cementitious hydrated systems contained at least a crystalline phase and a nanocrystalline/amorphous gel, which have both been characterized by several techniques, and chiefly by synchrotron total scattering using pair distribution functions. 2.1. Alite Pastes Since the alite hydration kinetics depends upon the average particle size [ 33 , 34 ], the particle size distribution for Ca 3 SiO 5 was measured (d v,50 = 13.8 µ m). The particle size distribution plot is shown in Figure S1a. Four alite pastes were prepared at water-to-solid (w/s) mass ratios of 0.46, 0.55, 0.65, and 0.80, for 34 days. After the hydration was arrested, as described in the methods section, Rietveld quantitative phase analyses, using the internal standard methodology, were performed for these pastes. The Rietveld analyses were carried out using the crystal structures reported for alite [ 35 ] Crystals 2017,7, 317 4 of 17 and portlandite [ 36 ]. Table S1 shows the quantitative phase analysis results for each paste. For these model samples, the ACn (Amorphous and Crystalline non-quantified content) values are mainly the C-S-H gel contents that include the nanocrystalline calcium silicate, plus any amorphous component (water in the pores and possibly calcium hydroxide). We recall that the free water is removed in the arresting process. The amount of unreacted alite ranges from 13 to 10 wt% for the samples w/s = 0.46 and 0.80, respectively. Consequently, the amount of crystalline portlandite was larger for the w/s = 0.80 sample, ~22 wt%, which decreased to ~18 wt% for the w/s = 0.46 sample. Crystalline calcite contents, from the partial carbonation of portlandite, were always lower than 1.5 wt%, and the amorphous content was close to 66 wt%. As an example, Figure 1shows the laboratory X-ray powder diffraction (LXRPD) Rietveld plot for the w/s = 0.80 paste with the main peaks labelled. Crystals 2017, 7, 317 4 of 17 model samples, the ACn (Amorphous and Crystalline non-quantified content) values are mainly the C-S-H gel contents that include the nanocrystalline calcium silicate, plus any amorphous component (water in the pores and possibly calcium hydroxide). We recall that the free water is removed in the arresting process. The amount of unreacted alite ranges from 13 to 10 wt% for the samples w/s = 0.46 and 0.80, respectively. Consequently, the amount of crystalline portlandite was larger for the w/s = 0.80 sample, ~22 wt%, which decreased to ~18 wt% for the w/s = 0.46 sample. Crystalline calcite contents, from the partial carbonation of portlandite, were always lower than 1.5 wt%, and the amorphous content was close to 66 wt%. As an example, Figure 1 shows the laboratory X-ray powder diffraction (LXRPD) Rietveld plot for the w/s = 0.80 paste with the main peaks labelled. Figure 1. Laboratory X-ray powder diffraction (LXRPD) Rietveld plot (CuKα 1 radiation, λ = 1.54059 Å) with internal standard for the alite w/s = 0.80 paste hydrated for 34 days. The crystalline phases are portlandite (high content, ICSD #202220) and alite (ICSD #94742) and calcite (low contents, ICSD #80869). The main peaks are labelled as follows: portlandite (•), alite ( ) and added internal standard, α-Al 2 O 3 ( , ICSD #73725). The thermogravimetric data for the alite pastes are shown in Figure S2. The water loss from the nanocrystalline C-S-H gel takes place between room temperature and 250 °C. The water loss from crystalline portlandite, Ca(OH) 2 , is measured between 400 °C to 600 °C, with a strong peak centered close to 460 °C. The water loss observed between 250 °C and 400 °C could have a contribution from the dehydration of amorphous calcium hydroxide, see below. Finally, the small weight loss above 600 °C is due to calcium carbonate. Two small signals are observed at 660 and 860 °C. The weight loss observed close to 860 is due to the decarbonation of crystalline calcite. We speculate that the minor loss observed close to 660 °C comes from the decarbonation of amorphous calcite, and it could also be due to remnant organic solvent absorbed in the C-S-H gel [37]. Table S2 gives full details of the weight losses from the TGA study for the four alite pastes. The raw PDF data for the four hydrated pastes are displayed in Figure 2 in the 1.5–10 Å selected range. No significant differences are observed between the samples in this low-r region of the PDF curves. Initially, the PDF pattern for the w/s = 0.80 sample, which had the highest alite reaction degree, was quantitatively studied using the PDFgui software. The main objective of this study was to quantify the C-S-H nanocrystalline gel content, and to determine which crystal structure fits better the PDF signal arising from this phase. The full PDF data for alite pastes are very complex due to the presence of different phases of different natures: crystalline (e.g., portlandite and unreacted Ca 3 SiO 5 ), nanocrystalline (e.g., C-S-H gel) and possibly amorphous phase(s). The following strategy was used for the PDF fit. Initially, a Figure 1. Laboratory X-ray powder diffraction (LXRPD) Rietveld plot (CuK α1 radiation, λ= 1.54059 Å ) with internal standard for the alite w/s = 0.80 paste hydrated for 34 days. The crystalline phases are portlandite (high content, ICSD #202220) and alite (ICSD #94742) and calcite (low contents, ICSD #80869). The main peaks are labelled as follows: portlandite ( ), alite ( H ) and added internal standard, α-Al2O3(F, ICSD #73725). The thermogravimetric data for the alite pastes are shown in Figure S2. The water loss from the nanocrystalline C-S-H gel takes place between room temperature and 250 ◦ C. The water loss from crystalline portlandite, Ca(OH) 2 , is measured between 400 ◦ C to 600 ◦ C, with a strong peak centered close to 460 ◦ C. The water loss observed between 250 ◦ C and 400 ◦ C could have a contribution from the dehydration of amorphous calcium hydroxide, see below. Finally, the small weight loss above 600 ◦ C is due to calcium carbonate. Two small signals are observed at 660 and 860 ◦ C. The weight loss observed close to 860 is due to the decarbonation of crystalline calcite. We speculate that the minor loss observed close to 660 ◦ C comes from the decarbonation of amorphous calcite, and it could also be due to remnant organic solvent absorbed in the C-S-H gel [ 37 ]. Table S2 gives full details of the weight losses from the TGA study for the four alite pastes. The raw PDF data for the four hydrated pastes are displayed in Figure 2in the 1.5–10 Å selected range. No significant differences are observed between the samples in this low-r region of the PDF curves. Initially, the PDF pattern for the w/s = 0.80 sample, which had the highest alite reaction degree, was quantitatively studied using the PDFgui software. The main objective of this study was to quantify the C-S-H nanocrystalline gel content, and to determine which crystal structure fits better the PDF signal arising from this phase. Crystals 2017,7, 317 5 of 17 The full PDF data for alite pastes are very complex due to the presence of different phases of different natures: crystalline (e.g., portlandite and unreacted Ca 3 SiO 5 ), nanocrystalline (e.g., C-S-H gel) and possibly amorphous phase(s). The following strategy was used for the PDF fit. Initially, a high r-region, i.e., 40–70 Å, was analyzed, where the contributions are mainly due to crystalline phases: portlandite, alite, and calcite. As there was a significant amount of unreacted alite, the PDF pattern of as-received Ca 3 SiO 5 was also collected and fitted using PDFGui, see Figure S3. The unit cell parameters and atomic displacement parameters (ADP) were optimized. These values were used for the analysis of the four pastes, and only the scale factors for alite were refined. This strategy has been previously reported [ 31 ]. The contribution of crystalline portlandite was computed by optimizing the scale factor, the unit cell parameters, the anisotropic ADPs values for Ca and O, and the atomic positions for O (Ca is located at a special position). As the sample contained crystalline calcium carbonate as an impurity, the scale factor of this phase was also refined in this region. The final R W value was 28.8%. The fit is displayed in Figure 3. The unit cell values for portlandite converged to a = 3.594 Å and c = 4.916 Å, and the final ADPs values and atomic positions for portlandite are reported in Table S3. Crystals 2017, 7, 317 5 of 17 high r-region, i.e., 40–70 Å, was analyzed, where the contributions are mainly due to crystalline phases: portlandite, alite, and calcite. As there was a significant amount of unreacted alite, the PDF pattern of as-received Ca 3 SiO 5 was also collected and fitted using PDFGui, see Figure S3. The unit cell parameters and atomic displacement parameters (ADP) were optimized. These values were used for the analysis of the four pastes, and only the scale factors for alite were refined. This strategy has been previously reported [31]. The contribution of crystalline portlandite was computed by optimizing the scale factor, the unit cell parameters, the anisotropic ADPs values for Ca and O, and the atomic positions for O (Ca is located at a special position). As the sample contained crystalline calcium carbonate as an impurity, the scale factor of this phase was also refined in this region. The final R W value was 28.8%. The fit is displayed in Figure 3. The unit cell values for portlandite converged to a = 3.594 Å and c = 4.916 Å, and the final ADPs values and atomic positions for portlandite are reported in Table S3. Figure 2. Experimental pair distribution function (PDF) patterns from 1.5 to 10 Å for alite, calcium aluminate, and ye’elimite pastes. Figure 3. Experimental (blue circles) and fitted (red solid line) PDF for Ca 3 SiO 5 w/s = 0.80 paste in the 40 to 70 Å r-range. Difference curve is shown as a grey line. Figure 2. Experimental pair distribution function (PDF) patterns from 1.5 to 10 Å for alite, calcium aluminate, and ye’elimite pastes. After this refinement, all of the previous parameters were kept fixed, and the contribution from the nanocrystalline C-S-H gel was studied in the intermediate r-region, from 10 to 25 Å. As there is controversy about the ‘average’ crystal structure of the precipitated C-S-H nanocrystalline gel, different crystal structures have been tested. Table 1gives the R W values for each PDF fit using selected structural descriptions for the nanocrystalline gel, and also includes the quantitative phase analysis results for the different phases. Firstly, it is quite important to distinguish which structure yields a better fit: tobermorite-14Å, tobermorite-11Å, or jennite. Secondly, several models for different defective tobermorites have been tested in this study. A summary of the results for the PDF study of the w/s = 0.80 sample, in the 10–25 Å intermediate r-region, is given in Table 1. It is evident that the jennite structure led to a worse fit than the tobermorite structures. Although some crystal structures for tobermorite led to relatively similar R W values, see Table 1, the defective structure clinotobermorite T3_14sc [9] was selected due to the lowest RWvalue, and also because it matched properly the peak at 15.8 Å, see Figure 4. The clinotobermorite T3_14sc structure represents a hypothetical ‘trimer’ derived from a staggered-chain clinotobermorite, yielding a formula of Ca 11 Si 9 O 28 (OH) 2· 8.5H 2 O with a calculated theoretical density of 2.49 g/cm 3 [ 9 ]. The Crystals 2017,7, 317 6 of 17 unit cell values for the clinotobermorite T3_14sc structure converged to a = 11.258 Å, b = 7.307 Å, c = 42.538 Å and β = 94.3 ◦ , and the isotropic ADPs were 0.0065, 0.0833, and 0.0160 Å 2 for Ca, Si, and O, respectively. Figure 4shows the PDF fit in the region from 10 to 25 Å without the clinotobermorite T3_14sc contribution (a), and with its contribution (b). The selected clinotobermorite structural description improved the PDF fit as the R W value decreased from 36.3% to 28.5%. It is worth noting that the average length of the silicate chains in this defective clinotobermorite is three, and the Ca/Si atomic ratio is 1.22. Crystals 2017, 7, 317 5 of 17 high r-region, i.e., 40–70 Å, was analyzed, where the contributions are mainly due to crystalline phases: portlandite, alite, and calcite. As there was a significant amount of unreacted alite, the PDF pattern of as-received Ca 3 SiO 5 was also collected and fitted using PDFGui, see Figure S3. The unit cell parameters and atomic displacement parameters (ADP) were optimized. These values were used for the analysis of the four pastes, and only the scale factors for alite were refined. This strategy has been previously reported [31]. The contribution of crystalline portlandite was computed by optimizing the scale factor, the unit cell parameters, the anisotropic ADPs values for Ca and O, and the atomic positions for O (Ca is located at a special position). As the sample contained crystalline calcium carbonate as an impurity, the scale factor of this phase was also refined in this region. The final R W value was 28.8%. The fit is displayed in Figure 3. The unit cell values for portlandite converged to a = 3.594 Å and c = 4.916 Å, and the final ADPs values and atomic positions for portlandite are reported in Table S3. Figure 2. Experimental pair distribution function (PDF) patterns from 1.5 to 10 Å for alite, calcium aluminate, and ye’elimite pastes. Figure 3. Experimental (blue circles) and fitted (red solid line) PDF for Ca 3 SiO 5 w/s = 0.80 paste in the 40 to 70 Å r-range. Difference curve is shown as a grey line. Figure 3. Experimental (blue circles) and fitted (red solid line) PDF for Ca 3 SiO 5 w/s = 0.80 paste in the 40 to 70 Å r-range. Difference curve is shown as a grey line. Table 1. Selected results for the synchrotron PDF analysis of the w/s = 0.80 alite paste in the 10–25 Å r-region, using different structural descriptions for the nanocrystalline C-S-H phase. Phase RW(%) Ca3SiO5(wt%) CaCO3(wt%) Cryst-Ca(OH)2(wt%) C-S-H (wt%) Jennite (t), ICSD #151413 33.0 20.3 1.4 34.5 43.7 Tobermorite-14 (m), ICSD #152489 32.9 20.9 1.5 35.5 42.2 Tobermorite-11 (o), ICSD #92941 30.3 18.6 1.3 31.6 48.5 Tobermorite-11 (m), ICSD #87690 30.5 17.6 1.2 29.8 51.4 Tobermorite-11 (o), ICSD #100405 29.9 19.7 1.4 33.5 45.5 Clinotobermorite (m), ICSD #90036 28.8 13.8 1.0 23.4 61.9 Clinotobermorite (t), ICSD #90034 28.5 14.1 1.0 23.9 61.0 Clinotobermorite (m), T5_14sc* 29.2 15.4 1.1 26.2 57.4 Clinotobermorite (m), T5_11sc* 30.4 17.8 1.2 30.2 50.8 Clinotobermorite (m), T3_14sc* 28.5 16.0 1.1 27.1 55.8 * Richardson, 2014 [9]; (m), (o) & (t) denotes monoclinic, orthorhombic and triclinic, respectively. Crystals 2017,7, 317 7 of 17 Finally, for this sample, all of the previous parameters were fixed to study the lowest r-region, 2–10 Å. In this case, the delta2 value (low-r correlated motion peak sharpening factor) was fixed to 2 Å 2 . The high R W value, 45.6%, indicated the presence of an additional scattering contribution in this region, see Figure 5. This unfitted contribution, highlighted in the difference curve, is consistent with the theoretical PDF signal of an isolated single layer of Ca(OH) 2 arranged as in portlandite, as very recently suggested [ 13 ]. Figure 5shows the PDF fit in the r-region, 2–10 Å, including the simulated PDF curves for the crystal structures that have been included in the fit: crystalline portlandite, alite, and clinotobermorite T3_14sc, as well as the simulated curves for an isolated layer and two consecutives layers of Ca(OH)2. For the remaining three alite hydrated samples, the same procedure was followed. The final RW values in the 40–70 Å region were 22.8%, 27.6%, and 26.8%, for w/s = 0.65, 0.55, and 0.46 samples, respectively. In the r-region, 10–25 Å, only clinotobermorite T3_14sc and jennite structural descriptions were tested in order to show whether the fits by using clinotobermorite T3_14sc were better than those using jennite. Table S4 shows the R W values and the quantitative phase analysis results for the three pastes, using both, or either, jennite and clinotobermorite, as crystal structures to fit the nanocrystalline gel contribution. Invariably, clinotobermorite T3_14sc gave a better fit. The refined unit cell values of portlandite and clinotobermorite for the three pastes were very similar, and they are reported in Table S5. Finally, the PDF fits in the 2–10 Å r-region were very similar to that reported above for the paste at w/s = 0.80, and hence they are not shown here. Crystals 2017, 7, 317 7 of 17 and clinotobermorite T3_14sc, as well as the simulated curves for an isolated layer and two consecutives layers of Ca(OH)2. For the remaining three alite hydrated samples, the same procedure was followed. The final RW values in the 40–70 Å region were 22.8%, 27.6%, and 26.8%, for w/s = 0.65, 0.55, and 0.46 samples, respectively. In the r-region, 10–25 Å, only clinotobermorite T3_14sc and jennite structural descriptions were tested in order to show whether the fits by using clinotobermorite T3_14sc were better than those using jennite. Table S4 shows the RW values and the quantitative phase analysis results for the three pastes, using both, or either, jennite and clinotobermorite, as crystal structures to fit the nanocrystalline gel contribution. Invariably, clinotobermorite T3_14sc gave a better fit. The refined unit cell values of portlandite and clinotobermorite for the three pastes were very similar, and they are reported in Table S5. Finally, the PDF fits in the 2–10 Å r-region were very similar to that reported above for the paste at w/s = 0.80, and hence they are not shown here. Figure 4. Experimental (blue circles) and fitted (red solid line) PDF patterns for Ca3SiO5 w/s = 0.80 paste in the 10–25 Å r-range. (a) Fit including only portlandite and alite phases, RW = 36.3% and (b) Fit including portlandite, alite, and clinotobermorite T3_14sc phases, RW = 28.5%. Difference curves are shown as grey lines. 10 12 14 16 18 20 22 24 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 G (r) (Å -2 ) r (Å) (a) (b) 10 12 14 16 18 20 22 24 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 1.2 Figure 4. Experimental (blue circles) and fitted (red solid line) PDF patterns for Ca 3 SiO 5 w/s = 0.80 paste in the 10–25 Å r-range. ( a ) Fit including only portlandite and alite phases, R W = 36.3% and ( b ) Fit including portlandite, alite, and clinotobermorite T3_14sc phases, R W = 28.5%. Difference curves are shown as grey lines. Crystals 2017,7, 317 8 of 17 Crystals 2017, 7, 317 8 of 17 Figure 5. Experimental (blue circles) and fitted (red solid line) PDF patterns for Ca3SiO5 w/s = 0.80 paste in the 2–10 Å r-range. The difference curve is shown as a grey line. Simulated PDF curves for a single layer and a double layer of calcium hydroxide, portlandite, alite, and clinotobermorite T3_14sc are also included. 2.2. Calcium Aluminate Pastes The particle size distribution was also measured for CaAl2O4, where the dv,50 value obtained was 3.3 µm, and the particle size distribution plot is shown in Figure S1b. The second type of cementitious system studied was calcium aluminate pastes, which were initially studied by the PDF methodology in a previous publication [32] with a fixed water–solid ratio of 1.0. Here, we expand the research by varying the water-to-solid ratio, which is known to affect the hydration. In this work, CaAl2O4 was hydrated for 30 days with a w/s ratio of 0.55 and 1.20 at 35 °C. In order to study the influence of temperature, CaAl2O4 was also hydrated, w/s = 0.55, at 45 °C. Rietveld analyses were also performed for these three samples. These pastes contain a crystalline phase, hydrogarnet, and a nanocrystalline phase, aluminum hydroxide gel, as previously reported for a related sample [32]. No anhydrous calcium aluminate was found in any paste. Table S6 shows the quantitative phase analysis results for each paste. Again, the ACn content chiefly includes the nanocrystalline aluminum hydroxide gel. No important differences were found between the samples. Figure S4 shows the thermogravimetric plot for the three CaAl2O4 pastes. The overall measured weight losses (up to 600 °C) were 32.2, 32.7, and 30.9 wt% for the pastes with a w/s = 0.55 at 35 °C, 1.20 at 35 °C, and 0.55 at 45 °C, respectively. The temperatures for the two main water loss steps, nanocrystalline gel and hydrogarnet, are located around 260 and 300 °C, and they are partly overlapped. The expected water loss depends upon the sample composition (and water content of the aluminum hydroxide gel), as previously reported [32]. The PDF methodology was applied to find possible differences in the precipitated nanocrystalline aluminum hydroxide gels in the three pastes. The raw PDF patterns from 1–10 Å were displayed in Figure 2. For the PDF fits of these three samples, the methodology already reported (Cuesta et al., [32]) was followed. Firstly, the PDF data in a high r-region, 65–80 Å, were fitted with the crystal structure of the crystalline hydrogarnet. After optimizing the scale factors, unit cell, and ADPs, the RW values converged to 13.6%, 16.9%, and 16.2% for the pastes hydrated with a w/s of 0.55 at 35 °C, 1.20 at 35 °C, and 0.55 at 45 °C, respectively. The unit cell values and the final ADPs values (for Ca, Al, and O) are shown in Table S7. Then, all of the parameters for the crystalline 246810 -2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 G (r) (Å-2) r ( Å ) 246810 G (r) (Å-2) single layer of Ca(OH) 2 two layers of Ca(OH) 2 Portlandite alite Clinotobermorite_T3_14sc Figure 5. Experimental (blue circles) and fitted (red solid line) PDF patterns for Ca 3 SiO 5 w/s = 0.80 paste in the 2–10 Å r-range. The difference curve is shown as a grey line. Simulated PDF curves for a single layer and a double layer of calcium hydroxide, portlandite, alite, and clinotobermorite T3_14sc are also included. 2.2. Calcium Aluminate Pastes The particle size distribution was also measured for CaAl 2 O 4 , where the d v,50 value obtained was 3.3 µ m, and the particle size distribution plot is shown in Figure S1b. The second type of cementitious system studied was calcium aluminate pastes, which were initially studied by the PDF methodology in a previous publication [ 32 ] with a fixed water–solid ratio of 1.0. Here, we expand the research by varying the water-to-solid ratio, which is known to affect the hydration. In this work, CaAl 2 O 4 was hydrated for 30 days with a w/s ratio of 0.55 and 1.20 at 35 ◦ C. In order to study the influence of temperature, CaAl2O4was also hydrated, w/s = 0.55, at 45 ◦C. Rietveld analyses were also performed for these three samples. These pastes contain a crystalline phase, hydrogarnet, and a nanocrystalline phase, aluminum hydroxide gel, as previously reported for a related sample [ 32 ]. No anhydrous calcium aluminate was found in any paste. Table S6 shows the quantitative phase analysis results for each paste. Again, the ACn content chiefly includes the nanocrystalline aluminum hydroxide gel. No important differences were found between the samples. Figure S4 shows the thermogravimetric plot for the three CaAl 2 O 4 pastes. The overall measured weight losses (up to 600 ◦ C) were 32.2, 32.7, and 30.9 wt% for the pastes with a w/s = 0.55 at 35 ◦ C, 1.20 at 35 ◦ C, and 0.55 at 45 ◦ C, respectively. The temperatures for the two main water loss steps, nanocrystalline gel and hydrogarnet, are located around 260 and 300 ◦ C, and they are partly overlapped. The expected water loss depends upon the sample composition (and water content of the aluminum hydroxide gel), as previously reported [32]. The PDF methodology was applied to find possible differences in the precipitated nanocrystalline aluminum hydroxide gels in the three pastes. The raw PDF patterns from 1–10 Å were displayed in Figure 2. For the PDF fits of these three samples, the methodology already reported ( Cuesta et al., [32] ) was followed. Firstly, the PDF data in a high r-region, 65–80 Å, were fitted with the crystal structure of the crystalline hydrogarnet. After optimizing the scale factors, unit cell, and ADPs, the R W values converged to 13.6%, 16.9%, and 16.2% for the pastes hydrated with a w/s of 0.55 at 35 ◦ C, 1.20 at 35 ◦ C, and 0.55 at 45 ◦ C, respectively. The unit cell values and the final ADPs values (for Ca, Al, Crystals 2017,7, 317 9 of 17 and O) are shown in Table S7. Then, all of the parameters for the crystalline phase were kept fixed, and the r-region, 1.5–70 Å, was used to fit the additional contribution of the nanocrystalline phase, aluminum hydroxide gel. The gibbsite crystal structure was used for fitting this nanocrystalline phase. The optimized parameters were the scale factors, unit cell parameters, ADPs, delta2, atomic parameters, and sp. diameter, the average diameter of the nanoparticles. Table S7 also includes the unit cell parameters and ADPs for the gibbsite crystal structure that was used to fit the contribution of aluminum hydroxide gel. A hemicarbonate hydrated phase was also added in the final PDF fit, as it was present as a side phase in these pastes. A summary of the results for the three PDF fits are reported in Table 2. As an example, Figure 6displays the PDF fit for the w/s = 0.55 paste hydrated at 35 ◦C split in four r-ranges for better visualization. Crystals 2017, 7, 317 9 of 17 phase were kept fixed, and the r-region, 1.5–70 Å, was used to fit the additional contribution of the nanocrystalline phase, aluminum hydroxide gel. The gibbsite crystal structure was used for fitting this nanocrystalline phase. The optimized parameters were the scale factors, unit cell parameters, ADPs, delta2, atomic parameters, and sp. diameter, the average diameter of the nanoparticles. Table S7 also includes the unit cell parameters and ADPs for the gibbsite crystal structure that was used to fit the contribution of aluminum hydroxide gel. A hemicarbonate hydrated phase was also added in the final PDF fit, as it was present as a side phase in these pastes. A summary of the results for the three PDF fits are reported in Table 2. As an example, Figure 6 displays the PDF fit for the w/s = 0.55 paste hydrated at 35 °C split in four r-ranges for better visualization. Figure 6. Experimental (blue circles) and fitted (red solid line) PDF for the CaAl2O4 paste hydrated with w/s = 0.55 at 35 °C (a) from 65–80 Å; (b) from 30–70 Å; (c) from 1.4–30 Å and (d) from 1.4–10 Å represents a better visualization of the first interatomic distance peaks. Difference curves are shown as grey lines. 4 8 12 16 20 24 28 -0.6 -0.3 0.0 0.3 0.6 30 35 40 45 50 55 60 65 70 -0.50 -0.25 0.00 0.25 0.50 68 72 76 80 -0.4 -0.2 0.0 0.2 0.4 G (r) (Å -2 ) r (Å) G (r) (Å -2 ) G (r) (Å -2 ) (c) (b) (a) 2345678910 -0.6 -0.3 0.0 0.3 0.6 (d) G (r) (Å -2 ) Figure 6. 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This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). 1 Supporting material Synchrotron Radiation Pair Distribution Function Analysis of gels in cements Ana Cuesta 1 , Jesus D. Zea-Garcia 2 , Diana Londono-Zuluaga 2 , Angeles G. De la Torre 2 , Isabel Santacruz 2 , Oriol Vallcorba 1 and Miguel A.G. Aranda 1 * 1 ALBA Synchrotron radiation facility, Carrer de la Llum 2-26. 08290 Cerdanyola del Vallès, Barcelona, Spain; 2 Departamento de Química Inorgánica, Cristalografía y Mineralogía. Universidad de Málaga, 29071 Málaga, Spain * Correspondence: migarci[email protected]; Tel.: (+34) 935924305 This supporting information contains: Description of every total scattering raw data set deposited open access. Table S1. Rietveld quantitative phase analysis results for the alite pastes after 34 hydration days. Table S2. Summary of the weight losses from the TGA study for the alite pastes. Table S3. Anisotropic atomic displacement parameters (ADPs) for portlandite in w/s=0.80 paste obtained in the PDF analysis. Note that the parameters for the hydrogen were not refined. Table S4. Quantitative phase analysis results obtained by PDF using two crystal structures for the nanocrystalline CSH gel in the alite w/s=0.46, 0.55, and 0.65 pastes. R W values are also included. Table S5. Refined unit cell parameters for portlandite and clinotobermorite T3_14sc in the alite w/s=0.46, 0.55, and 0.65 pastes by PDF analysis. Table S6. LXRPD Rietveld quantitative phase analysis results for the calcium aluminate pastes. Table S7. Refined unit cell parameters and ADPs for hydrogarnet and gibbsite in the calcium aluminate pastes obtained by the PDF analysis. Figure S1. Particle size distribution of the raw materials measured by laser diffraction (a) Ca 3 SiO 5 , (b) CaAl 2 O 4 and (c) Ca 4 Al 6 O 12 SO 4 . Figure S2. Thermogravimetric data for Ca 3 SiO 5 pastes hydrated for 34 days at room temperature: (a) w/s=0.46 sample; (b) w/s=0.55 sample; (c) w/s=0.65 sample; (d) w/s=0.80 sample. Figure S3. Experimental (blue circles) and fitted (red solid line) PDF patterns for the as-received Ca 3 SiO 5 . Difference curve as grey line. Figure S4. Thermogravimetric data for CaAl 2 O 4 pastes hydrated for 30 days: (a) w/s=0.55 sample hydrated at 35ºC; (b) w/s=1.20 sample hydrated at 35ºC; (c) w/s=0.55 sample hydrated at 45ºC. Figure S5. Thermogravimetric data for (a) ye'elimite–gypsum paste hydrated with w/s=1.2 for 21 days at room temperature and (b) ye'elimite–bassanite paste hydrated with w/s=1.2 for 14 days at room temperature 2 Figure S6. Experimental (blue circles) and fitted (red solid line) PDF patterns for the ye'elimite– bassanite paste hydrated with w/s=1.20 for 21 days at room temperature (a) high r-range: 30–50 Å, (b) low r-range: 1.6–35 Å, (c) enlarged view of: 1.6–10 Å. Difference curve as grey lines. Description of every total scattering raw data set deposited open access. All of the total scattering raw data underlying this article, including the nickel and Ca 3 SiO 5 data sets employed as standards and the empty capillary utilized for data processing, can be accessed on Zenodo at https://doi.org/10.5281/zenodo.890585, and used under the Creative Commons Attribution license. Files: Ni0p7_ALL.dat: Nickel sample employed as standard. C3S_2016_anh_ALL.dat: Anhydrous alite employed as standard empty0p7_ALL.dat: empty capillary C3S-046_ALL.dat: alite paste hydrated with a w/s mass ratio of 0.46. C3S-055_ALL.dat: alite paste hydrated with a w/s mass ratio of 0.55. C3S-065_ALL.dat: alite paste hydrated with a w/s mass ratio of 0.65. C3S-080_ALL.dat: alite paste hydrated with a w/s mass ratio of 0.80. CA_35C_055_ALL.dat: calcium aluminate paste hydrated with a w/s mass ratio of 0.55 at 35ºC CA_35C_120_ALL.dat: calcium aluminate paste hydrated with a w/s mass ratio of 1.20 at 35ºC CA_45C_055_ALL.dat: calcium aluminate paste hydrated with a w/s mass ratio of 0.55 at 45ºC C4A3s_G_120_ALL.dat: ye'elimite with gypsum paste hydrated with a w/s mass ratio of 1.20. C4A3s_B_RT_120_ALL.dat: ye'elimite with bassanite paste hydrated with a w/s mass ratio of 1.20. Table S1. Rietveld quantitative phase analysis results for the alite pastes after 34 hydration days. sample Alite (wt%) CaCO 3 (wt%) Portlandite (wt%) ACn # (wt%) alite_046 13.2 1.4 18.2 67.2 alite_055 11.2 1.2 21.8 65.8 alite_065 10.8 1.2 21.6 66.4 alite_080 10.1 1.5 21.8 66.7 # ACn accounts for the amorphous phase plus any crystalline not-quantified content. Table S2. Summary of the weight losses from the TGA study for the alite pastes. Weight loss (wt%) RT–250ºC 250ºC–400ºC 400ºC–600ºC 600ºC–1000ºC Full range alite_046 13.9 2.0 6.2 2.2 24.3 alite_055 14.1 1.7 6.5 3.1 25.4 alite_065 15.1 1.6 6.6 3.3 26.6 alite_080 15.0 1.6 7.1 3.2 26.9 3 Table S3. Anisotropic atomic displacement parameters (ADPs) for portlandite in w/s=0.80 paste obtained in the PDF analysis. Note that the parameters for the hydrogen were not refined. Atom Ca1 O1 H x 0 0.3333 0.3333 y 0 0.6667 0.6667 z 0 0.2161 0.4256 u11 0.0037 0.0084 0.0264 u22 0.0037 0.0084 0.0264 u33 0.0118 0.0212 0.0264 u12 0.0019 0.0042 0.0132 u13 0 0 0 u23 0 0 0 Table S4. Quantitative phase analysis results obtained by pair distribution function (PDF) using two crystal structures for the nanocrystalline CSH gel in the alite w/s=0.46, 0.55, and 0.65 pastes. R W values are also included. sample Rw (%) Ca 3 SiO 5 (wt%) CaCO 3 (wt%) CrystCa(OH) 2 (wt%) C-S-H (wt%) Alite_046 clinotobermorite T3_14sc 28.5 32.4 0.3 23.1 44.1 Jennite, ICSD #151413 30.0 36.8 0.4 26.3 36.5 Alite_055 clinotobermorite, T3_14sc 29.5 23.4 0.9 33.6 42.2 Jennite, ICSD #151413 32.5 24.9 0.9 35.9 38.3 Alite_065 clinotobermorite, T3_14sc 23.2 14.1 0.8 30.2 54.9 Jennite, ICSD #151413 27.4 13.2 0.8 28.5 57.5 Table S5. Refined unit cell parameters for portlandite and clinotobermorite T3_14sc in the alite w/s=0.46, 0.55, and 0.65 pastes by PDF analysis. Sample Portlandite Clinotobermorite_T3_14sc a (Å) c (Å) a (Å) b (Å) c (Å) β (º) alite_046 3.593 4.914 11.304 7.307 42.375 94.5 alite_055 3.594 4.914 11.180 7.356 42.003 93.0 alite_065 3.594 4.916 11.238 7.302 42.621 94.2 Table S6. Laboratory x-ray powder diffraction (LXRPD) Rietveld quantitative phase analysis results for the calcium aluminate pastes. sample Hydrogarnet (wt%) Hemicarbonate (wt%) ACn (wt%) CA_055_35ºC 42.0 1.6 56.4 CA_120_35ºC 42.7 1.7 55.6 CA_055_45ºC 45.0 2.2 52.8 4 Table S7. Refined unit cell parameters and ADPs for hydrogarnet and gibbsite in the calcium aluminate pastes obtained by the PDF analysis. Sample Hydrogarnet Gibbsite a (Å) ADPs (Å 2 ) a (Å) b (Å) c (Å) β (º) ADPs (Å 2 ) Ca Al O Al O CA_055_35ºC 12.579 0.0063 0.0072 0.0182 8.693 5.046 9.713 94.6 0.0050 0.0153 CA_120_35ºC 12.578 0.0054 0.0063 0.0152 8.668 5.075 9.699 94.6 0.0012 0.0062 CA_055_45ºC 12.573 0.0063 0.0071 0.0167 8.671 5.064 9.712 94.5 0.0021 0.0093 Figure S1. Particle size distribution of the raw materials measured by laser diffraction (a) Ca 3 SiO 5 , (b) CaAl 2 O 4 , and (c) Ca 4 Al 6 O 12 SO 4 . 0 10 20 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1.0 10.0 100.0 Volume (%) 0 10 20 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1.0 10.0 100.0 Volume (%) 0 10 20 0 10 20 30 40 50 60 70 80 90 100 0.01 0.1 1.0 10.0 100.0 size (µm) volume (%) Cumulative volume (%) (a) (b) (c) d v,50 =13.8 µm d v,50 =3.3 µm d v,50 =9.0 µm 5 Figure S2. Thermogravimetric data for Ca 3 SiO 5 pastes hydrated for 34 days at room temperature: (a) w/s=0.46 sample; (b) w/s=0.55 sample; (c) w/s=0.65 sample; (d) w/s=0.80 sample. 200 400 600 800 1000 40 60 80 100 0.2 0.1 0.0 -0.1 -0.2 200 400 600 800 1000 40 60 80 100 0.2 0.1 0.0 -0.1 -0.2 200 400 600 800 1000 40 60 80 100 0.2 0.1 0.0 -0.1 -0.2 200 400 600 800 1000 40 60 80 100 0.2 0.1 0.0 -0.1 -0.2 T(ºC) Weight (%) Diff. rel. weight (%/ºC) (a) (b) (c) (d) 6 Figure S3. Experimental (blue circles) and fitted (red solid line) PDF patterns for the as-received Ca 3 SiO 5 . Difference curve as grey line. 5 10 15 20 25 30 35 40 45 50 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 r (Å) G (r) (Å -2 ) R W = 16.7% 7 Figure S4. Thermogravimetric data for CaAl 2 O 4 pastes hydrated for 30 days: (a) w/s=0.55 sample hydrated at 35ºC; (b) w/s=1.20 sample hydrated at 35ºC; (c) w/s=0.55 sample hydrated at 45ºC. 20 40 60 80 100 0.4 0.2 0.0 -0.2 -0.4 20 40 60 80 100 0.4 0.2 0.0 -0.2 -0.4 200 400 600 800 1000 20 40 60 80 100 0.4 0.2 0.0 -0.2 -0.4 T(ºC) Weight (%) Diff. rel. weight (%/ºC) (a) (b) (c) 8 Figure S5. Thermogravimetric data for (a) ye'elimite–gypsum paste hydrated with w/s=1.2 for 21 days at room temperature and (b) ye'elimite–bassanite paste hydrated with w/s=1.2 for 14 days at room temperature 0 20 40 60 80 100 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 T(ºC) Weight (%) Diff. rel. weight (%/ºC) 200 400 600 800 1000 0 20 40 60 80 100 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6