Strontium Retention of Calcium Zirconium Aluminate Cement Paste Studied by NMR, XRD and SEM-EDS

This work concerns the hydration mechanism of calcium zirconium aluminate as a ternary compound appearing in the CaO-Al2O3-ZrO2 diagram besides the calcium aluminates commonly used as the main constitutes of calcium aluminate cements (CACs). Moreover, a state-of-the-art approach towards significant changes in hydraulic properties was implemented for the first time in this work, where the effect of structural modification on the hydration behavior of calcium zirconium aluminate was proved by XRD, 27Al MAS NMR and SEM-EDS. The substitution of Sr2+ for Ca2+ in the Ca7ZrAl6O18 lattice decreases the reactivity of Sr-substituted Ca7ZrAl6O18 in the presence of water. Since the original cement grains remain unhydrated up to 3 h (Ca7ZrAl6O18) or 72 h (Sr1.25Ca5.75ZrAl6O18) of curing period in the hardened cement paste structures, strontium can be considered as an inhibition agent for cement hydration. The complete conversion from anhydrous 27AlIV to hydrated 27AlVI species was achieved during the first 24 h (Ca7ZrAl6O18) or 7 d(Sr1.25Ca5.75ZrAl6O18) of hydration. Simultaneously, the chemical shift in the range of octahedral aluminum from ca. 4 ppm to ca. 6 ppm was attributed to the transformation of the hexagonal calcium aluminate hydrates and Sr-rich (Sr,C)3AH6 hydrate into the cubic phase Ca-rich (Sr,C)3AH6 or pure C3AH6 in the hardened Sr-doped cement paste at the age of 7 d. The same 27Al NMR chemical shift was detected at the age of 24 h for the reference hardened undoped Ca7ZrAl6O18 cement paste.


Introduction
Calcium aluminate cements (CACs) [1][2][3] and other related cementitious systems [4][5][6][7] are believed to play an important role in a wide range of specialist are as from some construction areas and civil engineering, to refractory materials industry, due to their ability to gain strength rapidly in the initial days after casting and to withstand aggressive environments and high temperatures. The temperature-time weight ratio of water-to-cement (the w/c ratio) dependencies of the cement hydration processes have been widely investigated so far and are presented elsewhere in detail [8][9][10]. It must be made clear at this stage that both temperature and a weight ratio of water-to-cement (the w/c ratio) affect the performances of calcium aluminate cements, especially at an early age. The phase composition, microstructure and other properties of the cement pastes can easily be designed by choosing appropriate curing parameters. Nevertheless, upon curing of CAC-based paste at not adequately controlled, ambient conditions, these materials are very sensitive to humidity, CO 2 , temperature and time. Due to the fact of not being able to predict their long-term behavior makes them practically not suitable for constructions but suitable for building chemistry. Nevertheless, the high-early-heat and high-early-strength gain makes CACs attractive, especially in the winter The resonance at ca. 12.36 ppm is due to tricalcium aluminate hexahydrate C 3 AH 6 [18,20], at ca. 10.2 ppm due to CAH 10 and C 4 AH 13 [18][19][20] and at ca. 10.3 ppm due to C 2 AH 8 [17].
The aim of this work is to implement the NMR technique to monitor the progress of hydration of both undoped-and strontium-doped calcium zirconium aluminate cement. Hence, the influence of structural modification with SrO on the hydraulic activity of Ca 7 ZrAl 6 O 18 phase has been demonstrated. This work fills in a significant gap in the literature on the exploiting ssNMR spectroscopy to probe the early stages of hydration of new types of cements belonging to the CaO-Al 2 O 3 -ZrO 2 and CaO-SrO-Al 2 O 3 -ZrO 2 systems.

Synthesis and Phase Identification
Low-cost synthesis of Ca 7  Phase identification for the sintered samples was carried out using X-ray diffraction (XRD, PANalytical, Malvern PANalytical, Malvern, UK) on a ProPANalytical X'Pert X-ray diffractometer, with Cu Kα radiation (λ = 0.15418 nm), with 0.02 • per step and 3s time per step (2theta range from 5 • to 45 • ).
The NMR spectra were recordedat room temperature on Bruker Avance III 400WB (9.4T) spectrometer, (Bruker BioSpin, Rheinstetten, Germany) using 4 mm MAS (Magic Angle Spinning), dual-channel (1H/BB) probe-head, operating at a resonance frequency of 104.26 MHz for 27 Al. The sample was spun at a MAS frequency of 8 kHz in the rotors made of zirconium dioxide (4 mm). 32 K data points and 1024 scans FIDs were accumulated with a Single Pulse Excitation (SPE) pulse sequence using the observed 90 • pulse ( 27 Al) set at 6.0 us with a relaxation delay of 200 ms. Note no proton decoupling was applied during the experiment. Prior to Fourier transformation, the data were zero-filled twice and 80 Hz apodization filter was applied. The 27 Al chemical shifts were referenced using a sample of AlCl 3 ·6H 2 O in 1M solution as an external reference (0 ppm).
The microstructures of fracture surfaces of hydrated cement pastes were investigated using a scanning electron microscope (SEM, FEI Nova Nano SEM 200, Kyoto, Japan.). The chemical compositions of the samples were determined with electron-probe microanalysis using an energy-dispersive X-ray spectrometer (EDAX, Sapphire Si(Li) EDS detector, Mahwah, NJ, USA).

Preparation and Treatment of Cement Paste
Comparison study of hydration characteristics between Ca 7 ZrAl 6 O 18, Sr 1.25 Ca 5.75 Al 6 O 18 and SrAl 2 O 4 cements were determined for cement pastes prepared with water-to-cement (w/c) ratios of 1.0 or 0.5. The water-to-cement ratio of 0.5 was applied to achieve plastic properties without any undesirable sedimentation of neat SrAl 2 O 4 paste. Whereas, both Ca 7 ZrAl 6 O 18 and Sr 1.25 Ca 5.75 Al 6 O 18 cements require w/c = 1.0 to obtain the well-homogenized cement pastes without any undesirable phenomena of sedimentation. The cement powders which were obtained by grinding the sintered Materials 2020, 13, 2366 4 of 15 pellets and necessary mass of water were mixed together in a glass beaker to obtain three homogeneous neat cement pastes. Each neat cement paste was then placed in a polyethylene bag and sealed until 14 d at 50 • C. According to Litwinek and Madej [5], the optimal synthesis temperature for C 3 AH 6 from different precursors through hydration is suggested to be 50 • C. Moreover, this period for curing cement pastes was accepted to attain the maximum degree of hydration, as concluded from the previous studies [13]. Moreover, as it was previously mentioned by Garcés et al. [26] and Zhang et al. [27], at temperatures as high as 60 • C, only the cubic phase and the gibbsite appear in the calcium aluminates-based cement pastes. At 24 h and 7 d, the microstructure of cement pastes was investigated by SEM. Acetone quenching was used to stop hydration at 15 min, 0.5 h, 1 h, 2 h, 3 h, 24 h, 48 h, 72 h, 7 d and 14 d ( Table 1). The use of cold acetone, aiming to withdraw free water and inhibit further reactions within cement paste is known from the Ref. [28]. Cold acetone is related to acetone stored under laboratory conditions. Quenched pastes were characterized by XRD and 27 Al MAS NMR (Bruker BioSpin, Rheinstetten, Germany), according to the procedures presented in Section 2.1.

X-ray Diffraction Analysis of Special Cements Hydration
According to X-ray diffraction analysis, the cement clinkers synthesized by the solid-state reactive sintering technique were all crystalline, single-phase aluminate phases Ca 7  Compared with the XRD patterns of unhydrated phases, the decreasing intensity of peaks corresponding to Sr 1.25 Ca 5.75 ZrAl 6 O 18 and Ca 7 ZrAl 6 O 18 , and new peaks of low intensity corresponding to hydration products formation were recorded using XRD (Figures 1-3). A low degree of hydrates crystallinity was indicated by a poor XRD pattern, especially in the early stage of hydration. Moreover, the hydration processes result in a corresponding change in the XRD patterns of the initial cement clinker phases and formation of amorphous material, besides the crystalline hydrates, as it can be concluded from the severe intensity reduction and peaks broadening in the XRD pattern. rich (Sr,C)3AH6 or C3AH6. This work has successfully shown the existence of the solid solution of strontium in the tricalcium hydrate C3AH6 lattice by direct verification using XRD. By reason of structural modification of C3AH6 through ionic substitution, the lattice parameter of the cubic phase was increased and the slight shift in XRD peaks belonging to (Sr,C)3AH6 solid solution towards lower 2θ value was observed (Figure 1b). This increase in the lattice parameter was due to the size of the ionic radius of Sr 2+ (132 pm) which is bigger than the ionic radius of Ca 2+ (114 pm). A brief summary of XRD results is given as Figures 2 and 3. This overview XRD spectra recorded from the Sr1.25Ca5.75ZrAl6O18 cement paste (Sample A) at different curing periods from 15 min to 14 d showed a progressive reduction in the peaks associated with Sr1.25Ca5.75ZrAl6O18 due to its hydration process, which led to the formation of hydration products ( Figure 2). The cement paste at the age of 15 min is a mixture of the unhydrated phase and amorphous or poorly crystalline hexagonal hydrates, whereas the cement paste at the age between 0.5 h and 72 h contained a mixture of the Srrich (Sr,C)3AH6 cubic phase, hexagonal hydrates and the still unhydrated residues of the Sr1.25Ca5.75ZrAl6O18 cement grains. It should be noted that the positions of the XRD peaks of C4AH19 The overview XRD spectra recorded from the reference Ca7ZrAl6O18 cement paste (Sample C) at different curing periods from 15 min to 14 d is shown in Figure 3. The hexagonal hydrates exist with the still unhydrated residues of the Ca7ZrAl6O18 in cement paste between 15 min and 3 h of curing period, whereas C3AH6 hydrated phase is formed at the curing age of 0.5 h. At the age of 24 h, the XRD pattern of the cement pastes exhibits profile similar to that of pure C3AH6 without any traces of unhydrated cement particles Ca7ZrAl6O18 and metastable hydrates.  From the X-ray diffraction results, it seems obvious that strontium doping affects the hydration behavior of the cement clinker mineral phase Ca7ZrAl6O18, and leads to changes in the hydration products properties. There is a relationship between the proportion of residual unhydrated cement particles and the properties of the particular cement clinker mineral phases involved. After 24 h of curing at 50°C, where the hardened cement paste (Sample C) consists primarily of C3AH6, the original Ca7ZrAl6O18 cement particles are no longer evident. In the Sr-doping of Ca7ZrAl6O18 case, there is inhibition of hydration, and the Sr1.25Ca5.75ZrAl6O18 cementitious particles exist in the hardened cement paste up to 72 h (Sample A). This material would need to cure over 72 h to reach complete hydration. Hence, XRD data for 0.5 h-7 d materials containing strontium indicates the appearance of  (Figure 1c). Hence, this XRD peak located at the position of 2θ = 17.00 which belongs to Sr-rich (Sr,C) 3 AH 6 needs to be considered between the reference C 3 AH 6 synthesized through Ca 7 ZrAl 6 O 18 hydration and other reference sample Sr 3 AH 6 synthesized from SrAl 2 O 4 precursor through hydration. As is evident from this figure, the formation of the intermediate Sr-rich (Sr,C) 3 AH 6 hydrate precedes the formation of the stable Ca-rich (Sr,C) 3 AH 6 hydrate at 7 d of curing. In this sample, two isostructural compounds with a hydrogarnet type crystal lattice were present. The position of the lower-intensity XRD line at ca. 17.00 • 2θ is situated between lines belonging to pure phases Sr 3 AH 6 (Sample B) and C 3 AH 6 (Sample C). The second position of the higher intensity XRD line at ca. 17.26 • 2θ is similar to that found for reference C 3 AH 6 (Figure 1a-c). In addition, it is worth noting that the Sr-rich (Sr,C) 3 AH 6 exists in the hardened cement paste between 0.5 h and 7 d of curing. This phase disappeared after longer curing times and became replaced by Ca-rich (Sr,C) 3 AH 6 or C 3 AH 6 . This work has successfully shown the existence of the solid solution of strontium in the tricalcium hydrate C 3 AH 6 lattice by direct verification using XRD. By reason of structural modification of C 3 AH 6 through ionic substitution, the lattice parameter of the cubic phase was increased and the slight shift in XRD peaks belonging to (Sr,C) 3 AH 6 solid solution towards lower 2θ value was observed (Figure 1b). This increase in the lattice parameter was due to the size of the ionic radius of Sr 2+ (132 pm) which is bigger than the ionic radius of Ca 2+ (114 pm).
A brief summary of XRD results is given as Figures 2 and 3. This overview XRD spectra recorded from the Sr 1.25 Ca 5.75 ZrAl 6 O 18 cement paste (Sample A) at different curing periods from 15 min to 14 d showed a progressive reduction in the peaks associated with Sr 1.25 Ca 5.75 ZrAl 6 O 18 due to its hydration process, which led to the formation of hydration products ( Figure 2). The cement paste at the age of 15 min is a mixture of the unhydrated phase and amorphous or poorly crystalline hexagonal hydrates, whereas the cement paste at the age between 0.5 h and 72 h contained a mixture of the Sr-rich (Sr,C) 3  Therefore, it is often difficult to clearly differentiate between C 4 AH 19 and C 2 AH 8 in the XRD patterns, as is clearly demonstrated with a red rectangle ( ) in Figure 2. However, at 7d second adjacent cubic phase, Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 , exists together with the initially formed cubic phase Sr-rich (Sr,C) 3 AH 6 and some residues of the hexagonal hydrates. As a general trend at the age of 14 d, XRD pattern of cement paste achieved profile similar to that of pure C 3 AH 6 (Figures 1c and 2) without any metastable hydrates and unhydrated cement residues, i.e., unhydrated cement clinker mineral Sr 1.25 Ca 5.75 ZrAl 6 O 18 .
The overview XRD spectra recorded from the reference Ca 7 ZrAl 6 O 18 cement paste (Sample C) at different curing periods from 15 min to 14 d is shown in Figure 3. The hexagonal hydrates exist with the still unhydrated residues of the Ca 7 ZrAl 6 O 18 in cement paste between 15 min and 3 h of curing period, whereas C 3 AH 6 hydrated phase is formed at the curing age of 0.5 h. At the age of 24 h, the XRD pattern of the cement pastes exhibits profile similar to that of pure C 3 AH 6 without any traces of unhydrated cement particles Ca 7 ZrAl 6 O 18 and metastable hydrates.
From the X-ray diffraction results, it seems obvious that strontium doping affects the hydration behavior of the cement clinker mineral phase Ca 7 ZrAl 6 O 18 , and leads to changes in the hydration products properties. There is a relationship between the proportion of residual unhydrated cement particles and the properties of the particular cement clinker mineral phases involved. After 24 h of curing at 50 • C, where the hardened cement paste (Sample C) consists primarily of C 3 AH 6 , the original Ca 7 ZrAl 6 O 18 cement particles are no longer evident. In the Sr-doping of Ca 7 ZrAl 6 O 18 case, there is inhibition of hydration, and the Sr 1.25 Ca 5.75 ZrAl 6 O 18 cementitious particles exist in the hardened cement paste up to 72 h (Sample A). This material would need to cure over 72 h to reach complete hydration. Hence, XRD data for 0.5 h-7 d materials containing strontium indicates the appearance of additional peaks adjacent to each of the reference C 3 AH 6 lines caused by the presence of an additional Sr-rich cubic phase. These results confirmed the strontium retention by calcium zirconium aluminate cement paste through the chemical bonding to C-A-H in the hydrated phase. The presence of strontium in the C-A-H matrix is also known to delay the transformation of hexagonal hydrates into the cubic phases. 27 Al NMR Study of the Hydration Reaction at 50 • C Figure 4a,b presents the 27 Al NMR spectra of synthesized Ca 7 ZrAl 6 O 18 and Sr-doped Ca 7 ZrAl 6 O 18 cements together with their products of hydration. The 27 Al MASNMR spectra of the starting unhydrated samples are shown by the red lines. The intense and broad peak at ca. 50 ppm is due to Ca 7 ZrAl 6 O 18 , which consists of orientationally disordered six AlO 4 tetrahedra linked together by sharing corners, to form [Al 6 O 18 ] rings [29]. The 27 Al MAS NMR spectra of the hydrated samples during the first 15 min all contain peaks near 4 ppm due to VI Al in the cement hydration reaction products (amorphous or poorly crystalline hexagonal hydrates C 4 AH 19 or C 2 AH 8 ) (Figure 4a,b).    For the partially reacted samples, the signals are in the ca. 4 ppm and ca. 46-61 ppm ranges due to both hydrates and unhydrated reactants, respectively (Figure 4c). For the totally hydrated (and converted) samples, all of the signalsarein the ca. 6 ppm range, consistent with total conversion of Al from tetrahedral coordination in the unhydrated Ca7ZrAl6O18 and Sr1.25Ca5.75ZrAl6O18 cements to octahedral coordination [19] in the final hydrates formed at 24 h (undoped cement) or 7 d (Sr-doped cement), as expected from previous works on the calcium aluminate cement hydration processes investigated by solid-state 27 Al MAS NMR studies [19,25]. The maximum for the VI Al peak alters slightly depending on the calcium aluminate hydrates present [18,19]. For the undoped Ca7ZrAl6O18 cement hydrated between 15 min and 3 h, in which the detected crystalline hydrates are mainly hexagonal phases, the peak maximum is at ca. 4 ppm and shifts to 6 ppm for this cement hydrated between 24 h and 14 d (Figure 4d), which by XRD contain cubic C3AH6 as the predominant phase. This type of shift is delayed up to 7 d in the hydrated Sr-doped cement (Figure 4e), where Ca-rich (Sr,C)3AH6 or pure C3AH6 begin to form. Hence, it can be summarized that the chemical shift occurring at ca. 4 ppm was due to octahedrally coordinated framework aluminum atoms in Sr-rich (Sr,C)3AH6 (Sr1.25Ca5.75ZrAl6O18 cement paste), poorly crystalline C3AH6 (Ca7ZrAl6O18 cement paste) and hexagonal hydrates (both cement pastes) formed at an early stage of hydration. The chemical shift occurring at ca. 6 ppm was due to octahedrally coordinated framework aluminum atoms in Carich (Sr,C)3AH6 or pure C3AH6 formed in the totally reacted Sr-doped sample, as it can be referenced in for pure C3AH6 formed in the reference fully hydrated and converted undoped cement paste. It is worth discussing that the maximum for the Al(6) peak belonging to C-A-H phases varies slightly from data presented in Ref. [18,19]. In those works, and many others, the peak maximum belonging to C3AH6 was located at about 12 ppm, whereas the peak maxima belonging to hexagonal hydrates were located at ca. 10-11 ppm. For the partially reacted samples, the signals are in the ca. 4 ppm and ca. 46-61 ppm ranges due to both hydrates and unhydrated reactants, respectively (Figure 4c). For the totally hydrated (and converted) samples, all of the signalsarein the ca. 6 ppm range, consistent with total conversion of Al from tetrahedral coordination in the unhydrated Ca 7 ZrAl 6 O 18 and Sr 1.25 Ca 5.75 ZrAl 6 O 18 cements to octahedral coordination [19] in the final hydrates formed at 24 h (undoped cement) or 7 d (Sr-doped cement), as expected from previous works on the calcium aluminate cement hydration processes investigated by solid-state 27 Al MAS NMR studies [19,25]. The maximum for the VI Al peak alters slightly depending on the calcium aluminate hydrates present [18,19]. For the undoped Ca 7 ZrAl 6 O 18 cement hydrated between 15 min and 3 h, in which the detected crystalline hydrates are mainly hexagonal phases, the peak maximum is at ca. 4 ppm and shifts to 6 ppm for this cement hydrated between 24 h and 14 d (Figure 4d), which by XRD contain cubic C 3 AH 6 as the predominant phase. This type of shift is delayed up to 7 d in the hydrated Sr-doped cement (Figure 4e), where Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 begin to form. Hence, it can be summarized that the chemical shift occurring at ca. 4 ppm was due to octahedrally coordinated framework aluminum atoms in Sr-rich (Sr,C) 3 AH 6 (Sr 1.25 Ca 5.75 ZrAl 6 O 18 cement paste), poorly crystalline C 3 AH 6 (Ca 7 ZrAl 6 O 18 cement paste) and hexagonal hydrates (both cement pastes) formed at an early stage of hydration. The chemical shift occurring at ca. 6 ppm was due to octahedrally coordinated framework aluminum atoms in Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 formed in the totally reacted Sr-doped sample, as it can be referenced in for pure C 3 AH 6 formed in the reference fully hydrated and converted undoped cement paste. It is worth discussing that the maximum for the Al(6) peak belonging to C-A-H phases varies slightly from data presented in Ref. [18,19]. In those works, and many others, the peak maximum belonging to C 3 AH 6 was located at about 12 ppm, whereas the peak maxima belonging to hexagonal hydrates were located at ca. 10-11 ppm.  Figure 5 shows the typical microstructure of this cement paste fragment after 24 h hydration at 50 • C. The presence of Sr in Sr 1.25 Ca 5.75 ZrAl 6 O 18 clinker affects formation of Sr-rich (Sr,C) 3 AH 6 ( Figure 5a-point 1) with a cubic/isometric crystal form [30,31]. The EDS spectrum presenting intensity vs. energy of the detected X-ray clearly identifies the peaks of Sr, Ca, Al and O (Figure 5b). The EDS spectrum from the hexagonal irregular flakes is shown in Figure 5c. The Ca, Al and O peaks are mainly due to C-A-H phases. EDS intensity ratio of calcium and aluminum peaks indicates the presence of C 4 AH 19 hydrate rather than C 2 AH 8 hydrate.

Microstructural Studies on the
Materials 2020, 13, x FOR PEER REVIEW 12 of 17

Microstructural Studies on the Hydrated Sr1.25Ca5.75ZrAl6O18Cement Paste
The development of Sr1.25Ca5.75ZrAl6O18 cement paste microstructure in time can directly be linked to the evolution of phase composition presented in Sections 3.1 and 3.2. Figure 5 shows the typical microstructure of this cement paste fragment after 24 h hydration at 50°C. The presence of Sr in Sr1.25Ca5.75ZrAl6O18 clinker affects formation of Sr-rich (Sr,C)3AH6 (Figure 5a-point 1) with a cubic/isometric crystal form [30,31]. The EDS spectrum presenting intensity vs. energy of the detected X-ray clearly identifies the peaks of Sr, Ca, Al and O (Figure 5b). The EDS spectrum from the hexagonal irregular flakes is shown in Figure 5c. The Ca, Al and O peaks are mainly due to C-A-H phases. EDS intensity ratio of calcium and aluminum peaks indicates the presence of C4AH19 hydrate rather than C2AH8 hydrate.  Most of the C 3 AH 6 or Ca-rich (Sr,C) 3 AH 6 crystals formed after a hydration time of 7 d attain the shape of cubes, pyritohedra or other more complex forms of the isometric system, which are reinforced with Al(OH) 3 crystals (Figure 6a,c). As observed before, those hydration products are strongly dependent on curing time and the Ca peak (Figure 6b) intensity increases relative to the Sr peak intensity (Figure 5b). Hence, as the curing time increases, the crystals belonging to a cubic or isometric system formed initially as a transient Sr-rich (Sr,C) 3 AH 6 were replaced with Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 . An unavoidable change of one form of calcium aluminate hydrate to another can be found elsewhere [32][33][34].  Most of the C3AH6 or Ca-rich (Sr,C)3AH6 crystals formed after a hydration time of 7 d attain the shape of cubes, pyritohedra or other more complex forms of the isometric system, which are reinforced with Al(OH)3 crystals (Figure 6a,c). As observed before, those hydration products are strongly dependent on curing time and the Ca peak (Figure 6b) intensity increases relative to the Sr peak intensity (Figure 5b). Hence, as the curing time increases, the crystals belonging to a cubic or isometric system formed initially as a transient Sr-rich (Sr,C)3AH6 were replaced with Ca-rich (Sr,C)3AH6 or pure C3AH6. An unavoidable change of one form of calcium aluminate hydrate to another can be found elsewhere [32][33][34].

Conclusion
According to the current research, many conclusions can be drawn: (1) The Sr-doped cement is developed through structural substitution for Ca ions by Sr ions in the Ca7ZrAl6O18 clinker phase. (2) Strontium was used as a retarding agent to block this cement clinker phase hydration at a curing temperature of 50°C. Hence, the residual unhydrated cement particles in the hardened Sr1.25Ca5.75ZrAl6O18 cement paste were present for much longer than for the undoped Ca7ZrAl6O18 clinker phase sample as it was observed by XRD. (3) The hydration of both Ca7ZrAl6O18 and Sr1.25Ca5.75ZrAl6O18 cements was also inspected using the 27 Al MAS NMR technique. This hydration is accompanied by a change of Al-coordination from tetrahedral to octahedral. This complete conversion from anhydrous 27 Al IV to hydrated 27 Al VI species was achieved during the first 24 h of hydration at 50°C for Ca7ZrAl6O18 and during 7 d of hydration at 50°C for Sr1.25Ca5.75ZrAl6O18. (4) The hexagonal phases were formed starting in the very first minutes of hydration of these cements. For each cement type tested, these unstable hydrates consist mainly of C4AH19 and probably of C2AH8 as it was observed by XRD.

Conclusions
According to the current research, many conclusions can be drawn: (1) The Sr-doped cement is developed through structural substitution for Ca ions by Sr ions in the Ca 7 ZrAl 6 O 18 clinker phase. (2) Strontium was used as a retarding agent to block this cement clinker phase hydration at a curing temperature of 50 • C. Hence, the residual unhydrated cement particles in the hardened Sr 1.25 Ca 5.75 ZrAl 6 O 18 cement paste were present for much longer than for the undoped Ca 7 ZrAl 6 O 18 clinker phase sample as it was observed by XRD. (3) The hydration of both Ca 7 ZrAl 6 O 18 and Sr 1.25 Ca 5.75 ZrAl 6 O 18 cements was also inspected using the 27 Al MAS NMR technique. This hydration is accompanied by a change of Al-coordination from tetrahedral to octahedral. This complete conversion from anhydrous 27 Al IV to hydrated 27 Al VI species was achieved during the first 24 h of hydration at 50 • C for Ca 7 ZrAl 6 O 18 and during 7 d of hydration at 50 • C for Sr 1.25 Ca 5.75 ZrAl 6 O 18 . (4) The hexagonal phases were formed starting in the very first minutes of hydration of these cements.
For each cement type tested, these unstable hydrates consist mainly of C 4 AH 19 and probably of C 2 AH 8 as it was observed by XRD. (5) The formation of a thermodynamically stable phase pure C 3 AH 6 or Ca-rich (Sr,C) 3 AH 6 in the hardened Sr 1.25 Ca 5.75 ZrAl 6 O 18 cement paste was preceded by that of a number of less stable phases, i.e.,Sr-rich (Sr,C) 3 AH 6 hydrate and other hexagonal Ca-Al hydrates. The Sr-rich (Sr,C) 3 AH 6 hydrate existing between 0.5 h and 7 d of curing was isostructural with the Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 formed at the age of 7 d. (6) The transformation of the hexagonal calcium aluminate hydrates and Sr-rich (Sr,C) 3 AH 6 hydrate into the cubic phase Ca-rich (Sr,C) 3 AH 6 or pure C 3 AH 6 was expressed in terms of chemical shift from ca. 4 ppm to ca. 6 ppm in the hardened Sr 1.25 Ca 5.75 ZrAl 6 O 18 cement paste at the age of 7 d.