Mineralogy of CSH belite hydrates incorporating Zn-AlTi layered double hydroxides

Recently, the belitic cements with low alite content were the subject of several research works which aimed to replace the Ordinary Portland Clinker (OPC) for ecological reasons (reduction of CO2 emissions), so to understand the reactivity of this cement, the hydration study of the C2S "dicalcium silicate" phase is primordial research step. As well for a clean environment, the TiO2 photocatalyst has been extensively applied in the science of building materials because of its ability to degrade the cement surface pollutants. New photocatalyst based layered double hydroxides (LDH) associated with zinc, aluminium and TiO2 was introduced to increase the compatibility with mortars. The present work is subjected to investigate the effect of the layered double hydroxides on the hydration of C2S in following the evolution of hydration by X-ray diffraction at 2, 7, 28 and 90 days and analyzing the calcium/silicon ratio of different formed hydrates.


Introduction
The ordinary Portland cement is a complex multiphase system, the hydration includes several simultaneous processes of dissolution/precipitation [1].As soon as the cement is contacted with the gauging water, the silicate phases C 3 S and C 2 S react rapidly with H 2 O to form less soluble hydrates than these phases.Nevertheless, despite of the almost immediate forming of the first hydrates, the hydration may continue for many years to come [2].When the cement is hydrated, the reactions during the gauging form the Calcium Silicate Hydrate C-S-H (notation in the cement industry C = CaO, S = SiO 2 , and H = H 2 O) by releasing Ca 2+ and OH -ions [3], these two species combine and form Portlandite. Dicalcium silicate C 2 S gives the same products with low quantities and slow kinetics compared to C 3 S.In Portland cement the dicalcium silicate has three polymorphic forms α-C 2 S, β-C 2 S and γ-C 2 S (metastable) with a composition which varies between 20% and 25% in the clinker.The slower hydration of C 2 S, after 28 days, leads to improved long-term strength.Tests on alite and belite showed that long term strengths are comparable and that only the hydration kinetics is different [4].
C-S-H is the predominant phase resulting from the hydration of dry cement, it is directly responsible for the evolution of mechanical properties.In the literature, several studies show that the tobermorite-like structure is the closest to that of C-S-H [5].There are three isomorphic structures of tobermorite which are distinguished by their basal space (distance between the two basic plans of two first lamellas neighbors): 9.3 Å, 11 Å [6,7] and 14 Å [8].According to the ratio of water/cement and Ca/Si, the C-S-H can adopt one of the isomorphous structures.Tobermorite is generally considered as a valid analogy for the C-S-H with low ratios Ca/Si, while jennite structure is used to describe the structure of C-S-H with high ratios Ca/Si [9,10].Similarities between C-S-H and tobermorite were also deduced from atomic simulations [11], although this model is the subject of criticism [12].The chemical composition and the unit cell of the clinotobermorite are closely linked to those of the tobermorite, it is very likely that the clinotobermorite is a stable polymorph at a lower temperature than the tobermorite [13].Studies of the structure and the kinetic of C-S-H are essential so as to understand the process of the cement setting and the "sticking" mechanism at the microscopic level, their chemical composition is variable.In particular, the ratio (Ca/Si) of calcium-silicon structure is commonly assumed to vary from 0.6 to 2.3, the highest ratio being found in the clean Portland cement, and the lowest in cements containing products such as fly ash or metakaolin [14,15].The layered double hydroxides LDH have spurred the interest many researchers, they are more likely useful as additions in cements.Cement based materials containing the nanoparticles have shown several benefits under distinct perspectives for sustainable construction practice, as well as, the kinetics of hydration acceleration [16].Photocatalytic phenomena have become an attractive field of studies in the last decade because of its great potentials for the purification of the environment and the ability to break down aquatic and air pollutants [17,18].

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Furthermore and as part of research on the reduction of pollutants in the atmosphere, the Portland cement is also studied for this aim by adding TiO 2 which allows the degradation of air pollutants by photocatalysis.The photocatalytic principles are based on the free-radical reaction initiated by light onto a concrete surface.Its efficiency depends on the mobility of electron-hole pairs that determines the probability of electrons and holes to achieve contact with active sites on the photocatalyst surface [19].The most commonly used photocatalyst is TiO 2 due to its strong oxidizing power, its photo-stability and its non-toxicity.This oxide is already widely marketed for its properties and use in surface state [20,21].The titanium oxide properties can be improved when using double hydroxides lamellar based Zn-Al-Ti [22].
The introduction of the photocatalytic active inorganic-inorganic nanocomposites to the cement-based mortars improved overall mortar properties (microhardness and crystallinity) and showed that the synergetic effect between TiO 2 as traditional photocatalytic and Zn-Al-LDH contributes to the overall photocatalytic performances, improving also the compatibility of the photocatalytic active phase with the mortar matrix [23].Zinc was selected as constituent LDH metal because of its photocatalytic and antimicrobial activity with the intention to possibly contribute to the overall activity of novel Ti-Zn-Al nanocomposite.ZnO is frequently looked as an alternative to TiO 2 , since it can absorb a larger energy fraction of the solar spectrum and more light quanta [24].In our work, the LDH compounds after synthesis are added under a calcined form with a quantity of ZnTiO 3 , the compounds, thus introduced under a dried active form, are more likely to react to form again LDH structures that are themselves trapped in hydrates dicalcium silicate.Thermal treatment provides important physicochemical properties to LDH compounds [25]: a "memory effect" of the hydroxide lattice, which allows different anionic species to be incorporated into the LDH interlamellar space, a larger surface area, increasing adsorption of anions and elimination of the interlayer carbonate (CO 3 2-), which strongly hinders anion exchange processes in LDHs [26,27].
In this paper we seek to use LDH compounds and exploit their memory effect by heat treatment and addition to the C 2 S phase to study the evolution of hydrates formed in these mixtures.The purpose is to make a mineralogical study of the hydrates of dicalcium silicate in the presence of varying amounts (1% and 3 wt%) of layered double hydroxides Zn-Al-Ti, monitoring of phases used is carried out by XRD after 2, 7, 28 and 90 days of hydration to compare the polymorphism of phase formation and determine the Ca/Si in the formed phases.

Synthesis of C 2 S
The synthesis of the C 2 S phase was performed by firing a silica source SiO 2 finely ground with calcite (CaCO 3 ) and of (NH 4 )H 2 PO 4 used as a dopant to stabilize the β-C 2 S variety, the mixture is subjected to a heat treatment at different temperatures 500 °C, 800 °C and 1000 °C for 24, 12 and 8h respectively, followed by rapid air cooling.The heat treatments are interspersed by milling with the addition of ethanol whose role is to increase the reactivity of the products.

Synthesis of Zn-Al-Ti Layered Double Hydroxides
In a beaker an acid solution of Zn(NO 3 ) 2 6H 2 O, Al(NO 3 ) 3 9H 2 O precursors and a basic solution of Na 2 CO 3 and NaOH were simultaneously added (4cm3/min) so as to adjust a constant pH between 9 and 9.5 at a constant temperature of 45°C for 10 hours.A Zn-Al LDH white precipitate is formed after aging from 12h at 100°C in a stove and calcined for 5 hours at 500 °C.The wet impregnation of TiO 2 on the Zn-Al layered double hydroxide was used for the preparation of Zn-Al-Ti LDH.The wet impregnation process was carried out using TiO 2 suspension diluted (3% by weight) in a base solution 0.67M Na 2 CO 3 and loaded on the calcined powder Zn-Al LDH.The excess water was removed in a stove at 100°C.The impregnated sample is dried for a second time 12 hours at 100°C and calcined for 5 hours at 500 °C to finally obtain the powder Zn-Al-Ti LDH.

Hydration of blended samples
The resulting binders and the hydrated samples were analyzed by the X-ray diffraction performed by a Siemens D5000 diffractometer.This unit uses the mounting BRAGG-BRENTANO (θ/2θ) and a radiation λ Kα Cu = 1.5406Å.The spectra 2θ interval is between 10 and 60 ° (with a step of 0.04°).
Two samples are prepared from the synthesized C 2 S phase and the addition of 1% and 3% by weight of Zn-Al-Ti LDH.The adopted nomenclature is given in table 1.The designation for example of sample C2S3LDH7d means that the addition of 3% LDH is performed to C 2 S and hydration of sample was followed for 7 days.3).After the second calcination the dominant phase detected is assigned to the mixed oxides of ZnO and Al 2 O 3 , since the origin of the ZnO phase is the Zn-Al LDH [28,29], we also noticed the disappearance of the LDH phase such as what occurred in the first calcination (Figure 2), which resulted in the formation of stable phases of ZnO, ZnAl 2 O 4 and Zn 2 TiO 4 .The reflections of the TiO 2 anatase phase were detected, probably due to the titanium oxide that did not react with ZnO (Figure 4).These results are consistent with the work of Milica Hadnadjev-Kostic et al. [30].The interest of the calcination of Zn 6 Al 2 (OH) 16 CO 3 .4H 2 O is to form Zn-Ti LDH by the reconstitution method, the formed LDH were calcinated at 500°C to remove nitrates and form the ZnO and Zn 2 TiO 4 which is the essential final product for the photocatalytic activity [30].The reconstruction of the LDH structure took place during the impregnation process under the effect of the mixed oxides of ZnAl who have the ability to re-establish the hydroxide lamellar structure when are exposed to water and anions [31], which will be used as an addition to the hydration of different samples of C 2 S.

X-ray diffraction of hydrated samples
Figure 5 shows the diffraction patterns of C 2 S samples + 1% of Zn-Al-Ti LDH hydrated for 2, 7, 28 and 90 days.Table 2 summarizes the semi-quantitative analysis of the formed hydrates and their Ca/Si ratios as a function of time.The global Ca/Si observed for total C-S-H phases is calculated as the sum of all proportions of each observed C-S-H phase multiplied by its own factor Ca/Si.   Analysis of results shows that the main products of hydration of the C 2 S phase + 1% Zn-Al C 2 S and β-C 2 S phases, Portlandite and hydrated calcium silicates.We notice that the C-S-H phases are present in forms of different solid solutions including different proportions of Al and Zn.On the 2nd day of hydration, we have a small amount of C-S-H phases whose pr is of formulas Ca 5 (Si 6 O Ca 14 (Si 24 O 58 )(OH) 8 (H 2 O) 2 .On the 7th day of hydration, there is the appearance of other hydrates of formulas Ca 6 Si 3 O 12 .H 2 O and CaZn(SiO 4 )(H 2 O) with decreasing percentage of Clinotobermorite, while the content of Portlandite continues to increase.After 28 days of hydration, we observe fluctuations in the amounts of this phase accompanied by appearance of other compounds such us hydrates incorporating the LDHs.These last phases have been identified in the forms of CaZn(SiO 4 )(H 2 O) and Ca 4 Al 8 Si 36 O 48 .16Halso the formation of the Ca 5 (SiO 4 ) 2 (OH) percentage of 12.78% with a slight decrease of C and the total consumption of the Portlandite Ca(OH) these results are consistent with the Didamony et al. [32].The hydrated phases formed at a young age, before 7 days, are essentially constituted by cristalline phases with low ratio Ca/Si less than 1 while in the medium term, 28 to 90 days, the identified phases become morerich in calcium and the Ca/Si ratio increases and may exceed 2. The hydration of C presence of crystallized C-S-H compounds with percentages that increase and become more abundant in the 90th day, this is explained by the known character of this phase whose hydration is slow and participates in the creation of long-term resistances [2].
Figure 6 shows the diffraction patterns of C 3% of Zn-Al-Ti LDH hydrated for 2, 7, 28 and 90 days.Table 3 summarizes the semi-quantitative analysis of th formed hydrates and their Ca/Si ratios as a function of time.The observations for the sample with 3% Zn LDH addition are similar to those attributed to the sample with 1% of the added.From the spectra of Figure 6 and table 3  From the tables 2 and 3 we plot versus time the Ca/Si ratios of different hydrates formed (Figure 7), which clearly shows that in dependence of time the global Ca/Si ratio increases with time hydration, and that the Ca/Si ratios with the addition of 1% of Zn-Al-Ti LDH are lower than those with 3% of addition.These resu with several studies showing that the variation of the stoichiometry of C-S-H in terms of the ratio Ca/Si varies

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Analysis of results shows that the main products of Al-Ti LDH are αphases, Portlandite and hydrated calcium H phases are present in forms of different solid solutions including different proportions of Al and Zn.On the 2nd day of hydration, H phases whose presence O 17 )(H 2 O) 5 et .On the 7th day of hydration, there is the appearance of other hydrates of formulas O) with decreasing percentage of Clinotobermorite, while the content of ortlandite continues to increase.After 28 days of hydration, we observe fluctuations in the amounts of this phase accompanied by appearance of other compounds such us hydrates incorporating the LDHs.These last phases have been identified in the forms of .16H 2 O.We observe (OH) 2 phase with a percentage of 12.78% with a slight decrease of C 2 S phase and the total consumption of the Portlandite Ca(OH) 2 , these results are consistent with the work of H. El-Didamony et al. [32].The hydrated phases formed at a young age, before 7 days, are essentially constituted by cristalline phases with low ratio Ca/Si less than 1 while in e identified phases in calcium and the Ca/Si ratio increases and may exceed 2. The hydration of C 2 S shows the H compounds with percentages that increase and become more abundant in the 90th day, this is explained by the known character of se whose hydration is slow and participates in the Figure 6 shows the diffraction patterns of C 2 S samples + Ti LDH hydrated for 2, 7, 28 and 90 days.quantitative analysis of the formed hydrates and their Ca/Si ratios as a function of time.The observations for the sample with 3% Zn-Al-Ti LDH addition are similar to those attributed to the sample with 1% of the added.From the spectra of Figure 6 and ion of the clinotobermorite H type and Portlandite Ca(OH) 2 which are the main hydration products calcium silicates.Depending on the hydration time, the content of the clinotobermorite is variable and accompanied by the (OH) 8 .6H 2 O (C-S-H).Al LDH phases also interacted in this hydration by hydrate formation where Zn and Al are incorporated, these hydrates appear since the second day of hydration, they have formulas of CaZn 2 (OH) 6 .2H 2 O From the tables 2 and 3 we plot versus time the Ca/Si ifferent hydrates formed (Figure 7), which clearly shows that in dependence of time the global Ca/Si ratio increases with time hydration, and that the Ca/Si Ti LDH are lower than those with 3% of addition.These results agree well with several studies showing that the variation of the H in terms of the ratio Ca/Si varies with the concentration of lime in the solution in which it is in equilibrium, so the higher lime concentration, the ratio Ca/Si of C-S-H is higher [33,34].

Conclusion
This study examines the mineralogical aspects of hydration of belite phase incorporating Zn double hydroxides in time when the proportions of Zn Al-Ti LDH varied of 1% and 3%.The results can be summarized as follows: -As a function of the hydration time we observe a slow increasing of the formed C-S Portlandite phase Ca(OH) 2 , t increase of the mechanical resistances of the C known having a slow hydration participates essentially in the creation of long resistance.
-The basic elements of Zn-Ti C-S-H phases that are presented in forms of different solid solutions including different proportions of Al and Zn.
-The hydrated phases formed (before 7 days), are essentially constituted by crystalline ratio Ca/Si less than 1 while in the medium term, 28 days, the identified phases become more rich in calcium and the Ca/Si ratio increases and may exceed 2 such as in the formation of Reinhardbraunsite Ca with the concentration of lime in the solution in which it is in equilibrium, so the higher lime concentration, the H is higher [33,34].The hydrated phases formed (before 7 days), are crystalline phases with low ratio Ca/Si less than 1 while in the medium term, 28 to 90 days, the identified phases become more rich in calcium and the Ca/Si ratio increases and may exceed 2 such as in the formation of Reinhardbraunsite Ca 5 (

Fig. 7 .
Fig. 7. Evolution of the global Ca/Si ratio of hydrated samples as a function of time Ca/Si ratio of hydrated samples as a function of time This study examines the mineralogical aspects of hydration of belite phase incorporating Zn-Al-Ti layered time when the proportions of Zn-Ti LDH varied of 1% and 3%.The results can be As a function of the hydration time we observe a slow S-H and a decrease in the this may contribute to the increase of the mechanical resistances of the C 2 S phase hydration character and which participates essentially in the creation of long-term Ti-Al LDH interacted in the are presented in forms of different solid solutions including different proportions of Al and

Table 1 . Nomenclature of prepared samples Hydration time C 2 S+ 1% Zn-Al-Ti LDH C 2 S+ 3% Zn-Al-Ti LDH
3.1 X-ray diffraction of the anhydrous samplesThe C 2 S phase is synthesized by reacting of CaCO 3 and SiO 2 at 800°C and 1000°C.The α-C 2 S form of The diffraction peaks demonstrates a LDH double layered hydroxides by forming two LDH phases: A stoichiometric phase Zn 6 Al 2 (OH) 16 CO 3 .4H 2 O and a nonstoichiometric phase Zn 0.56 Al 0.44 (OH) 2 (CO 3 ) 0.22 .xH 2 O was detected as the dominant phase.During the first calcination of Zn-Al LDH phase at 500°C we can note the disappearance of the diffraction peaks corresponding to LDH, the structure is destroyed which resulted in the appearance of ZnO, Al 2 O 3 and Zn 4 Al 22 O 37 and the disappearance of carbonates (figure 2).After impregnation of TiO 2 onto Zn-Al LDH phase, TiO 2 was not detected in diffractograms because of its low concentration.However the appearance of zinicite phase ZnO and aluminum oxide Al 2 O 3 as well as Zn 0.63 Al 0.37 (OH) 2 (CO 3 ) 0.185 .xH 2 O LDH phases were due to the hydration following the impregnation process (figure

Table 3 .
Semi-quantitative analysis of hydrated samples with 3% Zn-Al-Ti LDH we can see the formation of the clinotobermorite Ca 5 (Si 6 O 17 )(H 2 O) 5 C-S-H type and Portlandite Ca(OH) which are the main hydration products calcium silicates.Depending on the hydration time, the content of the clinotobermorite is variable and accompanied by the formation of jennite Ca 9 H 2 Si 6 O 18 (OH) The Zn-Ti-Al LDH phases also interacted in hydration by hydrate formation where Zn and Al are incorporated, these hydrates appear since the second day of hydration, they have formulas of CaZn and Ca 4 Al 8 Si 36 O 48 .16H 2 O.