18, Zona Industriale di Pile, L Aquila, Italy

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1 Advanced Materials Research Online: ISSN: , Vol. 629, pp doi: / Trans Tech Publications, Switzerland Synthesis of Ca(OH) 2 Nanoparticles Aqueous Suspensions and Interaction with Silica Fume V.Daniele a, G.Taglieri a*, A.Gregori b a Department of Chemistry, Chemical Engineering and Materials, University of L Aquila, Via Gronchi 18, Zona Industriale di Pile, L Aquila, Italy b Department of Structural, Water and Ground Engineering, University of L Aquila, Via Gronchi 18, Zona Industriale di Pile, L Aquila, Italy * Corresponding author. Tel.: ; fax: Keywords: Calcium-Silicate-Hydrate (C-S-H); Characterization; X-Ray Diffraction; Ca(OH) 2 ; Silica fume Abstract.In this work, interaction at ambient temperature between silica fume and artificially produced Ca(OH) 2 nanoparticles by two different methods was analyzed. Initial products and formed hydrated phases were characterized by several investigations including X-ray diffraction, thermal analyses, transmission and scanning electron microscopy. Synthesized Ca(OH) 2 nanoparticles appeared regularly shaped and hexagonally plated, with side dimensions from 200nm to less than 20nm. Characterization analyses showed that, by reducing particles dimensions, calcium silicate hydrate phase formation was evident already after 7 day of hydration and a nearly complete consuming of free Ca(OH) 2 after 28 days was observed. Besides, formed hydrate phases showed a highly wrinkled layer with marked crumple and rough-edge surfaces. 1. Introduction A significant number of research works concerning the interactions of silica fume (SF) with calcium hydroxide (CH) are found in the scientific literature, also considering the increased use of silica fume in concrete production during the last decades, both in form of addition or in partial replacement of the dosage of ordinary Portland cement (OPC). In fact, CH liberated from hydration of calcium silicates can react with SF to form more calcium silicate hydrates (C-S-H) precipitates in the large pores and voids [1]. The interaction of SF exposed to saturated CH solution revealed that SF persisted in form of agglomerates coated with amorphous C-S-H up to 120 days; in addition, up to 7 days, X-ray diffraction analysis revealed that there was any significant decrease in the main broad amorphous SF peak [2]. Results from investigations on lime-silica hydration process carried out at ambient temperature and in relation to different factors (such as the type of silica and the lime/silica molar ratio) can be found [3,4]. Beyond a slight decrease of free lime content in the early stage of hydration [5-10], a completely consumed free lime was observed only after 90 days. Some Authors investigated lime/silica fume interaction at higher temperature too [11]. In particular, hydration process was studied at 60 C and kinetics of hydration reaction, followed at time intervals of 1, 3, 7 and 28 days, in accordance to [11,12], revealed that free CH was consumed only after 28 days. From microscopic analysis, hydrated products were found, after 28 days, to mainly consist of calcium silicate hydrates in form of fine fibrous particles and crumpled foils. In the present work, for the first time, the interaction between silica fume (SF) and artificially synthesized CH nanoparticles (nano-ch or nanolime) dispersed in aqueous medium was investigated. Nano-CH were synthesized by a chemical precipitation process, in supersaturated aqueous conditions, and two different methods of synthesis were considered. In method A), nano- CH were obtained, starting from an aqueous sodium hydroxide (NaOH) solution added drop by drop in an aqueous calcium chloride (CaCl 2 ) one, maintained at high temperature [13, 14 and ref. in]. The supersaturation degree, the high temperature and the slow mixing time tend to favor Ca(OH) 2 nucleation rate respect to particle growth, promoting precipitation of nanosized particles [15]. In the second method (namely, method B)), nano-ch were produced by an original method All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (# , Pennsylvania State University, University Park, USA-18/09/16,18:06:35)

2 Advanced Materials Research Vol that allowed to obtain calcium hydroxide nanoparticles easily and drastically reducing time of synthesis, respect to method A). In particular, they was obtained pouring a non-ionic surfactant agent (Triton X-100) in the initial aqueous solutions of CaCl 2 and NaOH, maintained at high temperature, and mixing them simultaneously. The molecular units of surfactant, stick by weak forces on the growing crystal surfaces, together with supersaturation degree and high temperature, seem to prevent the growth of nano-ch crystals [16]. The produced nano-ch were characterized by X-ray diffraction (XRD), and transmission electron microscopy (TEM). Commercial SF was used in the experiments as taken from the market, with phase and chemical composition previously characterized by XRD, X-ray fluorescence (XRF) and scanning electron microscopy (SEM) analyses as well. Interaction among silica fume (SF) and nano-ch (synthesized by different methods) was carried out in water, at room temperature, considering a water/solid ratio (W/S) of 6, and a CH/SF ratio (CH/SF) of 1 in weight. Phase composition of the formed hydrates was examined by using XRD and differential thermal analyses (DTA) at different time intervals. Morphology of the formed C-S- H was visually investigated by means of scanning electron microscopy (SEM). 2. Experimental 2.1. Materials Materials used in this investigation were: calcium chloride (CaCl 2 ) and sodium hydroxide (NaOH) (supplied by Merck); Triton X-100 surfactant (supplied by Acros), all used without any further purification. Water was purified by a Millipore Organex system (R 18 MW cm); commercial SF (Si 44,32%, Fe 0,56%, K 0,60%, Mg<0,50%, as measured by XRF measurements - Spectro Xepos) Synthesis of nano-ch Method A). Two different aqueous solutions of 300ml, containing 0,3mol/l of CaCl 2 and 0,6mol/l of NaOH respectively, were prepared. The NaOH alkaline solution (used as precipitator) was added drop by drop into the CaCl 2 solution (speed 4 ml/min, temperature of 90 C). After about 24 hours two distinct phases were observed, a limpid supernatant solution and a white precipitated phase. The obtained aqueous nanolime suspension was defined as nl (initial concentration of 10mg/ml). Method B). The nanolime particles were obtained by adding Triton X-100 in the initial aqueous solutions (300ml in volume) containing 0,3mol/l of CaCl 2 and 0,6mol/l of NaOH respectively. Subsequently, these solutions are mixed together simultaneously, at the temperature of 90 C. A suspension of 600ml of final volume is obtained (nlt), with initial concentration of 10 mg/ml. For both the methods, after the synthesis, several deionized water washings were carried out, in order to remove the NaCl produced and the surfactant too Preparation of nano-ch/sf mixtures First of all, nl and nlt suspensions were concentrated leaving, for each one, a water content of about 50ml (concentration of 80mg/ml); as well, SF was added in each suspension considering a nano-ch/sf ratio of 1 (in weight) and a W/S ratio of 6. The two mixtures were mechanically stirred to prevent segregation, obtaining two different pastes denominated nl/sf and nlt/sf samples, respectively. Right after mixing, these samples were homogeneously divided in six parts in closed bottles, so allowing to later analyze the hydrates phases, formed after different time intervals (1, 7, 14, 28 and 45 days, respectively). At each time interval, hydration process was stopped by adding ethanol and drying at 110 C, until the solvent was completely evaporated; the dried samples were then ground and placed in closed bottles for characterization.

3 484 Material Sciences and Manufacturing Technology 2.4. Test procedures TEM (Philips CM100) was used to investigate nano-ch morphology and particles dimension. Basic materials and formed hydrates phase composition were examined by means of a Philips X Pert diffractometer (using Ni-filtered Cu Kα radiation); data were also elaborated by Origin Pro v.8 software. To obtain qualitative information about chemical reactions, phase transitions and structural changes of the considered samples, DTA analyses (Netzsch STA 409) were performed on 30mg of dried and ground samples heated in air from ambient temperature to 1000 C (heating rate of 20 C/min). Measurements were carried out on both the nl/sf and nlt/sf samples at the hydration intervals of 1, 7 and 28 days respectively. Finally, SEM observations (Philips XL30CP) were performed to examine nl/sf and nlt/sf morphology after 30 minutes and 28 days of hydration times. 3. Results and discussion TEM observations on nl sample ware reported in Fig.1a); particles were hexagonally plated and regularly shaped, with side dimensions generally 400 nm, with typical habit of crystalline Ca(OH) 2. Fig 1. TEM micrographs on: a) nl sample; b) nlt sample By TEM micrographs performed on nlt sample it can be observed that particles dimensions tend to decrease (Fig.1b)); in fact, that particles, regularly shaped and hexagonally plated, appeared characterized by side dimensions generally <100 nm. Besides, Ca(OH) 2 fine particles (<20nm in diameter), with spherical shape habit [17] were observed too. XRD anhydrous SF spectrum revealed amorphous feature with a predominant broad peak centred at (2θ). XRD spectra of nlt/sf mixtures after different hydration times were shown in Fig.2a). Data registered after 1 day of hydration contained a large background hump (due to the scattering from amorphous SF) and Bragg peaks typical of the Ca(OH) 2 pattern (2θ = 18.07, 28.68, 34.10, and ). A secondary phase was present, recognizable as calcium carbonate CaCO 3 (main peak at 2θ = 29.45, less intense peaks at 2θ = 23.07, 43.16, and ); CaCO 3 can be due to an initial carbonatation occurred during the drying step in air. After 3 days, similar XRD pattern was obtained and it was not reported in figure. After 7 days of hydration, Ca(OH) 2 peaks tended to decrease and the peak at about 29 slightly grew up; considering that the CaCO 3 less intense peaks were not visible, this fact could be explained with the initial formation of C-S-H. In fact, C-S-Hs peaks are weak and belong to ill-crystallized tobermorites reflections (2θ = , 32 and 49.8, and a broad basal reflection at about 7 ) [1,18]. Fig 2. a) XRD patterns of nlt/sf mixtures after different hydration times: (a) 1day; (b) 7 days; (c) 14 days; (d) 28 days; (e) 45 days; b) Zoom view of nlt/sf XRD patterns in the angular range between 5 and 36 (2θ). Legend: h = Ca(OH) 2 ; C = CaCO 3 ; s = SF; t = C-S-H.

4 Advanced Materials Research Vol The initial formation of C-S-H was confirmed by the zoom view in the angular range between 5 and 36, (Fig.2b)), where C-S-H reflection at 2θ =32 began to be visible. C-S-H formation proceeded after 14 days, and at 28 days of hydration all the C-S-H characteristic peaks were distinctly found, while Ca(OH) 2 was almost completely consumed. After prolonging the hydration time to 45 days, only C-S-H can be observed. Moreover, the C-S-H peaks growth occurred with a decrease of SF background hump. XRD results obtained by nl/sf mixtures were reported in Fig.3a). Fig 3. a) XRD patterns of nl/sf mixtures after different hydration times: (a) 1day; (b) 7 days; (c) 14 days; (d) 28 days; (e) 45 days; b) Overlapping and zoom view of nl/sf XRD patterns in the angular range between 5 and 36 (2θ). Legend: h = Ca(OH)2; C = CaCO3; s = silica fume; t = C-S-H. Comparing these results with those previously obtained in nlt/sf pastes, after 7 days of hydration, only Ca(OH) 2 and CaCO 3 Bragg peaks were visible, and the C-S-H characteristic peaks appeared only after 14 days of hydration (the broad basal reflection at about 7 was not recognizable yet). After 45 days, Ca(OH) 2 was not completely consumed. These considerations were better evidenced by the zoom view reported in figure 3b). Results from DTA analyses performed on nl/sf and nlt/sf pastes, at different curing ages, tended to confirm the XRD measurements. The obtained results could be explained considering Ca(OH) 2 particles dimensions coming out from the different synthesis. In fact, the smaller particles (<100nm) observed in nlt suspension seem to be very promising in obtaining C-S-H, at ambient temperature, already after 28 days of hydration. In both cases, however, the results seem to be competitive to previous reported in literature and discussed above. Microstructure of the formed hydrates from nl/sf and nlt/sf mixtures were compared by using SEM technique. Pictures taken after 30 minutes of hydration were shown in Fig.4; nlt/sf paste was characterized by a homogenous, fine and continuous matrix that completely covered the underneath particles (Fig.4(a)). Differently, nl/sf sample was found to be less homogeneous in terms of covering distribution and of dimensions (note the larger particles marked by a white arrow) (Fig.4(b)). Fig 4. Comparison between nlt/sf ((a)) and nl/sf ((b)) samples after 30 minutes of hydration.

5 486 Material Sciences and Manufacturing Technology SEM images taken on samples after 28 days were reported in Fig.5. A highly wrinkled layer covering all particles and constituted by marked crumple and rough-edge surfaces was recognized, particularly marked in nlt/sf sample (Fig.5(a)). Fig 5. Comparison between nlt/sf ((a)) and nl/sf ((b)) mixtures after 28 days of hydration. 4. Conclusion The present work analyzed hydrate products obtained by mixing water, calcium hydroxide nanoparticles, synthesized by two different methods, and commercial silica fume. In particular, the addition of a surfactant agent in the initial solutions of the reactants, involved a decrease in particles dimension ( 100nm). Interaction between silica fume and calcium hydroxide nanoparticles, synthesized by adding the surfactant agent, revealed the initial formation of C-S-H already after 7 days of hydration; after 28 days all the C-S-H characteristic peaks were distinctly found, while Ca(OH) 2 was almost completely consumed. SEM measurements on the same pastes after 28 days showed a highly wrinkled layer covering all particles and constituted by marked crumple and rough-edge surfaces. Acknowledgements The authors thank to Professor Roberto Volpe for his helpful comments during the investigation. References [1] Taylor HFW. The Chemistry of Cement, London, Academic Press; [2] Mitchell DRG, Hinczak I, Day RA. Interaction of Silica Fume with Calcium Hydroxide Solutions and Hydrated Cement Pastes, Cem. Concr. Res. 1998; 11, vol. 28: [3] Baltakys K, Jauberthie R, Siauciunas R, Kaminskas R, Influence of modification of SiO 2 on the formation of calcium silicate hydrate, Mater. Sci Poland 2007; 3, vol. 25: [4] El-Shimy E et al. Physico-Chemical and Thermal Characteristics of Lime-Silica Fume Pastes, J. Therm. Anal. Calorim. 2000; 60: [5] Huang CY, Feldman RF. Hydration reactions in Portland cement-silica fume blends, Cem. Concr. Res. 1985; 15: [6] Greenberg SA. Reaction between silica and calcium hydroxide solutions. Kinetics in the temperature range 30 to 85. J. Phys. Chem. 1961; 65: [7] Abo-El-Enein SA et al. Hydration of low porosity slag-lime pastes, Cem. Concr. Res. 1974; 4: [8] Abo-El-Enein SA, Shater MA, Nowar M. Il Cemento1981; 78: 137. [9] Abo-El-Enein SA et al.: ibid ; 87: 147.

6 Advanced Materials Research Vol [10] Mehta PK. Effect of lime on hydration of pastes containing gypsum and calcium aluminates or calcium sulfoaluminate, J. Am. Ceram. Soc. 1973; 56: [11] Hekal EE. Effect of some admixture on the hydration of silica fume and hydrated lime, J. Mater. Sci. Technol. 2000; 4, vol. 16: [12] Kondo R, Abo-El-Enein SA, Daimon M. Proc. 3rd Inter. Symp.,.Aut. Calcium Silicate Build. Prods.. Netherlands, p. 25; Bull. Chem. Soc., Japan 1975; 48: 222. [13] Daniele V, Taglieri G, Quaresima R. The nanolimes in Cultural Heritage Conservation: characterisation and analysis of the carbonatation process, J. Cul. Her. 2008; 9: [14] Daniele V, Taglieri G. Nanolime suspensions applied on natural lithotypes: the influence of concentration and residual water content on carbonatation process and on treatment effectiveness, J. Cul. Her. 2010; 11: [15] Ambrosi M, Dei L, Giorgi R, Neto C, Baglioni P. Colloidal Particles of Ca(OH) 2 : Properties and Applications to Restoration of Frescoes, Langmuir 2001; 17: [16] Daniele V, Taglieri G. Synthesis of a nanolime by using Triton X-100 and protective treatments on natural stones: preliminary results, J. Cul. Her. 2012; 13: [17] Ambrosi M, Giorgi R, Salvadori B, Dei L. Nanotecnologie per la conservazione di affreschi e materiali lapidei a base carbonatica, In: 1st International Workshop on Science, Technology and Cultural Heritage; Venezia, 29 Giugno 1 Luglio; 2004: [18] Lin RB et al. Characteristics and reactivities of Ca(OH) 2 /silica fume sorbents for lowtemperature flue gas desulfurization, Chemical Engineering Science 2003; 58: