CRYSTAL GROWTH & DESIGN
Calcite Morphology and Aggregation in the Presence of Comb-like Polymers Adsorbed on Cement Particles
2009 VOL. 9, NO. 5 2240–2247
Giuseppe Falini,*,† Simona Fermani,† Marco Goisis,‡ and Giuseppe Manganelli‡ Dipartimento di Chimica “G. Ciamician” Alma Mater Studiorum, UniVersita` di Bologna, Via Selmi 2, I-40126, Bologna, Italy, and CTG Italcementi Group, Via Camozzi 124, I-24121 Bergamo, Italy ReceiVed August 30, 2008; ReVised Manuscript ReceiVed February 9, 2009
ABSTRACT: Comb-like polymers have an important application in cement industries, where they are referred to as polymeric admixtures/superplasticizers. They are used to increase the workability of fresh cementitious mixtures as well as to improve the strength and durability of the hardened state. So far, the interaction of polymer/Portland-limestone cement particles has been poorly investigated. This paper describes the overgrowth of calcite on Portland-limestone cement particles, a mixture of ground clinker and calcite, in the presence of comb-like polymers. The morphological and aggregational variation of overgrown calcite crystals is used as a model to evaluate the interaction occurring between cement particles and polymeric admixtures. Rhombohedral, spherulitic, microtrumpet, or fibrous calcite crystals were observed as a function of the kind of cement particles and comb-like polymers used. These results suggest that comb-like polymers may assume diverse conformations when associated with the different particles that constitute Portland-limestone cement. Introduction In recent years, the use of polymeric additives to control crystal morphology and size has become widely diffuse. Many materials, organic and inorganic, have been (co)precipitated in the presence of polymers. They have enhanced properties and potential novel applications in materials science and medicine.1 However, polymers are also massively used as additives to cement, which is provided to the building companies to make concrete.2 The addition of polymers, referred to as polymeric admixtures (or superplasticizers) by cement industries, to concrete improves its workability by allowing the use of a low water/cement ratio.3 Recent polymeric admixtures have a structure similar to comb-like polymers, in which a carboxylated polymer, usually polyacrylate, is the main chain and methoxy polyethylene glycol makes the grafting chains.4 It is known that carboxylic groups of comb-like polymers are coupled with Ca2+ ions on the surface of cement particles or interact with their hydrated products.5 These superplasticizers induce in cement particles (or grains) less negative zeta potentials (a function of the charge of particles with adsorbed polymers) than those of the first generation, which were based on sulfonated naphthaleneformaldehyde condensates, sulfonated melamine-formaldehyde condensates, and modified lignosulphonates.6 Moreover, the presence of grafted chains on polymeric admixtures favors their adhesion on the cement and alumina particles, with respect to their corresponding ungrafted chains.7 The mechanism of dispersion of cement particles by these polymeric admixtures should be more related to the steric hindrance caused by the presence of the long grafted chains than to electrostatic repulsions, as it occurs in those of first generation. This observation has been confirmed by studies on rheological properties of concentrated aqueous suspensions of calcium carbonate containing comb-like polymeric admixtures.8 It is well-known, and it has been extensively proved, that when a polymer adopts conformations able to stabilize crystalline planes the crystal morphology changes. A specific interac* To whom correspondence should be addressed. Tel: (+39) 051 2099484; fax: (+39) 051 2099456; e-mail:
[email protected]. † Universita` di Bologna. ‡ CTG Italcementi Group.
tion occurs only when the polymer recognizes one or a few families of crystalline planes and stabilizes them; in this case a typical crystalline habit is conserved. For nonspecific interaction the polymer stabilizes several families of crystalline planes, and in this case usually the typical crystal habit is lost and rounded crystals are formed. High concentrations of polyelectrolyte may also give nonspecific interactions and lead to the formation of rounded crystals.9 In solution comb-like polymers assume random coil conformations and may rearrange in ordered and extended structures only when adsorbed on particles.10 In fact, comb-polymers specifically modified zinc oxide crystal morphology by adsorption on otherwise unstable crystalline faces.11 The aim of this research is to study the effects of comb-like polymers (polymeric admixtures) on calcite crystal growth and aggregation, which may occur by molecular recognition at the polymer/cement particles interface. This has been done overgrowing calcite crystals on calcite, ground clinker, or Portlandlimestone cement (ground clinker blended with calcite)12 seed particles in the presence of adsorbed, and in solution, comblike polymers. We suppose that calcite can interact with the polymer in a conformation close to that present on cement particles surface, while overgrowing on cement particles.13 The choice to overgrow calcite has been done because (i) its crystalline structure allows one to correlate morphological variations with the polymer conformation, and (ii) calcium carbonate is a component of Portland-limestone cement. Two comb-like polymers, used as polymeric admixtures, and their corresponding ungrafted polymers have been used. Control experiments have been carried out in the absence of polymers and in the presence of the grafted polymeric chain of comblike polymeric admixtures. Results Calcium carbonate was overgrown into aqueous comb-like polymeric admixture solutions seeded with (i) synthetic calcite particles (crystals) or (ii) ground clinker particles or (iii) a mixture of (i) and (ii). This mixture may represent a Portlandlimestone cement.12 Particles were sieved below 1 µm grain size. They were insoluble at the starting pH (8.0) of crystal-
10.1021/cg800963u CCC: $40.75 2009 American Chemical Society Published on Web 03/12/2009
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Table 1. Polymeric Admixtures Used in Overgrowth Experiments of Calcium Carbonate on Portland-Limestone Cement Particles code HSP214 HSP147 MPSP6 MPSP9 MPEG
chemical composition
A. weight (Mw (Da))
A. numbera (Mn (Da))
conc (µM)
41000 24300 4200 4500 1000c
18000 13800
0.04 0.06 0.02
b
MPEG grafted poly(acrylic acid) MPEG grafted polymethacrylic acidb polymethacrylic acid polyacrylic acid methoxy poly(ethylene glycol)
a
The polymeric admixtures are commercial products and for some of them the data on the average number is not available. along the main chain is random. c The average weight of the MPEG was the same of the molecules used in the grafting.
2.5 b
The MPEG grafting
Scheme 1. (A) Schematic Illustration of Molecular Structure of Comb-like Polymeric Admixtures. (B) Illustration of Interactions among Calcite Crystalline Units to Form Aggregates, This Process Is Mediated by Polymeric Admixtures. (C) Proposed Interaction between the HPS147 and the (011) Crystalline Face of Calcite. The Polymeric Carboxylate Groups Bind the Calcium Ions in the Calcite Structure, while the Grafted MPEG Chains Are Exposed toward the Solution. (D) Illustration of the MPEG Grafted Chains of HSP147 Emerging from Crystal Surfaces. They Should Deplete Crystal Aggregation
lization experiments, in the absence or presence of comb-like polymeric admixtures. Chemical features and concentrations of polymeric admixtures are reported in Table 1/Scheme 1 and are listed hereafter: sodium salts of polyacrylic acid (MPSP9) and polymethacrylic acid (MPSP6), and of their methoxy poly(ethylene glycol) grafted forms, HSP214 and HSP147, respectively. Control experiments have been carried out in the absence of polymeric admixtures and in the presence of methoxy poly(ethylene glycol) (MPEG). The final pH of crystallization solutions was in the range 8.4-8.6. The phase of calcium carbonate overgrown on the three kinds of particles, (i), (ii), and (iii), was investigated by X-ray powder diffraction. The diffraction patterns showed only the typical diffraction peaks of calcite (Figure 1). The presence of additional diffraction peaks, due to the ground clinker, was not observed. A few particles of clinker were observed during the crystallization experiments by optical microscope (data not shown); these particles were probably removed during the water washing of precipitates. The mass of calcite overgrown varied depending on the polymeric admixture used, and it was always lower than that one in the control experiments (data not reported). The morphological investigation on overgrown calcite crystals was carried out by scanning electron microscopy. The results are summarized in Table 2. In the absence of additives calcite crystals appeared as rhombohedra showing a stepped overgrowth, in which only {104} faces were present (Figure 2A-C),
independently from the kind of particles used as seeds. However, overgrown calcite crystals were rough on the {104} faces when ground clinker particles were used as seeds. These crystals had
Figure 1. X-ray powder diffraction patterns of calcium carbonate overgrown on the Portland-limestone cement particles in the presence of polymeric admixtures. (A) no additive; (B) MPEG; (C) MPSP9; (D) MPSP6; (E) HSP214; and (F) HSP147. X-ray powder diffraction patterns of calcium carbonate overgrown on the other seed particles, calcite or clinker, were similar. Diffraction peaks are labeled with Miller indexes according to the hexagonal unit cell of calcite.
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Table 2. Morphological Effects of Polymeric Admixtures on Calcite Crystals Overgrown on Calcite or Clinker or Portland-Limestone Cement Seed Particlesa calcite a. sizeb (µm) Ctrl MPEG MPSP9 MPSP6 HSP214 HSP147d
20 ( 5 25 ( 5 10 ( 5 20 ( 5 20 ( 5 45 ( 10
clinker ex. facesc
{104} {104} {104} {104} {104} {104}
-
{hkl} {hkl} {00l} {011}
a. sizeb (µm) 20 ( 5 25 ( 5 50 ( 10 25 ( 5 30 ( 5 40 ( 10
I. P. cement ex. facesc
{104} {104} {104} {104} {104} {104}
-
{hkl} {hkl} {00l} {011}
a. sizeb (µm) 20 ( 5 20 ( 5 100 ( 20 20 ( 5 25 ( 5 35 ( 10
ex. facesc {104} {104} {104} {104} {104} {104}
-
{hkl} {hkl} {001} {011}
a Average size (a. size) and exposed faces (ex. faces) of aggregates or single crystals are reported. b The average size refers to the average length of the longest axis of aggregates or single crystals. At least 50 items were measured. In parentheses the standard deviation are reported. c Crystal faces are denoted by a set of the Miller indices h, k, l, which defines their orientation relative to the crystal axes. {hkl} denotes the family of symmetry related faces (hkl). d In the presence of HSP147, and differently from other additives, single crystals overgrew.
Figure 2. Scanning electron microscope images of calcite crystals overgrown on Portland-limestone cement particles in the absence of additives (A-C) and in the presence of MPEG (D-F). Crystals with similar shapes and morphologies were obtained by overgrowth experiments using as seeds calcite or clinker particles. In (E) the arrow indicates an intercrystals region.
an average size of about 20 µm along the longest axis. In the presence of MPEG the overgrowth of calcite crystals with similar morphology has been observed (Figure 2D-F). These crystals had dimension along the longest axis around 20-25 µm and appeared more clustered than those ones obtained in the absence of additives. Polymer-like material was observed at the interface among these crystals (arrow in Figure 2E). Also in this case the {104} faces were rough when ground clinker particles were used as seeds. When polymeric admixtures were present, the features of overgrown calcite crystals were a function of the kind of particle used as seed. In the presence of MPSP9, crystals overgrew as polycrystalline aggregates (Figure 3) in which the crystal units had dimensions lower than that of crystals in the control experiments. In fact, aggregates of about 10 µm were observed when calcite particles were used as seeds (Figure 3A-C). The crystal units making these aggregates were randomly associated, showed additional faces almost parallel to the [001] direction, and were capped with {104} faces (inset in Figure 3C). The use as seeds of ground clinker particles provoked the aggregation of overgrown calcite crystals in complex shapes. They formed flat bulk microstructures (average size of about 50 µm) from which emerged conic, pyramidal, or linear crystalline aggregates (Figure 3D-F). The crystal units forming these aggregates have dimensions below 1 µm. The shape of these aggregates, such as the pyramidal one, seems to be generated by the preferential association of the crystal units along one direction, probably
the c-axis of calcite (Figure 3E,F). The scenario of crystal aggregation changed when the mixture of calcite and ground clinker particles was used as seeds. In this case the crystal aggregates showed flat and rounded shapes, which had an average radius of about 100 µm. One example is shown in Figure 3G. In the central region of this aggregate the crystal units are stacked one of the top of each other forming elongated columns (Figure 3I), while in the periphery the crystal units appear as elongated needles, closely packed and radially oriented (Figure 3H). They have a homogeneous thickness of about 0.3 µm and their length is up to 10 µm. In the presence of MPSP6 the overgrown calcite crystals still formed polycrystalline aggregates, which had dimensions of around 20-25 µm (Figure 4). The calcite crystal units that make the aggregate showed additional {hkl} faces, together with rhombohedral {104} faces (insets in Figure 4C,F,I). They are elongated along the crystallographic [001] direction and are laterally aggregated, forming spherical shapes in which superficial {104} and {hkl} faces are visible. Aggregation and {hkl} faces expansion of crystals were more pronounced using calcite than ground clinker as particle seeds. The use of Portland-limestone cement particles as seeds provoked morphological changes in overgrown calcite crystals in-between the effects of calcite and ground clinker seed particles. In the presence of HSP214, the methoxy poly(ethylene glycol) grafted form of the MPSP9, calcite crystals overgrew forming spherical aggregates (Figure 5). When calcite particles were used
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Figure 3. Scanning electron microscope images of calcite crystals overgrown in the presence of MPSP9 on (A-C) calcite, (D-F) ground clinker, and (G-I) Portland-limestone cement seed particles. The kind of particle used influences the assembly of calcite crystalline units and the shape of aggregates. The inset in (C) shows a magnification of the surface of an aggregate, in which the arrow indicates a crystal face that develops on the calcite crystal after interaction with the polymeric admixture.
as seeds these aggregates were compact and had an average size of about 20 µm (Figure 5A-C). Their superficial crystal units of calcite showed {104} and {00l} faces, the latter cutting calcite crystals normal to the c-axis (inset in Figure 5C). The use of ground clinker as seed provoked the formation of aggregates less compact and with bigger crystal units than those ones obtained on calcite particles. They had an average size of about 30 µm. Each crystalline unit clearly showed etched {104} faces and additional faces, with rounded crystal edges (Figure 5D-F). When Portland-limestone cement particles were used as seeds the crystalline aggregates were constituted by small and compact crystal units and had an average size of about 25 µm, similar to that observed using calcite particles. On the surface of these aggregates the crystals showed smooth {104} faces and additional crystalline faces, almost parallel to the c-axis of calcite. These faces gave a rounded aspect to the aggregates (Figure 5G-I). HSP147, differently from other studied polymeric admixtures, did not provoke the precipitation of spherical aggregates of calcite (Figure 6). Single crystals overgrew on the calcite particles. These crystals had an average size of around 35-45 µm. They showed smooth {104} faces and additional faces almost parallel to the c-axis of calcite. The analysis of the crystal interfacial angles and the graphical computer simulation of crystal morphology indicate that these faces have index {011} (inset Figure 6B).13 The overall shape of these crystals is a function of the relative expansion of the {104} and {011} faces. Calcite crystals overgrew with a similar morphology using clinker particles as seeds. However, in this condition crystals aggregated and the expansion of the {104} faces appeared wider than in the previous case (Figure 6D-F). In the presence of
Portland-limestone cement particles the calcite crystals also overgrew forming aggregates in which {104} and {110} faces were exposed (Figure 6G-I). Discussion Only calcite overgrew on particles of calcite or ground clinker or Portland-limestone cement (a mixture of calcite and clinker particles) in the presence of polymeric additives, as indicated by the X-ray powder diffraction patterns. Moreover, the final pH of the crystallization experiments, around 8.4, is in agreement with the precipitation of calcite.14 The mass of overgrown calcite was reduced by the presence of comb-like polymeric admixtures, suggesting that they inhibit calcium carbonate precipitation, as previously reported.15 This effect was very evident when calcite particles were used as seeds. Comb-like polymeric admixtures influenced aggregation and morphology of overgrown calcite crystals, as a function of the kind of particles used as seeds. In general, the variation of these two parameters was more evident in the presence of calcite particles than that with ground clinker particles. Since the effect of polymeric admixture on overgrown calcite crystals is related also to its concentration in solution,9 the above observation may indicate either a preferential adsorption on ground clinker particle, that reduces the concentration, and/or a different polymer conformation on the two kinds of particles. Indeed, clinker particles are a complex mixture of calcium aluminates and calcium silicates that in water undergoes hydration reactions giving a nanoparticles highly reactive gel system.1a,2b,12 These nanoparticles can preferentially adsorb polymeric admixtures in comparison to the stable calcite crystal surfaces and modify
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Figure 4. Scanning electron microscope images of calcite crystals overgrowth in the presence of MPSP6 on (A-C) calcite, (D-F) ground clinker, and (G-I) Portland-limestone cement seed particles. The insets in (C), (F), and (I) show magnifications of the surface of aggregates in which the arrows indicate crystal faces that develop on calcite crystals after interaction with the polymeric admixture.
their conformation.8,9 It is important to note that in each experiment the same amount of homogenously sized particles was used. Moreover, in the presence of the clinker particles overgrown calcite crystals show etched {104} faces. Thus, the reactive chemical species present in gel clinker particles may interact with growing calcite crystals, without altering the {104} faces rhombohedral habit. Garcı´a Ruiz et al. reported that the change of crystal habit of calcite in silica gel was associated with the presence of silicate anions at high pH,16 because the solubility of silicate ions, such as SiO(OH)3- and Si4O6(OH)62-, gradually increases above pH 9.17 A study on the precipitation of calcium carbonate in silica gel at different pHs indicated that silica species are active as a crystal morphology modifier only at pH values higher than 10.0.18 However, it has been shown that calcite showing rough faces precipitated in silica gels at pH 8.0.16a Since the final pH of the overgrowth experiments is about 8.4, it can be supposed that the contribution of silica species in the modification of the morphology of calcite is limited compared to that due to comb-like polymeric admixtures. Polymeric admixtures are comb-like polyelectrolytes (Table 1, Scheme 1A) that can adsorb on the surface of overgrowing calcite crystals stopping, specifically or nonspecifically, the growth of crystalline faces. These phenomena are well-known and widely described in the literature on crystal growth in the presence of organic and inorganic additives.1,9 The interaction comb-like polymeric admixture-calcite should occur through the polymeric carboxylate groups present along the main polymeric chain.19 In fact, in the presence of methoxy poly(ethylene glycol), the side chain of the comb-like polymer, crystals overgrew with the same morphology of those obtained in the absence of additives, in agreement with reported data.19a The interaction occurring between the growing calcite crystal
and polymer depends on the polymer conformation. The studied polymeric admixtures are polyacrylate and polymethacrylate, and their corresponding comb-like polymers are obtained by grafting with methoxy poly(ethylene glycol). Polymethacrylate differs from polyacrylate in the presence of an additional methyl group in the monomeric unit (Scheme 1A). This reduces conformational freedom of polymethacrylate and may favor its specific interaction with a crystalline plane. The presence of grafting influences the conformation of both polyelectrolyte reducing their coiling in solution and favoring their adsorption on minerals. Moreover, it is likely that following the interaction with mineral (e.g., clinker particles) polymeric admixtures change their conformation.20 The results obtained from the overgrowth experiments are in agreement with the above suppositions. MPSP6, MPSP9, and HSP214, differently from HSP147, induce the overgrowth of rounded aggregates of calcite, which are obtained through the association of crystalline units having different morphologies. This process of assembling has been described in terms of fractal growth of meso-crystals.1d,f,16 By the use of bock-co-polymer or inorganic ions (e.g., Mg2+) the progressive stages of growth of calcium carbonates and phosphates from elongated hexagonal prisms through dumbbell shapes to spheres have been observed.1g,9e,21 Comb-like polymers have regions with different hydrophilicity, analogous to block-co-polymers. Thus, similar to them, charged chains (main chains) can stabilize crystalline faces of different crystal units of calcite, bridging them and favoring their assembly to form aggregates (or meso-crystals). This effect appears favored by coiled conformations of polymers, which enhance the random exposure of hydrophilic carboxylate groups in regions spatially different (Scheme 1B).
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Figure 5. Scanning electron microscope images of calcite crystals overgrown in the presence of HSP214 on (A-C) calcite, (D-F) ground clinker, and (G-I) Portland-limestone cement particles. Rounded aggregates of crystalline units of calcite formed. The kind of particle affected the compactness of aggregates. The inset in (C) shows a sketch of the crystalline units which form the aggregates observed in (C) in which face Miller indexes are reported.
MPSP9 (polyacrylate) induces the overgrowth of calcite as aggregates with unusual shapes (Figure 3D-I) and needle-like crystals (Figure 3H) when clinker particles are used as seeds. The overgrowth of assemblies of calcite crystals in complex shapes in the presence of polyacrylate, the most charged and conformational free polymeric admixture used, agrees with that reported in the literature.22 Moreover, single crystals calcite fibers, similar to the needle-like calcite observed, and calcite microtrumpet have been reported to form from an amorphous precursor in an overgrowth experiment on calcite crystals in the presence of organic molecules, copolymers, or polypeptides.23 Here, the complexity of growth processes is increased by the presence of the numerous silica species that form in clinker hydrated particles,2,3 making it difficult to any comparison with the reported growth mechanisms.23 Calcitic spherical aggregates overgrew when MPSP6 was used. In them crystal units showed {104} faces and additional crystalline faces almost parallel to the [001] direction. Aggregates overgrown on clinker particles showed a higher expansion of the {104} faces with respect to those that overgrew on calcite or Portland-limestone cement particles. The absence of complex shapes (as observed using MPSP9), which are usually associated with nonspecific interactions,1,9 and the preferential stabilization of crystalline {hkl} calcitic faces, strengthen the supposition that methyl groups along the main chain reduce the conformational freedom of this polymer. The use of HSP214, the grafted form of MPSP9, provokes the stabilization of {00l} faces in the crystalline units of calcite in the aggregates. These {00l} crystalline planes are characterized by layers of calcium or carbonate ions (Scheme 1C). Since polymeric admixtures contain negatively charged carboxylate
groups on the main chain, the interaction with calcite should occur with calcium ions. In this process the main chain interacts with the crystalline plane and the grafted chains are exposed reducing crystal aggregation by steric hindrance. This effect was particularly marked in the presence of the clinker particles, implying that they can favor the polymeric admixture unfolding. HSP147 interacts in specific way with {011} faces of overgrown calcite crystals. The stabilization of these crystalline faces of calcite by carboxylate groups has been widely reported.24 HSP147 contains methyl groups along the main carboxylic chain and is MPEG grafted, both factors favoring highly unfolded conformations. A possible mode of interaction with calcite crystals is presented in the Scheme 1C, in which carboxylate groups interact with calcium ions in the (011) crystalline plane of calcite and grafted chains are exposed toward the solution. This exposition increases the sterical hindrance and almost avoids crystal aggregation (Scheme 1D). In fact, in the presence of HSP147 single crystals overgrew.
Conclusion The overgrowth of calcite on the particles that make Portlandlimestone cement has allowed monitoring of the conformational status of polymeric admixtures adsorbed on them. The results indicate that comb-like polymeric admixtures interact differently with the particles making cement properties a function of the rigidity of the main chain and the presence of grafted chains. This study can be relevant for the design of new polymeric
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Figure 6. Scanning electron microscope images of calcite crystals overgrown in the presence of HSP147 on (A-C) calcite, (D-F) ground clinker, and (G-I) Portland-limestone cement particles. The inset in (B) shows a sketch of the calcite crystal observed in (B) in which face Miller indexes are reported.
admixtures able to reduce the aggregation of particles and thus to improve the workability of cements. Experimental Procedures Materials. Calcium chloride dihydrate and anhydrous calcium chloride granular (for drying) manufactured by Merk Co. were used. Ammonium carbonate and calcium carbonate were supplied by Sigma Co. Water Millipore grade (18.2 MΩ) was used to prepare solutions. Synthetic calcite crystals were prepared as previously reported.15 These crystals were first grounded and then sieved below 1 µm. Clinker particles having a grain size below 1 µm were supplied by CTG Italcementi Group. A mixture that represents a Portland-limestone cement was obtained by mixing 10% (w/w) of calcite with 90% (w/w) of clinker particles.12 The polymeric admixtures used in the crystallization experiments are reported in Table 1 and were supplied by CTG Italcementi Group. Calcium carbonate overgrowth was carried out using microplates, 24 wells (dia. 16 mm) with lid, tissue culture treated polystyrene supplied by IWAK brand and glass cover round slides (12 mm). Calcium Carbonate Overgrowth. The 24 wells microplate was prepared introducing an ethanol-washed and post-dried glass cover slide in each well. 750 µL of a suspension of calcium carbonate, clinker or Portland clinker seed particles (5 mg/mL) in 10.0 mM calcium chloride solution was introduced to each well. The wells were covered with aluminum foil and punctured by a needle. Then the microplate was placed inside a sealed Plexiglass box containing ammonium carbonate and anhydrous calcium chloride. When effects of comb-like polymeric admixtures were studied 10 mM calcium chloride solutions containing the additive were used. The commercial polymeric admixture solutions were diluted with 10.0 mM calcium chloride solution to the required concentrations (Table 1). In each experiment the starting pH was adjusted to the value of 8.0 by addition of the required amount of a sodium hydroxide solution. The crystallization process started with the vapor diffusion of CO2 and NH3 from ammonium carbonate in the calcium chloride solutions at room temperature. After 6 days the crystallization process was stopped by opening the Plexiglass box. First
the obtained precipitates were washed with deionized water (750 µL for well) at least twice and then the microplates were air-dried for two days at room temperature. In each crystallization experiment control wells containing pure calcium chloride solution and methoxy poly(ethylene glycol) solution were present. Characterization Methods. Crystals were initially examined directly in wells by an optical microscope (Leika optical microscope connected to a CCD digital camera) and their orientation was checked using crosspolar. Optical microscope pictures were taken at different magnifications and the sizes of the crystals were measured. The dried glass coverslips covered with crystals were glued to SEM aluminum stubs. After gold coating, the crystals were observed in the SEM (Philips 515) using a tension of 15 kV. The calcite crystals overgrown in the presence of the polymeric admixtures showed new {hkl} faces. In order to identify these new faces, the crystals were viewed with their {hkl} and corresponding {104} faces both edge-on. In this position the crystallographic c-axis lies in the plane of the picture, allowing the measurement of the angle between the c-axis and the unknown {hkl} face. This angle unequivocally identifies the Miller indices of the face.13 A graphical computer simulation of the morphology of the calcite crystals expressing the additional {hkl} faces was carried out using the software SHAPE. X-ray powder diffraction (XRD) patterns were recorded using a Philips X’Celerator diffractometer with Cu KR (1.5814 Å) radiation. The samples were scanned for 2θ angles between 5° and 60°, with a resolution of 0.02°.
Acknowledgment. We thank CTG Italcementi Group, the Università di Bologna (Funds for Selected Topics), Consorzio Interuniversitario di Ricerca della Chimica dei Metalli nei Sistemi Biologici (CIRCMSB) and Ministero dell’Istruzione, dell’Universita` e della Ricerca for the financial support.
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