Article pubs.acs.org/IECR
Modified Tetraethoxysilane with Nanocalcium Oxalate in One-Pot Synthesis for Protection of Building Materials A. Verganelaki, C. Kapridaki, and P. Maravelaki-Kalaitzaki* Materials for Cultural Heritage Lab, School of Architectural Engineering, Technical University of Crete, University Campus, 73100 Chania, Greece S Supporting Information *
ABSTRACT: A nontoxic strengthening and protective agent for porous calcareous stones and cement mortars has been synthesized in a one-pot synthesis via the sol−gel method, incorporating nanoparticles of synthesized amorphous calcium oxalate monohydrate (ACO) in tetraethoxysilane (TEOS). Calcium hydroxide and oxalic acid added into TEOS produce ACO, which is then incorporated into the silica matrix, while oxalic acid also acts as catalyst for TEOS hydrolysis. The crack-free nanocomposite derived possesses a uniform microstructure with average pore diameter of 2.73 nm and particles of approximately 7−15 nm in size. The ACO incorporated into the silica matrix gives good interfacial compatibility between the nanocomposite and building materials, such as calcareous stones and cement mortars, and improves their mechanical properties. The hybrid nanocomposite can penetrate inside the lithic substrate and acts as a strengthening agent with protective effect against environmental loading. This one-pot synthesis allows an easy and cost-effective industrial scaling up.
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and nanolimes, have recently been developed.12−14 Calcium alkoxides, along with nanolimes, can easily be hydrolyzed through the influence of atmospheric moisture and carbon dioxide, leading to the formation of calcium carbonates. Although these promising nanomaterials are very effective, they are also very dependent on the atmospheric conditions, and this may make them less effective for treating building materials located in areas with low humidity and high temperatures. Accepting that inorganic consolidants confer compatibility to the treated porous inorganic materials and resistance to weathering, we proposed in a previous work a new inorganic nanocomposite based on silica and calcium oxalate.15 The synthesis proposed in that work included a synthetic route in two steps, to obtain the hybrid nanocomposite silica−nano calcium oxalate. TEOS was selected because of its low viscosity, nontoxicity, cost-effectiveness, and ability to act as the dispersing medium to produce the nanosized structure of silica. Calcium oxalate is the main component found in wellpreserved surface layers on monuments, the so-called “patinas”,16,17 and it has been proposed as protective treatment for building stones.18−21 Furthermore, calcium carbonates, calcium oxalate, and amorphous silica are the most common components identified in biomineralization.22 As an advancement over the previously described synthesis route,15 in the present work the one-pot synthesis of the calcium oxalate−silica nanocomposite, using calcium hydroxide (CH), oxalic acid dihydrate (Ox), and TEOS in isopropanol (ISP) as precursors, is proposed. Nanocalcium oxalate (nanoCaOx) is formed simultaneously with the polymerization of
INTRODUCTION Alkoxysilanes, and specifically tetraethoxysilane (TEOS), are the most widely used stone consolidants and form the basis of most existing commercial strengthening agents for protecting porous building materials against deterioration. These products can provide consolidation and water repellency simultaneously and are safe, controllable, and easy to apply.1 They are polymerized inside the pores of stone materials under the influence of atmospheric moisture, forming a silica matrix that stabilizes their structure. Despite their numerous advantages, alkoxysilanes present some practical drawbacks, the three most important being their inefficient bonding to calcite, their tendency to crack during the shrinkage and drying process, and biodeterioration.2−7 More specifically, calcareous substrates contain few hydroxyl groups, and the result of this is limited bonding with alkoxysilanes.5,6 The cracking that frequently occurs is generated by the high capillary pressure supported by the uneven pore size of the gel network during drying. The third major drawback of alkoxysilanes is related to the presence of synthetic organic compounds in their formulation; in some cases these compounds are not only degraded by microorganisms, but further damage to the building material is also promoted by the creation of an organic substrate that favors fungal growth.7 Several attempts to minimize the disadvantages of alkoxysilanes have been reported, including the addition of metal salts, metal colloidal oxides and alkoxides, and nonionic surfactants, as well as functional (3-glycidoxypropyl) trimethoxysilane (GPTMS) and polyhedral oligomeric silsesquioxane (POSS).8−10 Although the use of modified-TEOS prevents the gel from cracking during drying by reducing the capillary pressure and surface tension, the bonding of the silica matrix to calcareous substrates remains insufficient.1,11 According to the literature, alternative compounds to conventional consolidation materials, such as calcium alkoxides © 2015 American Chemical Society
Received: Revised: Accepted: Published: 7195
January 19, 2015 June 8, 2015 June 20, 2015 June 21, 2015 DOI: 10.1021/acs.iecr.5b00247 Ind. Eng. Chem. Res. 2015, 54, 7195−7206
Article
Industrial & Engineering Chemistry Research TEOS. CH, Ox, and TEOS are nontoxic and low-cost reagents, while Ox aids the sol−gel process by acting as acid catalyst for TEOS hydrolysis. Furthermore, the derived nano-CaOx incorporated into the silica matrix functions as a mechanism that enables an electrostatic interaction between the nanocomposite and the carbonaceous substrate. This feature overcomes the lack of affinity between TEOS-based consolidants and carbonaceous stones. Given that neither byproducts nor toxic agents are involved in the proposed synthesis, a simple, cost-effective, one-pot synthesis of the final product is achieved, allowing an easy and cost-effective industrial scalingup. The colloidal solution derived from this one-pot synthesis can penetrate deep into the lithic substrate where it acts as a strengthening agent with a protective effect, but without occluding the whole pore since it has been selectively deposited on the boundaries of the pore.
Scheme 1. One-Pot Synthesis of the TCO Nanocomposite
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EXPERIMENTAL SECTION Materials. TEOS (>99%) and isopropanol (ISP, puriss. p.a.) were purchased from Sigma-Aldrich, calcium hydroxide (CH) from Fluka, and oxalic acid dihydrate (Ox, > 99%) from Panreac. All chemicals were used as received, without further purification prior to use. Synthesis. A solution, designated A, of TEOS and ISP, with a TEOS-to-ISP molar ratio of 1/3, was prepared and left in a closed vessel for 120 min. A solution of CH in a mixture of ISP and H2O, designated B, was prepared under vigorous magnetic stirring for 120 min and ultrasonic agitation for 5 min. Next, solutions A and B were mixed to obtain the dispersion C, which was left under continuous stirring for 1200 min at room temperature. A solution of Ox in ISP was slowly added to C with the aim of yielding CaOx nanoparticles (solution D). CH was selected because it would act as both a potential reagent with Ox and a basic catalyst in the hydrolysis of TEOS. The final sol D, named TCO-s (from the initials of the raw materials used), was ultrasonically agitated for 60 min and then left under continuous stirring at room temperature for at least another 2880 min. The synthesis route is illustrated in Scheme 1. The molar ratio of the TCO-s sol was 1/12/4/0.1/0.1 TEOS/ISP/ H2O/CH/Ox. For TCO-s, the quantities of the reagents selected were based on individual experiments aimed at discerning the most appropriate formulation of a colloidal solution with a viscosity sufficiently low to enable the impregnation of stone and cement mortars. The rate of the hydrolysis and condensation of TEOS at various experimental stages, as well as the formation of CaOx nanoparticles, was monitored by FTIR, as described in detail in the Supporting Information. Additionally, in order to gain insights into the role of the nano-CaOx in the xerogel network (TCO-x) and the mechanism of reaction, a reference sol, defined as TH-s, was also synthesized; this contains TEOS, ISP, H2O, and Ox in a molar ratio TEOS/ISP/H2O/Ox equal to 1/12/4/0.026. Ox was deliberately selected in order to facilitate the TEOS hydrolysis, to reduce the gel time of the TH xerogel (TH-x), and, most importantly, to enable better comparisons between TCO-x and TH-x, thus emphasizing the role played by the CaOx formation in the designed xerogel. For TH-s the Ox quantity was regulated to a pH = 4, equal to that measured in the TCO-s. The TCO-s and TH-s sols “as prepared” were cast into transparent molds with dimensions of 2.5 cm diameter and 5 cm height. The molds were covered on top by a perforated
parafilm, in order to allow very gradual evaporation of the solvent. The cast sols were exposed to laboratory conditions (RH = 60 ± 5%, T = 20 ± 2 °C) until a constant weight was achieved. This procedure can be considered comparable to the application of sol in the lithic substrates and the phenomena that are related to the curing of sol inside the lithic matrix. Gel times were determined by a visual inspection of the gel transition inside the transparent molds. After completion of the polymerization and drying processes, a crack-free monolithic TCO-x and a crack-susceptible TH-x were obtained in a period of approximately 18 and 4 days, respectively. The properties of the sols and xerogels under study are listed in Table 1. The volume reduction was calculated by measuring the volume of the derived monolithic TCO xerogel and comparing it with the initial volume of the sol cast in the mold. Application Procedure on Building Materials. With the aim of evaluating the consolidation performance of the synthesized product TCO-s in a laboratory-scale case study and to compare its effectiveness with the reference sol TH-s, both TCO-s and TH-s were applied to three different types of building material: (a) a porous biomicritic limestone, called Alfas, with a porosity ranging from 25 to 35% and pore size distribution from 1 to 10 μm; (b) a porous limestone, designated PRC, with a porosity of approximately 35% and pore size ranging from 10 to 100 μm; and (c) cement mortars, designated CEM, with a porosity of approximately 20% and pore size distribution from 0.7 to 2 μm. Alfas is composed of micritic calcite, while the composition of PRC is 95% calcite, 4% dolomite, 0.6% quartz, and 0.4% goethite. Cement mortars with constant workability (165 ± 5 mm) were produced in the laboratory with a binder/aggregate ratio 1:3; the aggregate fraction consisted entirely of local 0−3 mm standard sand of carbonaceous nature. The choice of these materials was based on the need, first, to consolidate calcareous substrates, where the TEOS-based consolidants appear incompatible, and second, to strengthen and protect widely used modern building materials, such as cement. 7196
DOI: 10.1021/acs.iecr.5b00247 Ind. Eng. Chem. Res. 2015, 54, 7195−7206
Article
Industrial & Engineering Chemistry Research Table 1. Properties of the Sols under Study sample reference
reagents
viscosity (mPa·s)
volume reduction (%)
gelation time (days)
sol appearance
xerogel state
TCO-s TH-s
TEOS/ISP/H2O/CH/Ox 1/12/4/0.1/0.1 TEOS/ISP/H2O/Ox 1/12/4/0.026
3.35 2.62
71 -
18 4
white transparent
monolithic cracking
For this study, 12 cylindrical test specimens from each type of the above-described building materials were prepared. Half of these specimens were shaped with dimensions of 5 cm × 2.7 cm (diameter × height), while the rest of them had a 5 cm diameter and 5 cm height. Before treatment, all the specimens were rinsed with deionized water, dried in an oven at 80 °C for 3 days, and then cooled to ambient temperature in a desiccator. The application procedure consists of brushing the sols on the surface of the stone and cement samples. The treatment was considered completed when the surface remained wet for 1 min. The quantity of the product that was applied on the specimens studied was the minimum possible, in order to avoid the creation of a layer oversaturated in product on the stone surface. After treatment, all the test specimens were allowed to dry in laboratory conditions (RH = 60 ± 5%, T = 20 ± 2 °C), until they reached a constant weight. This was achieved within 30 to 45 days of drying; then, the amount of the deposited material in the stone specimens in terms of g/cm2, denoted as dry matter, was calculated. Test Methods. Nanocomposite. The synthesized nanocomposites TCO-s and the TH-s were assessed with the analytical techniques described below. Initially, the viscosity of the sols was measured with a Brookfield DV-II + Pro spindle: S18 viscometer. A Leica M125 optical microscope was used to study the surface morphology of the TCO-x. Vickers hardness of the TCO-x and TH-x nanocomposites was estimated as the average value of the four measurements under loading of 300 g with dwell time 5 s by means of a microhardness tester FutureTech FM-700. Fourier transform infrared spectroscopy (FTIR) absorption spectra were recorded on a PerkinElmer 1000 spectrometer, in the spectral range from 4000 to 400 cm−1, to investigate the evolution of the sol−gel process, the chemical bonds in the TH-x and TCO-x xerogels, and the stability of the TCO-x after aging in humidity and UV. Details of the FTIR analysis employed and spectra of the precursor materials used as references for comparison purposes can be found in the Supporting Information. More specifically, FTIR was used to assess the chemical stability and durability of the TCO-x cured in laboratory conditions for 1 yr, after aging in a UV chamber equipped with four 8 W black light blue lamps working at 365 nm (240 h of UV irradiation) and in a chamber with relative humidity RH = 90 ± 2% and temperature T = 20 ± 2 °C for 240 h. X-ray diffraction analysis (XRD) was performed in a Bruker D8 Advance Diffractometer, using Ni-filtered Cu Kα radiation (35 kV 35 mA) and a Bruker Lynx Eye strip silicon detector to identify crystal phases in the TCO-x nanocomposite and to compare the xerogels under study. The diffractograms are shown in the Supporting Information. The thermal behavior of the above-described xerogels, together with commercial calcium oxalate monohydrate (COM) as reference material, was determined by thermogravimetric analysis (DTA-TG) using a Setaram LabSys Evo 1600 °C at a heating rate of 10 °C/min under nitrogen atmosphere from 27 to 1000 °C. From the comparison of TG and DTA curves, significant observations were recorded in respect of the
weight loss, structural changes, and endothermal and exothermal effects. Specific surface area, pore volume, and pore size distribution were determined by isothermal nitrogen adsorption−desorption at 77 K in an Autosorb-1 Quantachrome automatic device. The TCO-x and TH-x xerogels were degassed under high vacuum (
