1. Introduction
⌅Owing
to its versatility in terms of modification, application and shape
formation, reinforced concrete is the most commonly used construction
material worldwide. Its mechanical properties and durability offer a
low-cost material to use for building structures in diverse
environments. However, during its fabrication, cement, the principal
component of reinforced concrete, generates a considerable quantity of
carbon dioxide, a greenhouse gas that affects the environment (1-41.
Andersson R, Stripple H, Gustafsson T, Ljungkrantz C. 2019. Carbonation
as a method to improve climate performance for cement based material.
Cem. Concr. Res. Pergamon.124:105819. https://doi.org/10.1016/j.cemconres.2019.105819
2.
Borges PC. 2010. Impacto del cambio climático global en la durabilidad y
el desarrollo sustentable de las construcciones de concreto ¿Qué es el
Calentamiento Global? Causantes del Calentamiento Global. pp. 69.
3.
Peñaloza D, Erlandsson M, Berlin J, Wålinder M, Falk A. 2018. Future
scenarios for climate mitigation of new construction in Sweden: Effects
of different technological pathways. J. Clean. Prod. 187:1025-1035. https://doi.org/10.1016/j.jclepro.2018.03.285
4.
Talukdar S, Banthia N. 2013. Carbonation in concrete infrastructure in
the context of global climate change: Development of a service lifespan
model. Constr. Build. Mater. 40:775-782. https://doi.org/10.1016/j.conbuildmat.2012.11.026
).
Another concern in the use of concrete is that owing to its porous
nature, it is prone to absorbing chloride and sulfate ions (5-85. Du H, Du S, Liu X. 2014. Durability performances of concrete with nano-silica. Constr. Build. Mater. 73:705-712. https://doi.org/10.1016/j.conbuildmat.2014.10.014
6.
Zahedi M, Ramezanianpour AA, Ramezanianpour AM. 2015. Evaluation of the
mechanical properties and durability of cement mortars containing
nanosilica and rice husk ash under chloride ion penetration. Constr.
Build. Mater. 78:354-361. https://doi.org/10.1016/j.conbuildmat.2015.01.045
7.
Behfarnia K, Salemi N. 2013. The effects of nano-silica and
nano-alumina on frost resistance of normal concrete. Constr. Build.
Mater. 48:580-584. https://doi.org/10.1016/j.conbuildmat.2013.07.088
8.
Tobón JI, Payá J, Restrepo OJ. 2015. Study of durability of Portland
cement mortars blended with silica nanoparticles. Constr. Build. Mater.
80:92-97. https://doi.org/10.1016/j.conbuildmat.2014.12.074
), CO2 ingress that results in carbonation phenomena, (99.
Isfahani FT, Redaelli E, Lollini F, Li W, Bertolini L. 2016. Effects of
nanosilica on compressive strength and durability properties of
concrete with different water to binder ratios. Adv. Mater. Sci. Eng.
2016(1):8453567. https://doi.org/10.1155/2016/8453567
, 1010. Lim S, Mondal P. 2015. Effects of incorporating nanosilica on carbonation of cement paste. J Mater Sci. 50:3531-3540. https://doi.org/10.1007/s10853-015-8910-7
) and other concerns that directly affect the durability of concrete (1111.
Wang D, Wu L, Shi C, Xiang S, Wu Z, Pan X. 2016. Effects of
nanomaterials on hardening of cement-silica fume-fly ash-based
ultra-high-strength concrete. Adv. Cem. Res. 28(9):555-566. https://doi.org/10.1680/jadcr.15.00080
, 1212.
Tobón JI, Payá J, Restrepo OJ. 2015. Study of durability of Portland
cement mortars blended with silica nanoparticles. Constr. Build. Mater.
80:92-97. https://doi.org/10.1016/j.conbuildmat.2014.12.074
).
Consequently,
improvements in the durability of concrete have attracted the attention
of many recent investigations. As a result, sustainable development and
the possibility of using alternative construction materials, which
reduce the use of conventional cement, generate a need for further
research in the field of supplementary cementitious materials (SCMs),
which will not only reduce costs but also enhance and modulate concrete
properties and resistance according to their application needs (1313.
Banthia N, Zanotti C, Sappakittipakorn M. 2014. Sustainable fiber
reinforced concrete for repair applications. Constr. Build. Mater.
67(PART C):405-412. https://doi.org/10.1016/j.conbuildmat.2013.12.073
).
To improve the durability of concrete, waste materials can be used as reinforcement (14-1614.
Mohajerani A, Suter D, Jeffrey-Bailey T, Song T, Arulrajah A,
Horpibulsuk S, et al. 2019. Recycling waste materials in geopolymer
concrete. Clean Technol. Environ. Policy. 21:493-515. https://doi.org/10.1007/s10098-018-01660-2
15.
Kishore K, Gupta N. 2020. Application of domestic & industrial
waste materials in concrete: A review. Mater. Today Proc.
26(2):2926-2931. https://doi.org/10.1016/j.matpr.2020.02.604
16.
Sandanayake M, Bouras Y, Haigh R, Vrcelj Z. 2020. Sustainable current
trends of using waste materials in concrete-a decade review.
Sustainability. 12(22):9622. https://doi.org/10.3390/su12229622
).
For example, the use of plant fibers to reinforce the cement matrix is a
strategy that has been applied to improve cement properties (17-2017.
Zhang T, Yin Y, Gong Y, Wang L. 2020. Mechanical properties of jute
fiber-reinforced high-strength concrete. Struct. Concr. 21(2):703-712. https://doi.org/10.1002/suco.201900012
18.
Farooqi MU, Ali M. 2018. Contribution of plant fibers in improving the
behavior and capacity of reinforced concrete for structural
applications. Constr. Build. Mater. 182:94-107. https://doi.org/10.1016/j.conbuildmat.2018.06.041
19.
Bittner CM, Oettel V. 2022. Fiber reinforced concrete with natural
plant fibers-investigations on the application of bamboo fibers in
ultra-high performance concrete. Sustainability. 14(19):12011. https://doi.org/10.3390/su141912011
20.
Zhao K, Xue S, Zhang P, Tian Y, Li P. 2019. Application of natural
plant fibers in cement-based composites and the influence on mechanical
properties and mass transport. Materials. 12(21):3498. https://doi.org/10.3390/ma12213498
).
The main advantages of using vegetable fibers to reinforce concrete are
that they are inexpensive, have suitable mechanical properties to
reinforce fragile materials, and, their degradation products have the
ability to seal pores in the studied concrete samples (21-2321.
Ardanuy M, Claramunt J, Toledo Filho RD. 2015. Cellulosic fiber
reinforced cement-based composites: A review of recent research. Constr.
Build. Mater. 79:115-128. https://doi.org/10.1016/j.conbuildmat.2015.01.035
22.
Onuaguluchi O, Banthia N. 2016. Plant-based natural fibre reinforced
cement composites: A review. Cem. Concr. Compos. 68:96-108. https://doi.org/10.1016/j.cemconcomp.2016.02.014
23.
Talavera-pech WA, Montiel-rodr D, Paat-estrella JDLA, Ruth L, Jos TP,
Tezozomoc P. 2021. Improvement in the carbonation resistance of
construction mortar with cane bagasse fiber added. Materiales.
14(8):2066. https://doi.org/10.3390/ma14082066
).
The use of sugarcane bagasse fibers to reinforce concrete offers
advantages in terms of its final properties, as an increase in the
resistance to impact and toughness in a hardened state was previously
reported, in addition to the ability to control plastic shrinkage while
setting in the fresh state (15-1715.
Kishore K, Gupta N. 2020. Application of domestic & industrial
waste materials in concrete: A review. Mater. Today Proc.
26(2):2926-2931. https://doi.org/10.1016/j.matpr.2020.02.604
16.
Sandanayake M, Bouras Y, Haigh R, Vrcelj Z. 2020. Sustainable current
trends of using waste materials in concrete-a decade review.
Sustainability. 12(22):9622. https://doi.org/10.3390/su12229622
17.
Zhang T, Yin Y, Gong Y, Wang L. 2020. Mechanical properties of jute
fiber-reinforced high-strength concrete. Struct. Concr. 21(2):703-712. https://doi.org/10.1002/suco.201900012
).
Waste glass could be used as a filler for ordinary Portland cement or as a main component of concrete (2424.
Afshinnia K, Rangaraju PR. 2016. Impact of combined use of ground glass
powder and crushed glass aggregate on selected properties of Portland
cement concrete. Constr. Build. Mater. 117:263-272. https://doi.org/10.1016/j.conbuildmat.2016.04.072
, 2525.
Jiang M, Chen X, Rajabipour F, Hendrickson CT. 2014. Comparative life
cycle assessment of conventional, glass powder, and alkali-activated
slag concrete and mortar. J. Infrastruct. Syst. 20(4):04014020. https://doi.org/10.1061/(asce)is.1943-555x.0000211
). The use of this waste material is important from the perspectives of both the environment and the circular economy (2626.
Nodehi M, Mohamad Taghvaee V. 2022. Sustainable concrete for circular
economy: a review on use of waste glass. Glass Struct. Eng. 7:3-22. https://doi.org/10.1007/s40940-021-00155-9
, 2727. Bellopede R, Zichella L, Marini P. 2020. Glass waste3: a preliminary study for a new industrial recovery processing. Sustainability.12(5):1997. https://doi.org/10.3390/su12051997
)
because the production of glass waste is approximately 100 million tons
annually, of which only 26% is recycled, producing 16.9 M.J. (million
Joule) of waste heat and 0.57 kg of CO2 for 1 kg of glass (2828. Schmitz A, Kamiński J, Maria Scalet B, Soria A. 2011. Energy consumption and CO2 emissions of the European glass industry. Energy Policy. 39(1):142-155. https://doi.org/10.1016/j.enpol.2010.09.022
).
The traditional application of waste glass in construction materials
involves grinding or crushing the glass to obtain particles that range
from 100 μm to approximately 5 mm (2929.
Gorospe K, Booya E, Ghaednia H, Das S. 2019. Effect of various glass
aggregates on the shrinkage and expansion of cement mortar. Constr.
Build. Mater. 210:301-311. https://doi.org/10.1016/j.conbuildmat.2019.03.192
);
however, it was previously demonstrated that nanostructured silica
materials could be used as SCMs to improve the durability of concrete (30-3330.
Liu C, Su X, Wu Y, Zheng Z, Yang B, Luo Y, Yang J, Yang J. 2021. Effect
of nano-silica as cementitious materials-reducing admixtures on the
workability, mechanical properties and durability of concrete.
Nanotechnol Rev. 10(1):1395-1409. https://doi.org/10.1515/ntrev-2021-0097
31.
Jiang W, Li X, Lv Y, Jiang D, Liu Z, He C. 2020. Mechanical and
hydration properties of low clinker cement containing high volume
superfine blast furnace slag and nano silica. Constr. Build. Mater.
238:117683. https://doi.org/10.1016/j.conbuildmat.2019.117683
32.
Liu H, Yu Y, Liu H, Jin J, Liu S. 2018. Hybrid effects of nano-silica
and graphene oxide on mechanical properties and hydration products of
oil well cement. Constr. Build. Mater. 191:311-319. https://doi.org/10.1016/j.conbuildmat.2018.10.029
33.
Mendoza Reales OA, Duda P, Silva ECCM, Paiva MDM, Filho RDT. 2019.
Nanosilica particles as structural buildup agents for 3D printing with
Portland cement pastes. Constr. Build. Mater. 219:91-100. https://doi.org/10.1016/j.conbuildmat.2019.05.174
),
and it is also possible to synthesize hexagonal MCM-41 mesoporous
silica nanoparticles using precursor products obtained from waste glass (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
).
Most
of the research on nanotechnologies applied to construction has focused
on two main topics: 1) the development of nanoparticle materials with
greater strength than steel and 2) the elaboration of ultrahigh-strength
concrete through the modification of its cementitious matrix structure,
resulting in changes in the fracture mechanism. Currently, silica
nanoparticles have been demonstrated to be an SCM that improves many
durability properties of concrete (1212.
Tobón JI, Payá J, Restrepo OJ. 2015. Study of durability of Portland
cement mortars blended with silica nanoparticles. Constr. Build. Mater.
80:92-97. https://doi.org/10.1016/j.conbuildmat.2014.12.074
, 35-3935.
Ganesh P, Murthy AR, Kumar SS, Reheman MMS, Iyer NR. 2016. Effect of
nanosilica on durability and mechanical properties of high-strength
concrete. Magaz. Concr. Res. 68(5):229-236. https://doi.org/10.1680/jmacr.14.00338
36.
Fallah S, Nematzadeh M. 2017. Mechanical properties and durability of
high-strength concrete containing macro-polymeric and polypropylene
fibers with nano-silica and silica fume. Constr. Build. Mater.
132:170-187. https://doi.org/10.1016/j.conbuildmat.2016.11.100
37. Rong Z, Sun W, Xiao H, Jiang G. 2015. Effects of nano-SiO2 particles on the mechanical and microstructural properties of
ultra-high performance cementitious composites. Cem. Concr. Compos.
56:25-31. https://doi.org/10.1016/j.cemconcomp.2014.11.001
38.
Rao S, Silva P, De Brito J. 2015. Experimental study of the mechanical
properties and durability of self-compacting mortars with nano materials
(SiO2 and TiO2). Constr. Build. Mater. 96:508-517. https://doi.org/10.1016/j.conbuildmat.2015.08.049
39.
Tobón JI, Payá J, Restrepo OJ. 2015. Study of durability of Portland
cement mortars blended with silica nanoparticles. Constr. Build. Mater.
80:92-97. https://doi.org/10.1016/j.conbuildmat.2014.12.074
).
Some examples of the use of these nanomaterials to improve concrete
include the development of self-cleaning surfaces, photocatalytic
coatings that give rise to surfaces with bactericidal and self-cleaning
properties, concrete with greater resistance and enhanced
physical‒mechanical properties through the application of nanofibers or
nanoparticles, and coatings based on nanoparticle incrustations to
obtain several functional surfaces (40-4240.
Cruz-Moreno D, Fajardo G, Flores-Vivian I, Orozco-Cruz R, Ramos-Rivera
C. 2020. Multifunctional surfaces of Portland cement-based materials
developed with functionalized silicon-based nanoparticles. Appl. Surf.
Sci. 531:147355. https://doi.org/10.1016/j.apsusc.2020.147355
41.
Pan X, Shi Z, Shi C, Ling TC, Li N. 2017. A review on concrete surface
treatment Part I: Types and mechanisms. Constr. Build. Mater.
132:578-590. https://doi.org/10.1016/j.conbuildmat.2016.12.025
42.
Pan X, Shi Z, Shi C, Ling TC, Li N. 2017. A review on surface treatment
for concrete - Part 2: Performance. Constr. Build. Mater. 133:81-90. https://doi.org/10.1016/j.conbuildmat.2016.11.128
).
Previously, our group explored the reinforcement of concrete with sugarcane bagasse fiber (SBF) (2323.
Talavera-pech WA, Montiel-rodr D, Paat-estrella JDLA, Ruth L, Jos TP,
Tezozomoc P. 2021. Improvement in the carbonation resistance of
construction mortar with cane bagasse fiber added. Materiales.
14(8):2066. https://doi.org/10.3390/ma14082066
).
Another group synthesized MCM-41-type mesoporous silica nanoparticles
(MSNs) via sodium silicate extracted from glass waste and obtained the
same properties as MCM-41 synthesized from commercial
tetraethylortosilicate (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
).
This study focused on evaluating the effects of the addition of
sugarcane bagasse fibers, MCM-41-MSN nanoparticles synthesized from
recycled glass, or a combination of both materials, i.e., fibers and
nanoparticles as SCMs, on the physicomechanical and electrochemical
properties of reinforced concrete structures.
2. Materials and methods
⌅2.1. Synthesis of MCM-41 silica nanoparticles from recycled glass (NP-MCM-41)
⌅The synthesis of NP-MCM-41 followed a previously reported method with some variations (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
).
For this purpose, NP-MCM-41 was synthesized from a sodium silicate
solution previously obtained by mixing waste glass-NaOH at a molar ratio
of 1:3, taking the glass as pure SiO2, and the mixture was heated for 1 h at 500 °C in a muffle furnace. Then, the sodium silicate was extracted with H2O,
and the solution was kept for further steps. For the synthesis, the
silica nanoparticles were placed in a balloon flask;
hexadecyltrimethylammonium bromide (CTAB) (200 mg), distilled water (96
mL) and 2M NaOH (0.7 mL) were stirred at 80 °C. Then, 20 mL of the
sodium silicate solution was added, the pH was adjusted to 11, the
mixture was maintained at that temperature for 2 h, and then the mixture
was vacuum filtered and dried in an oven. Finally, the cationic
surfactant was eliminated through acidic ethanol extraction at reflux
for 24 h.
2.2. Characterization of NP-MCM-41
⌅The
surfactant extraction method used in this study was chemical extraction
instead of the previously utilized calcination method (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
),
which can cause differences in the pore structure of the materials, so
the characterization was focused on the textural properties that could
cause this change. Tests for surface area analysis employing nitrogen
physisorption and the Brunauer-Emmett-Teller (BET) method (4343. Brunauer S, Emmett PH, Teller E. 1938. Adsorption of gases in multimolecular layers. J. Am. Chem. Soc. 60(2):309-319. https://doi.org/10.1021/ja01269a023
)
were performed with Micromeritics TriStar II model equipment, where a
0.2 g sample was placed in a quartz cell, degassed at 300 °C for one
hour and placed under vacuum in a nitrogen bath. The analysis was
performed in a relative pressure (RP) range (x = P/P0) of 0.01 to 1.0.
The variations in the relative pressure against the volume of the pore
to determine the isotherm were measured via BEL Japan software.
Transmission electron microscopy (TEM) images were obtained using a JEOL
2100 microscope operated at an accelerating voltage of 200 kV and
equipped with a LaB6 filament.
2.3. Sugarcane bagasse fibers (SBFs)
⌅The
collection, characterization, and preparation of samples of sugarcane
bagasse fibers were carried out according to procedures described in a
previous work from our group (4444.
Talavera-Pech WA, Montiel-Rodríguez D, Paat-Estrella JA,
López-Alcántara R, Pérez-Quiroz JT, Pérez-López T. 2021. Improvement in
the carbonation resistance of construction mortar with cane bagasse
fiber added. Materials. 14(8):2066. https://doi.org/10.3390/ma14082066
).
2.4. Preparation and tests of the concrete samples
⌅Portland
cement composite used as a binder was CPC 30 R, Maya trademark (Mérida,
Mexico), and the cement properties meet the specifications of the
NMX-C-414-ONNCCE-2017 and ASTM-C-150-2007 standards. The aggregates used
are characteristic of the region and are a gravel product produced by
crushing limestone rock typical of the Yucatan Peninsula and mixing with
sea sand obtained from a materials quarry in Campeche, México; both
materials were washed before their use. Tap water was mixed according to
the NMX-C-122-ONNCCE-2006 standard. Additionally, a fluidizing additive
of concrete or mortar, SikaCem fluidifier, which increases tempering or
decreases water consumption by approximately 10%, was used (4545. PROCONSA ® GDC& B/ P. Hoja técnica_ Producto: Dispercon ‘AL-100’. pp. 1.
).
All the concrete samples were combined with a water/cement ratio (w/c)
of 0.56, and for the different analyses, they were cast into
conventional molds measuring 7.5 cm in diameter and 15 cm in height with
and without reinforcing steel for electrochemical and compressive
strength tests; additionally, cylinders of 7.5 cm in diameter and 10 cm
in height were cast for the carbonation test, and beams with square
sections of 5 cm and 20 cm in length were used for flexural strength
testing. After being cast, the samples remained in the laboratory for 24
hours before they were cured. All the materials used as concrete
components were subjected to chemical characterization via X-ray
photoelectron spectroscopy (XPS), for this, fine aggregate (AF), coarse
aggregate, and composite Portland cement CPC30 were analyzed with an
X-ray Photoelectron Spectrometer (XPS, K-Alpha, Thermo Scientific,
Waltham, MA, USA) equipped with a monochromatic Al K-alpha radiation
source (1486.6 eV). The pass energy was 100 eV, and the energy step size
was 1 eV for XPS full spectra (Figure 3d), while for XPS high-definition spectra (Figure 4),
they were 50 eV and 0.1 eV, respectively. The XPS spectra were obtained
after sputtering the specimens’ surface for removing environmental
contamination with a scanning argon-ion gun during 15 s. The spectra
were calibrated by setting the main line for the C 1s signal of carbon
at 284.8 eV; Also, was conducted crystallographic characterization via
X-ray diffraction (XRD), the powder X-ray diffraction analysis were
conducted for all concrete components using a BRUKER D8 Advance X-ray
diffractometer with CuKα radiation (λ=1.5418 Å) with a step size of 0.02
° and a step time of 3 s, from 10 to 70 ° of 2θ.
The samples obtained using the different SCMs were as follows: control samples (C), samples with the addition of sugarcane bagasse fibers at a quantity of 1% (1F), samples with the addition of 0.8% NP-MCM-41 (0.8N), and samples with the addition of both 1% SBF and 0.8% NP-MCM-41 (0.8N-1F). All percentages were with respect to the mass of cement used to compare their characteristics and properties.
2.5. Characterization of the concrete samples
⌅2.5.1. Determination of the volume of permeable voids
⌅The porosity of concrete is a controlling factor that influences the diffusion of pollutants such as various ions and CO2. This factor directly influences concrete durability. The percentage of the volume of permeable voids in the concrete samples was determined according to the ASTM C-642-2006 standard. The weight of each sample was measured with a balance (Precise model XB 2200C, Precise, Dietikon, Switzerland) with 0.01 g of sensitivity. The samples were dried at 100 °C in a laboratory stove (RIOSSA Monterrey, Mexico). Tap water was used for all wet steps.
2.5.2. Compressive strength tests
⌅The quality of concrete (both fresh and hardened) depends on its properties. The most important property of concrete is the compressive stress strength. Therefore, the compressive strength of cylindrical samples obtained with different SCMs was determined at 7, 14, and 28 days. This value was calculated from the breaking load divided by the area of the section that resists the load and is reported in units of kg/cm2 or MPa as established by the Mexican standard NMX-C-083-ONNCCE-2014 and the international standard ASTM C-39-2018.
2.5.3. Bending strength tests
⌅This analysis was performed using samples with a square section of 5 cm × 20 cm length with the reinforcement materials described previously. The test was conducted by applying the NMX-C-191-ONNCCE-2015 standard.
2.6 Chemical deterioration of concrete by accelerated carbonation
⌅To
study the carbonation protection imparted by the SCMs added to the
concrete samples, the carbonation process was accelerated. For this
purpose, the 28-day-cured samples were placed in an adapted carbonation
chamber with the capacity to maintain the experimental conditions at 25 ±
2 °C, 65 ± 5% relative humidity, and a constant concentration of 3% CO2 inside the chamber equipped with a commercial CO2 tank (25 kg) with a flowmeter installed; this environment has a higher concentration of CO2 than a normal ambient environment (2323.
Talavera-pech WA, Montiel-rodr D, Paat-estrella JDLA, Ruth L, Jos TP,
Tezozomoc P. 2021. Improvement in the carbonation resistance of
construction mortar with cane bagasse fiber added. Materiales.
14(8):2066. https://doi.org/10.3390/ma14082066
).
The stable carbonation process was controlled for 8 hours a day. The
progress of carbonation was determined via the wet method. The procedure
consisted of carefully making a cross-section of the samples
approximately 2.5 cm thick, and immediately after the samples were cut
and the surface was cleaned, each of the acid-base indicators,
phenolphthalein, and thymolphthalein-were applied uniformly, using an
atomizer. After the application of the indicators, the surface of the
samples that did not present coloration was measured with a Vernier
measurement instrument.
2.7. Electrochemical tests
⌅2.7.1. Corrosion potential measurement (Ecorr)
⌅The measurement of potential is an indicator of the probability that the corrosion phenomenon is occurring. The corrosion potential was measured following ASTM C 876 using a copper/copper sulfate reference electrode (Cu/CuSO4) connected to a voltmeter.
3. Results and discussion
⌅3.1. Characterization of NP-MCM-41 from sodium silicate extracted from recycled glass
⌅3.1.1. N2 physisorption
⌅From
the surface area analysis using a nitrogen physisorption test via the
BET (Brunauer-Emmett-Teller) method, a specific surface area of 840.142 m2/g, with an average pore diameter of 1.628 nm and a pore volume of 0.629 cm3/g, was obtained. Importantly, the only change in the synthesis in our previous work (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
)
was the surfactant extraction method, and with this new method, the
surface area was overtaken by approximately 120%, which can indicate
better surfactant extraction, resulting in a large surface area inside
the pore structure. On the basis of the classification presented in
previous studies, the obtained structural parameters confirm that the
synthesized samples have a mesoporous structure (4646.
Rosenholm JM, Sahlgren C, Lindén M. 2010. Towards multifunctional,
targeted drug delivery systems using mesoporous silica nanoparticles -
Opportunities & challenges. Nanoscale. 2(10):1870-1883. Retrieved
from https://pubs.rsc.org/en/content/articlelanding/2010/nr/c0nr00156b
). Figure 1 shows the isotherms of N2 adsorption/desorption for the synthesized samples, resulting in type IV
isotherms, which, according to the IUPAC classification, correspond to
mesoporous materials (4747.
Kinashi K, Kambe Y, Misaki M, Koshiba Y, Ishida K, Ueda Y. 2012.
Synthesis, characterization, photo-induced alignment, and surface
orientation of poly(9,9-dioctylfluorene-alt-azobenzene)s. J. Polym. Sci.
A. Polym. Chem. 50(24):5107-5114. https://doi.org/10.1002/pola.26338
), with the tipping point at approximately P/P0 = 0.22. This isotherm has three well-defined ranges: at a low RP, a
gradual increase caused by adsorption, in mono- and multilayer forms, on
the pore walls is observed; then, a pronounced rise at medium P/P0 values is observed, associated with capillary condensation; finally, at high values of P/P0, a gradual linear increase caused by adsorption on the outer surface of the samples is observed (4848.
Serrano MR, Saracho AMPG, Acosta DE, Bonini NA, Gonzo EE, Parentis ML.
2018. Síntesis de sílice mesoestructurada: determinación de las
condiciones óptimas de extracción del templato. Matéria (Rio de
Janeiro). Laboratório de Hidrogênio, Coppe - Universidade Federal do Rio
de Janeiro, em cooperação com a Associação Brasileira do Hidrogênio,
ABH2. 23(2). https://doi.org/10.1590/s1517-707620180002.0465
, 4949.
Majchrzak-Kucȩba I, Nowak W. 2011. Characterization of MCM-41
mesoporous materials derived from polish fly ashes. Int. J. Miner.
Process. 101(1-4):100-111. https://doi.org/10.1016/j.minpro.2011.09.002
).
3.1.2 TEM
⌅The
TEM images obtained confirmed the MCM-41 ordering of the pore
structure, which was discussed previously as the [100], [110], and [200]
crystallographic planes in low-angle XRD diffractograms; these
correspond to the hexagonal order of the pores and confirms the
interplanar space and the cell parameters (3434.
Ramírez-Arévalo MS, Pérez-López T, Quintana-Owen P, Fajardo-San Miguel
GJ, Talavera-Pech WA. 2022. Comparative study of physicochemical
properties of MCM-41 silica nanoparticles obtained from recycled glass
and TEOS. Silicon. 15:2653-2661. https://doi.org/10.1007/s12633-022-02206-4
). As shown in Figure 2,
the TEM micrograph clearly reveals an ordered line array of pores
associated with the hexagonal arrangement of a honeycomb-like structure,
characteristic of the MCM-41 order of pores (5050.
Lin YS, Tsai CP, Huang HY, Kuo CT, Hung Y, Huang DM, et al. 2005.
Well-ordered mesoporous silica nanoparticles as cell markers. Chem.
Mater. 17(18):4570-4573. Retrieved from https://pubs.acs.org/doi/suppl/10.1021/cm051014c/suppl_file/cm051014csi20050711_121101.pdf
).
These results confirm that an ordered porous structure can be obtained
through the synthesis of silica nanoparticles utilizing sodium silicate
extracted from recycled glass. The porous arrays for samples obtained
from waste glass are equivalent to those found in silica nanoparticles
synthesized with commercial TEOS (5151.
Bernal YP, Alvarado J, Juárez RL, Méndez Rojas MÁ, de Vasconcelos EA,
de Azevedo WM, et al. 2019. Synthesis and characterization of MCM-41
powder and its deposition by spin-coating. Optik. 185:429-440. https://doi.org/10.1016/j.ijleo.2019.03.117
, 5252.
Cazula BB, Oliveira LG, Machado B, Alves HJ. 2021. Optimization of
experimental conditions for the synthesis of Si-MCM-41 molecular sieves
using different methods and silica sources. Mater. Chem. Phys.
266:124553. https://doi.org/10.1016/j.matchemphys.2021.124553
) because of their similar textural properties.
3.2. Characterization of the concrete samples
⌅3.2.1. Physicochemical characterization of concrete components
⌅The results of XRD analysis are presented in figure 3 a-c where it is possible to observe that the fine and coarse aggregates are
composed principally of calcite which is the main component of the
stones of the region. On the other hand, the components present in the
CPC are the expected for this kind of cements presenting as the main
phase alite, and another phases like gypsum, portlandite and larnite
prevenient from the clinker. Figure 3d shows the full XPS spectra for CPC30, fine aggregate, and coarse
aggregate samples. In these, signals corresponding to the C1s, O1s,
Ca2p, Si2p and Al2p bonds can be observed for the three samples (53-5553.
Zakaznova-Herzog VP, Harmer SL, Nesbitt HW, Bancroft GM, Flemming R,
Pratt AR. 2006. High resolution XPS study of the large-band-gap
semiconductor stibnite (Sb2S3): Structural contributions and surface
reconstruction. Surf. Sci. 600(2):348-356. https://doi.org/10.1016/j.susc.2005.10.034
54. Briggs D. X-ray photoelectron spectroscopy (XPS). 2005. Handbook of Adhesion: Second Edition. pp. 621-622. https://doi.org/10.1002/0470014229.ch22
55. NIST X-ray Photoelectron Spectroscopy database [cited 2024 Jul 21]. https://doi.org/10.18434/T4T88K
), while for the CPC30 sample, signals corresponding to Mg1s, Na1s, Cl2p, S2p and Fe2p additionally appeared (54-5654. Briggs D. X-ray photoelectron spectroscopy (XPS). 2005. Handbook of Adhesion: Second Edition. pp. 621-622. https://doi.org/10.1002/0470014229.ch22
55. NIST X-ray Photoelectron Spectroscopy database [cited 2024 Jul 21]. https://doi.org/10.18434/T4T88K
56.
Chubar N, Gilmour R, Gerda V, Mičušík M, Omastova M, Heister K, et al.
2017. Layered double hydroxides as the next generation inorganic anion
exchangers: Synthetic methods versus applicability. Adv. Colloid
Interface Sci. 245:62-80. https://doi.org/10.1016/j.cis.2017.04.013
).
Figure 4 shows the
high-definition XPS spectra for the three samples. These XPS spectra
show C1 and O1 bond signals. The first has three signals attributed to
C-C, carbonate, and carbide bonds at 284.8, 290, and 283.5 eV,
respectively. The second indicates the presence of Al2O3 due to the signals of O1s at 531eV and Al2p at 74.1 eV (5757. Nohira H, Tsai W, Besling W, Young E, Petry J, Conard T, et al. 2002. Characterization of ALCVD-Al2O3 and ZrO2 layer using X-ray photoelectron spectroscopy. J. Non. Cryst. Solids. 303(1):83-87. https://doi.org/10.1016/s0022-3093(02)00970-5
).
Likewise, the O1s signal at 531 eV is attributed to carbonate,
specifically calcium because it presents a Ca2p signal at 347.1 eV and
the C1s at 290 eV, mentioned above. The O1s signal has another peak at
533 eV corresponding to SiO2 (Si2p at 103.5 eV). The last O1s
signal (at 530 eV) corresponds to the formation of oxides, which was
identified with the signals Ca2p at 346.6 eV (CaO), Mg1s at 1303.63 eV
(MgO), Na1s at 1072.5 eV (Na2O), and Fe2p at 709.94 eV (Fe2O3).
On the other hand, the additional O1s signal to the metal oxides also
corresponds to the sulfur anions S2p at 169.6 and 167.4 eV, which form
CaSO4 and CaSO3 (Ca2p at 348 eV) (5353.
Zakaznova-Herzog VP, Harmer SL, Nesbitt HW, Bancroft GM, Flemming R,
Pratt AR. 2006. High resolution XPS study of the large-band-gap
semiconductor stibnite (Sb2S3): Structural contributions and surface
reconstruction. Surf. Sci. 600(2):348-356. https://doi.org/10.1016/j.susc.2005.10.034
),
respectively. The last signals correspond to the peaks of Na1s at
1071.57 eV and Cl2p at 198.3 eV attributed to the formation of NaCl (5454. Briggs D. X-ray photoelectron spectroscopy (XPS). 2005. Handbook of Adhesion: Second Edition. pp. 621-622. https://doi.org/10.1002/0470014229.ch22
, 5555. NIST X-ray Photoelectron Spectroscopy database [cited 2024 Jul 21]. https://doi.org/10.18434/T4T88K
).
3.2.2. Determination of the volume of permeable voids
⌅ Figure 3 shows the
volume percentages of permeable voids obtained for each sample. The
reduction in voids in all the treated samples is due to the space
occupied by the fibers or nanoparticles or a mixture of both. The sample
containing both SBF and NP-MCM-41 had the greatest reduction in
permeable voids, with a value for the 0.8N-1F sample being 4.33% less
than that in the control sample; this decrease makes it more difficult
for the carbonation front to advance. These results confirmed the
pore-sealing mechanism of SBF proposed by Talavera-Pech et al. (2021) (2727. Bellopede R, Zichella L, Marini P. 2020. Glass waste3: a preliminary study for a new industrial recovery processing. Sustainability.12(5):1997. https://doi.org/10.3390/su12051997
).
In their study, highly alkaline environments, such as concrete samples,
were shown to cause chemical changes in the polymers that compose
bagasse fibers, which in turn causes changes in their structural
properties, resulting in a breakdown of the polymer chains and an
increase in their volume, promoting the sealing of pores. On the other
hand, nanosilica used as an SCM has two effects on concrete samples: 1)
it can act as a catalytic activator (seed) for the hydration reactions
of OPC (5858. Reches Y. 2018. Nanoparticles as concrete additives: Review and perspectives. Constr. Build. Mater. 175:483-495. https://doi.org/10.1016/j.conbuildmat.2018.04.214
),
and 2) the nonreactive silica nanoparticles can act as pore fillers,
allowing a more homogeneous and denser structure to be obtained (5959. Balapour M, Joshaghani A, Althoey F. 2018. Nano-SiO2 contribution to mechanical, durability, fresh and microstructural
characteristics of concrete: A review. Constr. Build. Mater. 181:27-41. https://doi.org/10.1016/j.conbuildmat.2018.05.266
).
In terms of the second effect, Said et al. reported that the use of 6%
colloidal silica reduces the number of critical pores by up to 40% and
increases the number of micropores by 4% (6060.
Said AM, Zeidan MS, Bassuoni MT, Tian Y. 2012. Properties of concrete
incorporating nano-silica. Constr. Build. Mater. 36:838-844. https://doi.org/10.1016/j.conbuildmat.2012.06.044
),
whereas Oltulu and Sahin reported that the use of 0.5% and 1.25%
reduced the porosity by 48% with respect to the control sample; however,
when 2.5% nanosilica was used, the porosity increased by 43% due to the
agglomeration of nanoparticles (6161.
Oltulu M, Şahin R. 2014. Pore structure analysis of hardened cement
mortars containing silica fume and different nano-powders. Constr.
Build. Mater. 53:658-664. https://doi.org/10.1016/j.conbuildmat.2013.11.105
).
In another investigation, Du et al. reported that using a small amount
of nanosilica (0.9%) does not change the total porosity, as they
reported almost identical percentages of porosity for reference and
nanoparticle samples, reducing the number of capillary pores and
increasing the number of medium capillary pores (6262. Du H, Du S, Liu X. 2014. Durability performances of concrete with nano-silica. Constr. Build. Mater. 73:705-712. https://doi.org/10.1016/j.conbuildmat.2014.10.014
);
these results coincide with those obtained in this investigation with a
very similar addition of MSNs (0.8%). However, the use of fibers and
nanoparticles could improve the concrete pore structure, and it is
possible to observe the synergistic effect of SBF and NP-MCM-41, as the
combination of these methods results in the greatest void reduction.
3.2.3. Mechanical tests
⌅The results of the compressive strength tests of the obtained concrete samples at 7, 14, and 28 days are presented in Figure 4a. In general, with increasing curing time of the concrete samples, the compressive strength increased for all the samples. For samples containing NP-MCM-41, the maximum compressive strength was obtained for the 0.8N sample aged for 28 days and reached 20.306 MPa, which is similar to that of the control sample (20.578 MPa). On the other hand, for samples containing SBFs, the maximum compressive strength was also obtained for the sample aged for 28 days, which was almost 6% greater than that of the control and 0.8N samples. Additionally, for samples containing a combination of SBF and NP-MCM-41, the maximum compressive strength was obtained after 28 days of aging, with an increase of approximately 16% compared with that of the control sample, confirming the enhanced effect of the combination of fibers and nanoparticles on the mechanical performance of the concrete samples.
The data
obtained show that the addition of 0.8% NP-MCM-41 does not improve the
compressive strength of the concrete samples; however, as discussed
previously regarding permeable voids, it is possible that the NPs
function as a catalyst to accelerate the hydration reactions in
concrete, resulting in a reduction in the pore size. This nanoparticle
effect can be observed in the compressive strength of this sample, which
approaches a maximum at 14 days, with a slight increase in strength at
28 days when combined with cane bagasse fiber (0.8N-1F), meaning this
addition provides greater compressive strength than that of the control
sample. This slight improvement can be explained by the fact that at
alkaline pH, the fibers break and their volume increases, causing a
pore-sealing effect in the concrete pastes (4444.
Talavera-Pech WA, Montiel-Rodríguez D, Paat-Estrella JA,
López-Alcántara R, Pérez-Quiroz JT, Pérez-López T. 2021. Improvement in
the carbonation resistance of construction mortar with cane bagasse
fiber added. Materials. 14(8):2066. https://doi.org/10.3390/ma14082066
); additionally, nanoparticles can increase the compressive strength of the concrete (99.
Isfahani FT, Redaelli E, Lollini F, Li W, Bertolini L. 2016. Effects of
nanosilica on compressive strength and durability properties of
concrete with different water to binder ratios. Adv. Mater. Sci. Eng.
2016(1):8453567. https://doi.org/10.1155/2016/8453567
, 3939.
Tobón JI, Payá J, Restrepo OJ. 2015. Study of durability of Portland
cement mortars blended with silica nanoparticles. Constr. Build. Mater.
80:92-97. https://doi.org/10.1016/j.conbuildmat.2014.12.074
).
Because the fibers can enhance the bending properties of concrete (13)13.
Banthia N, Zanotti C, Sappakittipakorn M. 2014. Sustainable fiber
reinforced concrete for repair applications. Constr. Build. Mater.
67(PART C):405-412. https://doi.org/10.1016/j.conbuildmat.2013.12.073
,
only the samples containing SBF fibers as SCMs, both with and without
nanoparticles, were evaluated. The maximum bending stress is called the
breaking modulus (BM) and was determined via a flexural strength test
using a simple beam with a load in the center of the clearing, as
stipulated in Mexican Standard NMX-C-303 - ONNCCE-2010. The bending
strength normally accounts for 12 to 15% of the compressive strength;
for the resistance f ́c = 27.459 MPa designed in this work, a rupture
modulus of 3.236 to 4.112 MPa is expected.
Figure 4b shows that the BM value for the control sample is within this range of expected values. The samples evaluated in this section exclude the 0.8N sample , which shows mechanical behavior similar to that of the control sample at the last curing time (28 days, data not shown), and the main change in the mechanical resistance properties is attributed to the fibers of cane bagasse. For the 0.8N-1F sample, the BM value is equal to 6.549 MPa, a value much higher than that expected in conventional concrete. Notably, this characteristic can be used to analyze the feasibility of using this type of concrete for pavements whose resistance to bending is specified.
These results indicate that,
again, the optimal value of acquired flexural strength is acquired with
the addition of a combination of fibers and nanoparticles. These
flexural strength data are due to the presence of fiber networks, which
act as stress transfer bridges. The fiber network strengthens the
concrete against the spread of cracks, controlling their openings and
possibly delaying the cracking/fracturing of the concrete (6363.
Ribeiro B, Uchiyama T, Tomiyama J, Yamamoto T, Yamashiki Y. 2020.
Development of interlocking concrete blocks with added sugarcane
residues. Fibers. 8(10):61. https://doi.org/10.3390/fib8100061
).
However, the effect of the nanoparticles is clear, as the samples with
SBF and NPs (0.8N-1SBF) present a maximum flexural strength that
surpasses 2.184 MPa in the sample with only the addition of SBF (1F).
3.3. Chemical deterioration of concrete by accelerated carbonation
⌅ Figure 5 shows the
results of the carbonation measurements. The values are the average of
eight measurements, which were taken in the radial direction on a
cylindrical surface for each sample (6464.
Moreno EI, Ixtepan DS, Sarabia EC. 2005. Barras de acero galvanizado:
Una opción contra la corrosión inducida por la carbonatación.
Ingeniería. 9(2):17-24.
). According to the
carbonation data obtained, at this exposure time, neutralization (pH
< 9) had not yet reached the reinforcing steel rod, which was
approximately 20 mm long in the control samples. However, an increased
resistance to the carbonation process is obtained for the samples
produced with SCMs in the following order: 0.8N-1SBF > 1F > 0.8N.
Although the use of nanoparticles alone did not produce a change in the
percentage of permeable voids, it does reduce the carbonation advance
compared with that of the reference sample, which could be related to a
variation in the kind of pores resulting in a change in the permeability
of the concrete samples, as was stated previously when mesoporous
silica nanoparticles were used. However, the best result is for the
material reinforced with the combination of fibers and nanoparticles,
corroborating the action of the seal pores of the fibers and the
improvement of the concrete pore structure by the effect of the
nanoparticles (2323.
Talavera-pech WA, Montiel-rodr D, Paat-estrella JDLA, Ruth L, Jos TP,
Tezozomoc P. 2021. Improvement in the carbonation resistance of
construction mortar with cane bagasse fiber added. Materiales.
14(8):2066. https://doi.org/10.3390/ma14082066
, 3030.
Liu C, Su X, Wu Y, Zheng Z, Yang B, Luo Y, Yang J, Yang J. 2021. Effect
of nano-silica as cementitious materials-reducing admixtures on the
workability, mechanical properties and durability of concrete.
Nanotechnol Rev. 10(1):1395-1409. https://doi.org/10.1515/ntrev-2021-0097
),
which is consistent with the results obtained for permeable voids and
the mechanical performance of the concrete described in the previous
sections (Figure 7).
3.4. Electrochemical tests
⌅3.4.1. Corrosion potential measurement (Ecorr)
⌅ Figure 6 presents
the results of the measurements of the average corrosion potential
(Ecorr) during accelerated carbonation. The values obtained indicate
that the steel remained active during the exposure period (values less
than -350 mV vs. Cu/CuSO4) (6464.
Moreno EI, Ixtepan DS, Sarabia EC. 2005. Barras de acero galvanizado:
Una opción contra la corrosión inducida por la carbonatación.
Ingeniería. 9(2):17-24.
). Changes in Ecorr were
observed toward more positive values because the carbonation process is
not enough to achieve the rod level. However, the samples with the
addition of fibers with and without nanoparticles, i.e., 0.8N-1F and 1F,
respectively, have lower negative values than the control and 0.8N
samples do, indicating that the bagasse fibers reduce the probability
that the steel bars suffer corrosion. It could be expected that once the
carbonation level reaches the rods, the corrosion potentials would show
a trend toward activation (more negative values) (6565.
Chávez-Ulloa, E., Camacho-Chab, R., Sosa-Baz, M., Castro-Borges, P.,
& Pérez-López, T. (2013). Corrosion Process of Reinforced Concrete
by Carbonation in a Natural Environment and an Accelerated Test Chamber.
Inter. J. Electrochem. Sci. 8(7):9015-9029. https://doi.org/10.1016/S1452-3981(23)12946-4.
).
Moreover, by adding NPs, FBC and the combination of these materials, a
decrease in the size of the pores is observed, which in turn decreases
the CO2 transport capacity toward the rod through the concrete (6565.
Chávez-Ulloa, E., Camacho-Chab, R., Sosa-Baz, M., Castro-Borges, P.,
& Pérez-López, T. (2013). Corrosion Process of Reinforced Concrete
by Carbonation in a Natural Environment and an Accelerated Test Chamber.
Inter. J. Electrochem. Sci. 8(7):9015-9029. https://doi.org/10.1016/S1452-3981(23)12946-4.
) (Figure 8).
4. Conclusions
⌅In this work, the ability to obtain NP-MCM-41 from residual glass waste was investigated via transmission electron microscopy (TEM), and N2 sorption analysis was used to explore 1) its honeycomb-like pore structure and 2) its enhanced textural properties via chemical extraction instead of calcination for surfactant extraction, as a large surface area, 120% greater than that obtained through the calcination process, was obtained. According to the analysis of the results obtained from the physicochemical and electrochemical characterization of reinforced concrete, the following conclusions can be drawn: the compressive strength and bending tests slightly improved with the addition of fibers or nanomaterials, but a significant increase was reached when a combination of SBF and NP-MCM-41 materials was used, which was attributed to the synergistic effect of both additive materials. Additionally, the presence of SBF has a greater effect on decreasing carbonation advancement than the addition of nanomaterials; however, the combination of SBF and NP-MCM-41 materials results in an even greater decrease in the advancement of the neutralization of the concrete paste, which is in line with the pores being sealed by both materials, as determined by the volume of permeable voids. In addition, the corrosion potential values indicate that the 0.8N-1F sample has a lower probability of corrosion than the control sample does, which could indicate that the steel surface continues to pass through, which could increase the durability of the reinforced concrete. All the performance tests indicated that the addition of sugarcane bagasse fiber and NP-MCM-41 mesoporous silica nanoparticles, particularly the combination of both 0.8N-1F samples, improved the properties of the concrete samples.
Acknowledgements
⌅Albertho Michel thanks the National Council of Science and Technology (CONACYT) for the scholarship (1032132) granted to realize his master's studies. Also, all the authors thank CONACYT Grant No. A1-S-34533 for the maintenance of TEM equipment. Bacelis-Jiménez thanks CONAHCYT for the National Postdoctoral Fellowship 2022 agreement number: I1200/320/2022. Likewise, all authors thank the National Laboratory of Nano and Biomaterials of CINVESTAV-IPN Mérida for their technical support to the project, especially Wilian Cauich for his contribution to the XPS analysis, and Daniel Aguilar for the XRD analysis.
Funding Sources
⌅This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Author Contributions
⌅Albertho Michel: Conceptualization, Research.
Josefa A. Paat-Estrella: Methodology, Data cleansing, Formal analysis.
Ángel A. Bacelis-Jiménez: Methodology, Data cleansing, Formal analysis, Writing - review & editing.
Alfredo R. Vilchis-Nestor: Methodology, Data cleansing, Formal analysis.
Tezozomoc Pérez-López: Conceptualization, Resources, Writing - review & editing.
William A. Talavera-Pech: Conceptualization, Methodology, Data cleansing, Formal analysis, Resources, Writing - original draft.
Declaration of competing interest
⌅The authors of this article declare that they have no financial, professional or personal conflicts of in-terest that could have inappropriately influenced this work.