10 research outputs found
Fractal Structure Evolution during Cement Hydration by Differential Scanning Calorimetry: Effect of Organic Additives
Low-temperature
differential scanning calorimetry (LT-DSC) is used to investigate
the microstructure of tricalcium silicate pastes, hydrating in pure
water and in the presence of comb-shaped polycarboxylate ether superplasticizers.
LT-DSC is shown to be a powerful technique, able to provide important
information on the porosity and on the fractality of the porous evolving
matrices by means of rapid and nondestructive measurements. In particular,
LT-DSC gives a semiquantitative estimation of the evolving porosity
(capillary, small gel, and large gel pores), the depercolation threshold
of the capillary pores, and the fractal dimension associated with
the probed porosity. The results are in good agreement with those
obtained by small-angle scattering methods ensuring that this approach,
based on the well-established and easily accessible DSC technique,
can provide valuable information on the evolving porosity and the
fractal nature of hydrating cement pastes
Tricalcium Silicate Hydration Reaction in the Presence of Comb-Shaped Superplasticizers: Boundary Nucleation and Growth Model Applied to Polymer-Modified Pastes
The Boundary Nucleation and Growth Model (BNGM), developed
for
the analysis of the hydration reaction of tricalcium silicate, has
been applied to study the kinetic behavior of pastes containing chemical
admixtures. Four comb-shaped polycarboxylate ether (PCE) superplasticizers
with well-known molecular structures have been added to tricalcium
silicate. The BNGM analysis performed on this series of additives
allows insights into the effect of the molecular architecture of the
PCEs on the induction time and rate constants. The results show that
decreasing the length of the polyethylene oxide side chains of the
PCE molecules increases the induction time. Also, the side chain density,
which highly influences the adsorption of the polymer to the C<sub>3</sub>S unreacted grains, is shown to significantly affect the duration
of the induction period: in particular, molecules with low side chain
density delay the setting of the paste to a greater extent than molecules
with denser side chains. Moreover, the chemical admixtures influence
the rate constants of the nucleation and growth processes, both reducing
them and affecting their temperature dependence
Pore Size Effect on Methane Adsorption in Mesoporous Silica Materials Studied by Small-Angle Neutron Scattering
Methane adsorption in model mesoporous
silica materials with the
size range characteristic of shale is studied by small-angle neutron
scattering (SANS). Size effect on the temperature-dependent gas adsorption
at methane pressure about 100 kPa is investigated by SANS using MCM-41
and SBA-15 as adsorbents. Above the gasâliquid condensation
temperature, the thickness of the adsorption layer is found to be
roughly constant as a function of the temperature. Moreover, the gas
adsorption properties, such as the adsorbed layer thickness and the
specific amount of adsorbed gas, have little dependence on the pore
size being studied, i.e., pore radius of 16.5 and 34.1 Ă
, but
are mainly affected by the roughness of the pore surfaces. Hence,
the surface properties of the pore wall are more dominant than the
pore size in determining the methane gas adsorption of pores at the
nanometer size range. Not surprisingly, the gasâliquid condensation
temperature is observed to be sensitive to pore size and shifts to
higher temperature when the pore size is smaller. Below the gasâliquid
condensation temperature, even though the majority of gas adsorption
experiments/simulations have assumed the density of confined liquid
to be the same as the bulk density, the measured methane mass density
in our samples is found to be appreciably smaller than the bulk methane
density regardless of the pore sizes studied here. The mass density
of liquid/solid methane in pores with different sizes shows different
temperature dependence below the condensation temperature. With decreasing
temperature, the methane density in larger pores (SBA-15) abruptly
increases at approximately 65 K and then plateaus. In contrast, the
density in smaller pores (MCM-41) monotonically increases with decreasing
temperature before reaching a plateau at approximately 30 K
State of Water in Hydrating Tricalcium Silicate Pastes: The Effect of a Cellulose Ether
Time-dependent quasi-elastic neutron
scattering (QENS) and differential
scanning calorimetry (DSC) were applied to study water dynamics and
hydration kinetic of the hydration reaction of tricalcium silicate
in the presence of a methyl hydroxyethyl cellulose (MHEC) additive.
The translational dynamics of the water confined in the developing
hydrated calcium silicate matrix was probed at the molecular scale
by QENS during the first 4 days, while the evolution of the matrix
porosity and the hydration kinetics were determined up to 28 days
of hydration by differential scanning calorimetry. The application
of the boundary nucleation and growth model consistently improved
the hydration kinetics picture, usually obtained from the application
of the classical Avrami-Eroveâev model, allowing the evaluation
of the individual contributions of nucleation and growth over the
entire hydration process. In the presence of the cellulose ether the
nature of the nucleation process is strongly modified, approaching
a âspatially randomâ hydration mechanism. The water
contained in the nanometric porosity of the hydrated calcium silicate
matrix, which is fundamental for the efficiency of the hydration process,
results increased when MHEC is added, leading to a delay of the onset
of the hydration process and the enhancement of the efficiency of
the reaction
Effect of the Alkyl Chains and of the Headgroups on the Thermal Behavior of Ascorbic Acid Surfactants Mixtures
The role of the alkyl chain length
and of the headgroup on the
thermal behavior of mixtures of ASC8 (ascorbyl octanoate) and ASC16
(ascorbyl hexadecanoate) was investigated through differential scanning
calorimetry, small- and wide-angle X-ray scattering, and Fourier transform
infrared spectroscopy experiments. The formation of two eutectics
and of a peritectic point was found from the phase diagram, and their
structural properties were studied. The results were compared by investigating
the thermal behavior of mixtures of octanoic acid and hexadecanoic
acid. The findings provide insights into the role of the ascorbyl
headgroups on the intermolecular interactions that determine the phase
behavior of the two ascorbic acid based surfactants in the solid state
Comb-Shaped Polymers as Nanostructure Modifiers of Calcium Silicate Hydrate: A <sup>29</sup>Si Solid-State NMR Investigation
Calcium silicate hydrate gel (CâSâH)
is the complex
phase mostly responsible for the binding properties and the mechanical
resistance of Portland cement. The clarification of the CâSâH
nanostructure and how the presence of organic additives affects it
is still an intriguing and not trivial task, especially due to CâSâH
scarce crystallinity and intrinsic complexity. In this work, we exploited <sup>29</sup>Si solid-state nuclear magnetic resonance (NMR) to investigate
the effects of different comb-shaped superplasticizers on the silicate
structure. The analysis of <sup>29</sup>Si solid-state NMR spectra
shows that the additives increase the degree of polymerization and
hence the average length of the silicate chains in CâSâH.
This finding correlates well with the increase of the globule dimensions
estimated by means of small angle scattering techniques showing that
the comb-shaped polymers are able to tune the overall dimension of
the CâSâH globule. This effect is dependent on the molecular
architecture of the superplasticizer and allows a molecular imprinting
to the globular structure of the CâSâH gel
Complex Fluids Confined into Semi-interpenetrated Chemical Hydrogels for the Cleaning of Classic Art: A Rheological and SAXS Study
The
removal of aged varnishes from the surface of easel paintings
using the common conservation practice (i.e., by means of organic
solvents) often causes pigment leaching, paint loss, and varnish redeposition.
Recently, we proposed an innovative cleaning system based on semi-interpenetrated
polymer networks (SIPNs), where a covalently cross-linked polyÂ(hydroxyethyl
methacrylate), pHEMA, network is interpenetrated by linear chains
of polyÂ(vinylpyrrolidone), PVP. This chemical gel, simply loaded with
water, was designed to safely remove surface dirt from water-sensitive
artifacts. Here, we modified the SIPN to confine complex cleaning
fluids, able to remove aged varnishes. These complex fluids are 5-component
water-based nanostructured systems, where organic solvents are partially
dispersed as nanosized droplets in a continuous aqueous phase, using
surfactants. The rheological behavior of the SIPN and the nanostructure
of the fluids loaded into the gel were investigated, and the mechanical
behavior of the gel was optimized by varying both the cross-linking
density and the polymer concentration. Once loaded with the complex
fluids, the hydrogels maintained their structural and mechanical features,
while the complex fluids showed a decrease in the size of the dispersed
solvent droplets. Two challenging case studies have been selected
to evaluate the applicability of the SIPN hydrogels loaded with the
complex fluids. The first case study concerns the removal of a surface
layer composed by an aged brown resinous patina from a wood panel,
the second case study concerns the removal of a homogeneous layer
of yellowed varnish from a watercolor on paper. The results show that
the confinement of complex fluids into gels allowed unprecedented
removal of varnishes from artifacts overcoming the limitations of
traditional cleaning methods
Methane Adsorption in Model Mesoporous Material, SBA-15, Studied by Small-Angle Neutron Scattering
The
understanding of methane adsorption is important for many industrial
applications, especially for the shale gas production, where it is
critical to understand the adsorption/desorption of methane in pores
even as small as a few nanometers. Using small-angle neutron scattering
(SANS), we have studied the adsorption of deuterated methane (CD<sub>4</sub>) into one model mesoporous material, SBA-15, with pore diameter
approximately 6.8 nm at the temperature range from 20 to 295 K at
low pressure (â100 kPa). A new scattering model is developed
to analyze the SANS patterns of gas adsorption in SBA-15. The surface
roughness of the SBA-15 matrix is estimated. The gas adsorption behaviors
on the surface regions are extracted from the fitting. The rough surface
of the pores is found to retain a large amount of CD<sub>4</sub> at
the temperature above the capillary condensation temperature (<i>T</i><sub>c</sub>). At temperatures below <i>T</i><sub>c</sub>, the confined liquid and solid methane are estimated
to be less dense than the corresponding bulk liquid and solid methane.
Detailed theoretical analysis and experimental verification also show
that SANS patterns at temperatures higher than <i>T</i><sub>c</sub> are much more sensitive to the change of the excess adsorption,
Δ<sub>ads</sub>, rather than the average density of adsorbed
layers commonly used in many studies. The model we establish can be
used to analyze future SANS/SAXS data for gas confined in similar
model porous materials
Innovative Hydrogels Based on Semi-Interpenetrating p(HEMA)/PVP Networks for the Cleaning of Water-Sensitive Cultural Heritage Artifacts
Water-based detergent systems offer several advantages,
over organic
solvents, for the cleaning of cultural heritage artifacts in terms
of selectivity and gentle removal of grime materials or aged varnish,
which are known to alter the readability of the painting. Unfortunately,
easel paintings present specific characteristics that make the usage
of water-based systems invasive. The interaction of water with wood
or canvas support favors mechanical stresses between the substrate
and the paint layers leading to the detachment of the pictorial layer.
In order to avoid painting loss and to ensure a fine control (layer
by layer) of grime removal, water-based cleaning systems have been
confined into innovative chemical hydrogels, specifically designed
for cleaning water-sensitive cultural heritage artifacts. The synthesized
hydrogels are based on semi-interpenetrating chemical polyÂ(2-hydroxyethyl
methacrylate)/polyÂ(vinylpyrrolidone) networks with suitable hydrophilicity,
water retention properties, and required mechanical strength to avoid
residues after the cleaning treatment. Three different compositions
were selected. Water retention and release properties have been studied
by quantifying the amount of free and bound water (from differential
scanning calorimetry); mesoporosity was obtained from scanning electron
microscopy; microstructure from small angle X-ray scattering. To demonstrate
both the efficiency and versatility of the selected hydrogels in confining
and modulating the properties of cleaning systems, a representative
case study is presented
Self-assembled monolayers of Prussian blue nanoparticles with photothermal effect
<p>A photo-responsive antibacterial surface was prepared grafting non-toxic Prussian blue nanoparticles on a functionalized glass surface. Colloidal Prussian blue was synthesized as nanoparticles with cubic shape and grafted on a polyamine-functionalized SiO<sub>2</sub> surface, obtaining a good coverage and a homogeneous distribution of the nanocubes. Irradiation of these samples in the so-called âbio-transparent windowâ of the near-infrared allows to exert a triggered antibacterial effect.</p