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 C₃S 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 gas–liquid 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 Å, 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 gas–liquid condensation temperature is observed to be sensitive to pore size and shifts to higher temperature when the pore size is smaller. Below the gas–liquid 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-Erove’ev 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 “spatially random” 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 ²⁹Si Solid-State NMR Investigation

Calcium silicate hydrate gel (C–S–H) is the complex phase mostly responsible for the binding properties and the mechanical resistance of Portland cement. The clarification of the C–S–H nanostructure and how the presence of organic additives affects it is still an intriguing and not trivial task, especially due to C–S–H scarce crystallinity and intrinsic complexity. In this work, we exploited ²⁹Si solid-state nuclear magnetic resonance (NMR) to investigate the effects of different comb-shaped superplasticizers on the silicate structure. The analysis of ²⁹Si solid-state NMR spectra shows that the additives increase the degree of polymerization and hence the average length of the silicate chains in C–S–H. 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 C–S–H globule. This effect is dependent on the molecular architecture of the superplasticizer and allows a molecular imprinting to the globular structure of the C–S–H 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 poly­(hydroxyethyl methacrylate), pHEMA, network is interpenetrated by linear chains of poly­(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 (CD₄) 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 (≈100 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 CD₄ at the temperature above the capillary condensation temperature (<i>T</i>_c). At temperatures below <i>T</i>_c, 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>_c are much more sensitive to the change of the excess adsorption, ε_ads, 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 poly­(2-hydroxyethyl methacrylate)/poly­(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 SiO₂ surface, obtaining a good coverage and a homogeneous distribution of the nanocubes. Irradiation of these samples in the so-called ‘bio-transparent window’ of the near-infrared allows to exert a triggered antibacterial effect.</p