Silicate Chains Research Articles

Sulfate resistance of cement composites containing Nano-Fibrillated Cellulose (NFC)

In this study, the effect of Nano-Fibrillated Cellulose (NFC) on the sulfate resistance of cement composites was investigated. Plain and 0.5% steel fiber reinforced cement paste and mortar mixtures (FRM) modified with 0.1% NFC were evaluated. Changes in mass and tensile strength of specimens subjected to alternate immersion in 5% Na2SO4 solution and drying cycles were monitored for about six months. At the end of the test period, microstructural changes in cement pastes were analyzed via X-ray Diffraction (XRD), Fourier Transform Infrared – Attenuated Total Reflectance (FTIR-ATR) spectroscopy, Scanning Electron Microscopy (SEM), and solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. Mass change and tensile strength results showed that the sulfate attack resistance of the cement composites prepared with the 0.1% NFC was superior to that of the Plain mixture. XRD and FTIR-ATR analyses indicated reduced dissolution of portlandite and calcium aluminosilicate phases from the NFC-modified cement matrices. NMR results also showed that the transformation of monosulfate to ettringite, the decalcification of the C–S–H and polymerization of the silicate chains was lowest for cement pastes incorporating the NFC. Moreover, SEM imaging confirmed that the NFC and the steel fiber helped mitigate the propagation of cracks in cement pastes.

<sup>29</sup>Si NMR Investigation of the Effect of Acetic and Oxalic Acids on Portland-Limestone Cement Hydration

The use of carboxylic acids in mix design alters the hydration process of cement, the resulting pore structure of the obtained cement paste, and, consequently, the mechanical properties of concrete. All these changes are directly related to the structure of the calcium silicate hydrate phase. In the present study, the effect of acetic acid and oxalic acid on the hydration of Portland-limestone cement was monitored using solid state 29Si NMR spectroscopy. The results showed that acetic acid facilitated alite and belite hydration, however, the formation of polymerized silicate chains, incorporating Q2p species, begun later than in pure cement paste. Oxalic acid accelerated the polymerization, but slowed down alite and belite hydration. Such behaviors may correspond to decreased porosity (acetic acid addition) and increased strength (oxalic acid addition). Both acids accelerated belite hydration, compared to the pure paste, likely due to an increased acidity of the pore solution. The findings provide structural information about C─S─H phase, to be considered for thaumasite sulfate attack investigations on Portland-limestone cement pastes containing carboxylic acids.

Preferred orientation of calcium silicate hydrate and its implication to concrete creep

Calcium silicate hydrate (C–S–H) is the primary binding phase of cement-based and alkali-activated materials. The preferred orientation of C–S–H under non-hydrostatic pressure (e.g., uniaxial/biaxial load) is overlooked yet crucial in understanding concrete's multiscale mechanical performance. Here, we unveil the texture formation of C–S–H under compressive deviatoric stress, S, from 0 to ∼200 MPa, using high-pressure X-ray diffraction. Texture initiated at S < 12 MPa: the c-axis (normal to the basal plane) of C–S–H nanocrystallites preferentially re-oriented towards the direction of the principal compressive stress. Below S ∼100 MPa, the preferred orientation intensified through translation and rotation of C–S–H nanocrystallites; above ∼100 MPa, the texture stopped growing then weakened, suggesting internal transformations of C–S–H nanocrystallites. The time-dependence of the preferred orientation development is unveiled by the texture weakening after full unloading. The findings implicate that concrete creep under service loads is contributed by the intergranular preferential re-orientation of C–S–H nanocrystallites, not interlayer sliding or silicate chain breakage.

Al uptake in calcium silicate hydrate and the effect of alkali hydroxide

The effect of Al and of pH on the structure of calcium silicate hydrate (C-S-H), the most important hydration product in Portland and blended cement, was studied at Ca/Si ~1.0. The presence of Al led to higher amounts of secondary phases, larger interlayer distances, longer silicate chains, higher concentrations of dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases progressively decreased with increasing pH; that of strätlingite and Al(OH)3 dominated at low pH, and katoite at high pH values. High pH was also associated with increased Al and Si concentrations, lowered Ca concentrations, shortened silicate chain length and higher maximum Al/SiC-S-H ratios. The Al uptake in C-A-S-H increased with aqueous aluminium concentration. XANES analysis showed the presence of both Al(IV) and Al(VI) in different coordination environments in C-S-H.

Understanding the aging mechanism of calcium silicate hydrates under the action of solar radiation

This work examines the influencing mechanism of light radiation on performances and structures of calcium silicate hydrates (C-S-H) through exposing synthetic C-S-H samples to high-power Xenon radiation. The compositions, chemical structures and nanomorphology of C-S-H were investigated using XRD, TG, 29Si NMR, Ca XANES, Si XANES and TEM. Xenon radiation reduces the chemically bound water content in C-S-H and the drying effect induced by Xenon radiation is more pronounced in C-S-H with lower Ca/Si ratio. Xenon radiation increases the polymerization of C-S-H and elongates the silicate chain by driving separated Si–O tetrahedron to connect with each other without changing its Ca/Si ratio. Furthermore, Xenon radiation reduces the coordination numbers of Ca–O and causes the self-organization of chemical structure at molecular and atomic scales. The existence of portlandite improves the resistance of C-S-H to compositional and structural changes induced by Xenon radiation. These findings pave the way for understanding aging mechanism of cement-based materials under the action of solar radiation.

Preparation and characterization of a novel alkali-activated magnesite cement

The present paper focuses on synthesizing an alkali-activated binder, in which magnesium carbonate (MC) was the only precursor. MC was individually activated with sodium hydroxide (NaOH), sodium silicate (Na2SiO3) and sodium aluminate (NaAlO2). Increasing Na2O content (as NaOH) up to 10 wt% enhanced the compressive strength slightly. At constant Na2O content (10 wt%), the sample activated by (Na2SiO3) recorded the highest compressive strength. Although the hardened mixture activated by NaAlO2 showed the higher compressive strength compared to NaOH-activated sample, it still recorded strength lower than Na2SiO3-activated-mixture. XRD, TG/DTG, FTIR, and SEM techniques indicated that phase composition depends mainly on the Na2O source. 29Si MAS-NMR spectroscopy verified the formation of magnesium silicate hydrate with Q1, Q2, and Q3 silicate connectivity, when MC was activated with Na2SiO3. Incorporating lead glass sludge (LGS) as a silicate source, enhances the formation of a cross-like silicate chain (Q3) coinciding with an improvement of the compressive strength and a retardation of Pb-leachability. As proved by 27Al MAS-NMR, Al inside hydrotalcite phase, formed within NaAlO2-activated-MC, has six coordination bonds. Activating MC with NaOH produces magnesium hydroxide (Mg(OH)2) and thermonatrite Na2CO3·H2O phases. Owing to the diversity of the hydration products, a wide range of 90-day compressive strengths (9–53 MPa) was recorded. A preliminary result showed that the CO2-emission generates from optimum mixture containing 30 wt% LGS is lower than resulted from Portland cement manufacturing.

Degradation characteristics of Portland cement mortar incorporating supplementary cementitious materials under multi-ions attacks and drying-wetting cycles

The multiple corrosive ions in seawater cause more severe concrete corrosion in tidal and splash zones . To understand the mechanisms of harmful ions in seawater under the real field conditions, four different synthetic multi-ions solutions (NaCl, NaCl + MgCl 2 , NaCl + Na 2 SO 4 and NaCl + MgCl 2 +Na 2 SO 4 ) were prepared. This study investigated the effects of external multi-ions solutions and repeated drying-wetting cycles on the degradation mechanisms of Portland cement mortar/paste with and without supplementary cementitious materials (SCMs) by using different microstructural techniques. It was found that the multi-ions solutions of Cl − + SO 4 2− and Cl − + Mg 2+ +SO 4 2− aggravated the deterioration of mortar samples compared to the solution with Cl − alone under drying-wetting cycles. The cracks and larger pores were induced due to the formation of secondary ettringite , and the enlarged size of microvoids in mortar obviously affected the damage evolution and resulted in an accelerated deterioration of mortar samples. In addition, the incorporation of ground blast furnace slag (GBFS) significantly improved the corrosion resistance of cement-based materials and showed more efficient than other pozzolanic materials . This may be due to that aluminum incorporation could bridge the defective silicate chains and increase the polymerization degree of silicate tetrahedra in the C–S–H gel. Moreover, the matrix incorporated GBFS could effectively reduce pore defects and volumes of connected pores and large pores, which enhanced the resistance to multi-ions attack. These results can be provided a theoretical basis on the improvement of concrete durability under severe marine environmental attacks.

Influences of leaching on the composition, structure and morphology of calcium silicate hydrate (C–S–H) with different Ca/Si ratios

Leaching of hydration products in cement-based materials may reduce the service life of concrete constructions, such as radioactive waste disposal containers and hydro structures. C–S–H is the most important cement hydration product, and systematic research on its nanostructures change under leaching is necessary to clarify the leaching mechanism in cement-based materials. This work aims to disclose the leaching mechanism of C–S–H with different Ca/Si ratios by immersing them in NH4Cl solution. The chemical structure, composition and morphology of samples were investigated by XRD, TG, ICP-OES, FTIR, 29Si NMR and TEM. Results show that decalcification occurs both in interlayer and intralayer of C–S–H, reducing the Ca/Si ratio to 0.67–0.70. Although initial Ca/Si ratios are different, the eventual decalcification behavior of C–S–H tends to be congruent dissolution. Additionally, Ca(OH)2 buffers the decalcification of C–S–H. Ca leaching breaks the charge balance of C–S–H and therefore changes the chemical state of silicate tetrahedra, which increases the length of silicate chains. Meanwhile, the silicate chain in adjacent layers was linked and the amorphous silica gel was formed. Leaching coarsens the pore structure of C–S–H and transforms partial foil-like C–S–H into silica gels with flocculent morphology.

Synthesis and characterization of an intermediate for C-S-H structure tailoring

The intermediate describes the stage prior to final product, which is the key step for structural design and functional groups tailoring at nanoscale. Inspired by this concept, a sol-gel method for the C-S-H intermediate preparation in volatile organic solution was firstly proposed in this work, and the feasibility of inducing the formation of C-S-H with targeted structure from the intermediates was also evaluated. The results show that the volatilization of TEA and ethanol initiates the formation of C-S-H intermediates, of which the structure is a hybrid of organic and inorganic silicate chains. C-S-H intermediates upon the excitation of NaOH solutions can eventually evolve into C-S-H gels. The nanostructure of evolved C-S-H can be tailored by excitation of different C-S-H intermediates. This evolved C-S-H was characterized by the stacking of tobermorite-like nanocrystals. And those evolved C-S-H particles are hollow shape, and their shell includes outer sheet-resembling C-S-H and inner dense C-S-H.

Eco-friendly recycling of silicon−rich lye: Synthesis of hierarchically structured calcium silicate hydrate and its application for phosphorus removal

Silicon−rich lye (SRL), a byproduct generated from pre−treatment of coal−based solid waste (CSW), was considered as a preponderant silicon source to prepare hierarchically nanostructured calcium silicate hydrate (C-S-H). Through the novel mild−causticization synthesis strategy, C-S-H was prepared under optimal caustic process conditions at time of 3 h, temperature of 80 °C, Ca/Si of 1.25:1, and active CaO to obtain a conversion rate of Si up to 97.33 % during the high−value utilization of SRL. The synthesized C-S-H possesses abundant mesoporous structure and massive exchangeable active sites, whose formation is advanced through an appropriate elevation regulation of caustic temperature and time. The silicate chain depolymerization occurs to C-S-H prepared in the highly alkaline system at higher caustic temperature, longer caustic period, especially at existence of massive sodium ions, but it presents higher polymerization degree at more aluminum co-existing. The adsorption capacity up to 119.27 mg/g for C-S-H presents a valid removal performance toward phosphorus in the wastewater than massive present reports. The removal mechanism of phosphorus can be identified as the surface chemisorption and formation of calcium phosphate co-precipitation. This study can provide considerable and potential guidance to the coordinated disposal between industrial solid wastes and wastewater purification.

Upscaling degradation of cementitious calcium (aluminate) silicate hydrate upon ultra-low temperature attack: A multiscale insight and a bottom-up enhancement route

Low-carbon calcium (alumino)silicate hydrate (C-(A)-S-H) based composites (e.g., blended cement-based materials and alkali-activated materials) have broad application potential in ultra-low-temperature engineering areas. This study devotes to reveal the structural stability of C-(A)-S-H under ultra-low temperature attack (−170 °C) and evaluating the role of aluminum as a bottom-up enhancement strategy for C–S–H. The atomic structure, interlayer structure, internal basic building block structure, pore network, and micro-morphology were experimentally studied using 29Si MAS NMR, XRD, helium pycnometry, nitrogen adsorption, and SEM. These results indicate that an ultra-low temperature attack can deteriorate C-(A)-S-H structure, even though without the presence of bulk water, whereas the incorporation of aluminum positively stabilizes C-(A)-S-H structure. The silicate chains of Al-free C–S–H are ruptured with more defective vacant sites, which consequently initiates a decline in its d002 interlayer space and the volume collapse of basic building blocks, further resulting in the formation of microcracks. In contrast, aluminosilicate chains of C-A-S-H are more stable and even polymerized with longer chain length, stabilizing the micro/mesostructure. An upscaling degradation route from the atomic scale to mesoscale was proposed, providing a multiscale view to understand the degradation of concrete composites upon ultra-low temperature attack. We also confirm that incorporating aluminum in the silicate chains is an effective bottom-up strategy to enhance the cryogenic stability of C-(A)-S-H.

Effect of B2O3 content on viscosity and structure of SiO2−MgO−FeO-based slag

The effect of B2O3 content on the viscosity of SiO2−MgO−FeO-based molten slag system was investigated using the rotating cylinder method. The evolution process of the melt structure under different contents of B2O3 was comprehensively studied via FTIR spectroscopy and a model for calculating the degree of polymerization was developed. The results showed that the viscosity of the molten slag decreased with the addition of B2O3, which had a slight effect when its content exceeded 3 wt.%. As the addition of B2O3 increased from 0 to 4 wt.%, the break temperature of the slags decreased from 1152 to 1050 °C and the apparent activation energy decreased from 157.90 to 141.84 kJ/mol. The addition of B2O3 to the molten slag destroyed the chain silicate structure to form a more cyclic borosilicate structure. The Urbain model was improved to calculate the viscosity of the SiO2−MgO−FeO-based slags, and the values were in good agreement with the experimentally measured values.

Solubility and characterization of synthesized 11 Å Al-tobermorite

The structure and solubility of hydrothermally prepared 11 Å Al-tobermorite with Al/(Al + Si) = 0.1 has been studied. NMR and FTIR data indicated the formation of a cross-linked, well crystalline 11 Å Al-tobermorite and preferential uptake of Al in the branching sites of the silicate chains. The incorporation of Al in branching sites of 11 Å tobermorite increased the basal spacing of the 11 Å Al-tobermorite by ~0.18 Å. Solubility experiments indicated little effect of temperature on the solubility of 11 Å Al-tobermorite and a weak stabilization of 11 Å Al-tobermorite with regard to 11 Å tobermorite. The experimental data confirmed the strong similarity in structure, Al-uptake, and solubility with poorly ordered C-A-S-H gel, although C-A-S-H gels showed a lower degree of ordering, the absence of cross-linking, an increased basal spacing, and a higher solubility.

Granulometric effect of silica nanoparticles on hydration kinetics and microstructure of cement based materials

Effect of particle size of two types of silica nanoparticles (SNPs) on hydration kinetics of tricalcium silicate (C3S) and Portland cement was investigated. Two different types of SNPs were used in the present study, first one is the synthesized by sol-gel method (SNPs-sg) having particle size 30–70 nm, while the second one is the synthesized by Flame Spray Pyrolysis method (SNPs-fsp) having particle size <10 nm. The dosages of SNPs used in this study varied from 1–10% and water/binder ratio used for the hydration was 0.4. Rietveld analysis of XRD results show that at 4 h of hydration, the unhydrated content in control samples at 4 h hydration was ∼88% and in presence 1% SNPs-sg, it was ∼75%, however, in presence of SNPs-fsp, the content of unhydrated C3S was ∼60%. Further, FTIR results reveal that in case of control samples at 1 h of hydration, the characteristic peaks of C3S at 850 cm−1 and 940 cm−1 are present, while, with 1% SNPs-sg addition, reduction in the intensity C3S peaks were observed showing acceleration in dissolution rate and a broader peak is observed at ∼1110 cm−1 with 1% SNPs-fsp samples indicating the starting of polymerization in silicate chain of C-S-H gel.

Time-varying structure evolution and mechanism analysis of alite particles hydrated in restricted space

Restricted space could lead to the structure evolution of calcium silicate hydrate (C–S–H) especially during the later hydration stage, which essentially affects the strength and durability of Portland cement based materials. In order to get insights into the effect of restricted space on C–S–H microstructure, the alite powders were pressed into discs for hydration. The pre pressing treatment provides a relatively denser microstructure, which simulates the restricted space during the late hydration of alite or cement. The structure of hydrates grown in restricted space was revealed by Fourier transform infrared spectra, Raman spectra, 29Si nuclear magnetic resonance spectra, scanning electron microscope and energy dispersive X-ray spectroscopy. The results show that restricted space leads to the formation of unstable Ca–O bonds and Si–O bonds in early hydrates, and influences the silicate chains in C–S–H by the formation of high polymerized hydrates (Q3 silicates) at early stage. The content of Q3 silicates in the hydrates of restricted space is the highest at 7 d and then decreases, which is opposite to the control with the lowest Q3 silicates content at 7 d and the highest content at 28 d. The hydrates which form at 28 d show a remarkable higher chemical shift of Q3 sites and a lower Ca/Si ratio than that of the control. The findings indicate that the restricted space may lead to stress, which rearranges the nanostructure of C–S–H and induces a varied structure in Q3 silicates of C–S–H. This work lies a theoretical foundation for exploring the structure evolution of C–S–H in cement concrete after long service.

Reactive molecular simulation of shockwave propagation in calcium–silicate–hydrate gels

Despite its wide use, very little is known about the behavior of calcium–silicate–hydrate (C–S–H) gel under shock loading. Based on reactive molecular dynamic simulations, we investigate the effect of shock propagation on the structure of C–S–H. Specifically, we analyze the shock response of C–S–H structures along and normal to the layered structure, with piston velocities ranging from 0.1 to 7 km/s. The analysis of these structures reveals that the compressive shockwave propagation through the C–S–H significantly affects the short and medium-range order of the structure. These results are consistent with the increased depolymerization of the silicate chain observed in C–S–H under shock loading. Further, we observe that the Hugoniot elastic limit (PHEL) of C–S–H lie in the range of 7-10 GPa, irrespective of the shock direction. Altogether, these insights obtained from reactive molecular simulations can potentially enable the design of improved cement compositions that exhibit improved resistance to shock-induced damage.

Damage mechanism of engineered cementitious composites after exposed to elevated temperatures: Experimental and molecular dynamics study

Replacing cement with large amounts of fly ash in engineered cementitious composites (ECC) is one of the win-win measures to enhance matrix ductility and effectively reduce carbon emissions. However, the high content of fly ash undoubtedly makes the mechanical properties of ECC lower, resulting in its performance at high temperature to be questioned. To solve this problem, this paper combines experiments and molecular dynamics study to fully explore and innovatively explain the performance of ECC exposed to different temperatures (20 °C, 200 °C, 400 °C, 600 °C and 800 °C) from the aspects of macroscopic mechanical properties, microstructural characteristics and molecular-scale interfaces. From 20 °C to 800 °C, the mechanical properties first increase and then decrease significantly. SEM shows the changes in the microscopic morphology at elevated temperatures, such as the increase of cracks and pores in the matrix, and the melting of the PVA fibers. TG/DSC and XRD illustrate the phase transformation at different temperatures. 29Si NMR demonstrates the decomposition of hydration products and the depolymerization of silicate chains at higher temperatures. Additionally, molecular dynamics study is carried out to characterize the interaction between PVA fibers and C–S–H gel at a molecular scale as a function of temperature. With the increase of temperature, the morphology of C–S–H gradually changes from layered structure to spherical structure, the water molecules between the C–S–H layers gradually diffuse to the C–S–H channel, and PVA chains are present by disordered distribution. The simulated results have a good agreement with experimental results and further explain the damage mechanism of ECC at elevated temperatures on the nano-scale level.

Influence mechanism of Na2O on the network structure depolymerization of ZD coal ash silicate under high temperature

Na2O is the basic alkali metal oxide in Zhundong (ZD) coal, the content of Na2O directly affects the ash fusion temperatures (AFTs) of coal ash. Fourier transform infrared (FTIR) spectroscopy and molecular dynamics (MD) simulation were used to study the structural characteristics of synthetic coal ash under Na2O concentration of 1–13 mol%. Both FTIR experiments and MD simulation show that Na+ attacks the tri-coordinate oxygen (Ot) and the bridge oxygen (Ob) formed by [SiO4]4− and [AlO4]5− tetrahedron structure, generates more non-bridge oxygen (Onb), the shelf-like network structure(Q4) in fused coal ash is depolymerized to form layered silicate structure (Q3), chain or cyclic silicate structure (Q2, Q1). O(Al, Al, Si) and O(Al, Al, Al) in Ot decreased their trend by 3% each, Si–O–Si was reduced from 12% to 7%. Al–O–Si was reduced from 36% to 29%. The intricate structures are gradually depolymerized into plain structures, which leads to the decrease of AFTs.

Molecular insight in the wetting behavior of nanoscale water droplet on CSH surface: Effects of Ca/Si ratio

The distribution state and wettability of water molecules on the Calcium silicate hydrate (CSH) surface closely affect the transport behavior of water on CSH gel pores, thus affecting the durability of concrete. To understand the wetting behavior of water droplets on the CSH gel interface with different Ca/Si ratios, this paper focuses on the local structure, ionic pairs, hydrogen bonds, orientation characteristics and dynamic properties of water molecules on the CSH interface based on molecular dynamics simulation. At the molecular level, the water droplets have a larger contact angle at the CSH surface with a low Ca/Si ratio because the CSH with a low Ca/Si ratio has a dense structure and makes it difficult for water molecules to penetrate the silicate chain. Analysis reveals that the dense silicate chains tend to absorb and restrict water molecules in the form of static electricity. The orientation distribution of water molecules at different positions on the CSH interface reveals the driving force of wetting behavior. The results of dynamic properties analysis show that CSH with a high Ca/Si ratio has better wettability to water droplets.

Waste Glass-Derived Tobermorite Carriers for Ag+ and Zn2+ Ions

In this study, the layer-lattice calcium silicate hydrate mineral, tobermorite, was synthesized from waste green or amber container glass and separately ion-exchanged with Ag+ or Zn2+ ions under batch conditions. Hydrothermal treatment of stoichiometrically adjusted mixtures of waste glass and calcium oxide in 4 M NaOH(aq) at 125 °C yielded tobermorite products of ~75% crystallinity with mean silicate chain lengths of 17 units after one week. Maximum uptake of Zn2+ ions, ~0.55 mmol g−1, occurred after 72 h, and maximum uptake of Ag+ ions, ~0.59 mmol g−1, was established within 6 h. No significant differences in structure or ion-exchange behavior were observed between the tobermorites derived from either green or amber glass. Composite membranes of the biopolymer, chitosan, incorporating the original or ion-exchanged tobermorite phases were prepared by solvent casting, and their antimicrobial activities against S. aureus and E. coli were evaluated using the Kirby–Bauer assay. S. aureus and E. coli formed biofilms on pure chitosan and chitosan surfaces blended with the original tobermorites, whereas the composites containing Zn2+-substituted tobermorites defended against bacterial colonization. Distinct, clear zones were observed around the composites containing Ag+-substituted tobermorites which arose from the migration of the labile Ag+ ions from the lattices. This research has indicated that waste glass-derived tobermorites are functional carriers for antimicrobial ions with potential applications as fillers in polymeric composites to defend against the proliferation and transmission of pathogenic bacteria.