Porous Calcium Silicate Hydrate Research Articles

Hierarchical three-layered fibers for bioaerosol and CO2 capture, and antimicrobial performance

As the duration of indoor activities has increased owing to environmental issues and the prevalence of respiratory diseases, it has become important to reduce indoor air pollutants. In this study, air filters with structurally modified surfaces are fabricated to effectively capture and remove indoor air pollutants. High-temperature alkali treatment is applied to a smooth fiber filter to convert it into porous calcium silicate hydrate and create a hierarchical three-layered (H3L) fibrous structure. The outer, middle, and inner layers of the H3L fiber are designed for bioaerosol capture and disinfection, CO2 capture, and maintenance of mechanical strength, respectively. An antimicrobial agent ((3-(trimethoxysilyl)propyldimethyloctadecyl ammonium chloride, TPMMC) is covalently grafted onto the surface of H3L fibers to impart antimicrobial performance. The H3L-TPMMC filter outperforms the bare filter in capturing E. coli and S. aureus aerosols, displaying improved quality factors of 31.9 % and 63.9 %, respectively. It achieves a 99.999 % reduction and 99.790 % for the collected E. coli and S. aureus aerosols, respectively. The H3L-TPMMC filter demonstrates selective CO2 capture and maintains consistent performance and antimicrobial efficacy even in environments with bioaerosols and CO2. This hierarchical filter membrane provides an efficient approach for reducing various indoor air pollutants using a single filter system.

Conversion of Phosphogypsum into Porous Calcium Silicate Hydrate for the Removal and Recycling of Pb(II) and Cd(II) from Wastewater.

The discharge of lead and cadmium wastewater, along with the pollution caused by phosphogypsum, represents a particularly urgent environmental issue. This study employed a straightforward hydrothermal method to convert phosphogypsum into porous calcium silicate hydrate (P-CSH), which was then used to remove and recover Pb(II) and Cd(II) from wastewater. The adsorption capacities of P-CSH for Pb(II) and Cd(II) were notably high at 989.3 mg/g and 290.3 mg/g, respectively. The adsorption processes adhered to the pseudo-second-order kinetics model and the Langmuir isotherm model. Due to identical adsorption sites on P-CSH for both Pb(II) and Cd(II), competitive interaction occurred when both ions were present simultaneously. Additionally, the adsorption efficacy was minimally impacted by the presence of common coexisting cations in wastewater. The dominant mechanisms for removing Pb(II) and Cd(II) via P-CSH were chemical precipitation and surface complexation. Moreover, the adsorbed heavy metals were efficiently separated and reclaimed from the wastewater through a stepwise desorption process. The primary components of the residue from stepwise desorption were quartz and amorphous SiO2. Following dissolution via pressurized alkaline leaching, this residue could be recycled for synthesizing P-CSH. This research offered a new strategy for the resourceful use of phosphogypsum and heavy metal wastewater.

Recycling of blast furnace slag to prepare calcium silicate hydrate by mechanical-chemical co-activation and its application to calcium silicate fireproof board

The high-value utilization of water quenched slag (WQS), one of the bulk byproducts originating from the iron manufacturing industry, remains an important subject in the environment. Through a mechano-chemical co-activation strategy, WQS can be used to prepare porous calcium silicate hydrate (C-S-H), which was applied as gelling filler to prepare fiber-reinforced calcium silicate fireproof board (FCSFB) and extremely improved its high-temperature performance and non-combustibility. The optimal activation conditions for WQS are ball milling time of 30 min, reaction temperature of 90 ℃, reaction time of 5 h, and Ca/Si molar ratio of 1.5:1. The thermal conductivity for FCSFB prepared by the optimal A-WQS is 0.17 W/(m•K), and its overheating time of the applied temperature at 803.7 ℃ is 1335.5 s. The improvement mechanism of the performance of FCSFB can be attributed to the transformation of C-S-H toward more high crystallinity flaky tobermorite, which uniformly and closely combined with pulp fiber to form a micro-porous cavity structure to delay flame penetration period. The application of WQS in FCSFB can provide essential guidance for the utilization of other similar solid wastes.

Synthesis and humidity control performances of natural opoka based porous calcium silicate hydrate

In this work, a porous calcium silicate hydrate (CSH) humidity control material was prepared by hydrothermal synthesis with opoka (ROP) and slaked lime. The microstructures and humidity control properties of the prepared sample were characterized and analyzed in detail. Results show that CSH possesses plenty of mesoporous structures with the pore size range of 3–20 nm, which are superior to ROP. The maximum moisture adsorption capacity of CSH is about 2–3 times of ROP. The superior humidity control performance of CSH can be attributed to the increase of specific surface area and pore volume and the more reasonable and uniform pore size distribution. Moreover, CSH also exhibits good reusability within three cycles of adsorption/desorption. Furthermore, the synthesized CSH was added to an interior wall coating to test its humidity control performance in practical building materials. The result indicated that the moisture adsorption capacity of the coating with CSH in 24 h at high humidity can reach 110 g/m2, and the moisture desorption capacity can also reach 70 g/m2. The as received CSH shows excellent humidity control performance and can be used as a smart indoor humidity control material for various construction applications.

Evaluation of porous calcium silicate hydrate derived from carbide slag for removing phosphate from wastewater

This study investigated the phosphate removal performance of a porous calcium silicate hydrate (P-CSH) derived from carbide slag, a kind of highly alkaline and calcium-rich industrial by-product. The P-CSH was synthesized under 150 °C for 6 h in an autoclave. The prepared P-CSH exhibited distinct skeleton structures and abundant pores with an average radius of 10.88 nm, which provided more active sites. Batch experiments showed that the phosphate removal capacity (PRC) could reach 95% in 4 h. The Ca:P molar ratio in reaction product was 1.49–1.64 according to ICP-OES and EDS results, and XRD patterns confirmed that the product was mainly composed of Ca3(PO4)2 and Ca5(PO4)3OH. As for mechanism study, FTIR and XPS spectra revealed that the Ca-OH group of P-CSH was directly associated with phosphate removal, the Ca-OH will be transformed to Ca-P group after reacting with phosphate. The molar ratio of Ca:Si, equilibrium pH and coexisting ions were three important factors affecting the performance of P-CSH. Increasing Ca:Si ratio can improve the Ca2+ release amount and PRC of P-CSH. The optimal equilibrium pH was determined around 8.5 by experimental and Ksp-based theoretical calculation results. The influence of coexisting SO42− and HCO3− can be avoided by adjusting the equilibrium pH between 5.3 and 10.2. Furthermore, the reuse performance of P-CSH was investigated, the PRC could be accumulated to 14.79 mg P g−1 after nine reuse cycles. Besides, calcination at 700 °C was found to exhibit a certain regeneration effect on P-CSH.

Chloride ions transport and adsorption in the nano-pores of silicate calcium hydrate: Experimental and molecular dynamics studies

Water and ions transport in the nano-meter channel of the porous calcium silicate hydrate (C-S-H) gel was investigated by the experiments and molecular dynamics. The C-S-H gel was first synthesized and the purity, composition and chemical structure were characterized systematically by X-ray diffraction (XRD), Thermogravimetric analysis (TG) and Fourier transform infrared spectroscopy (FTIR). The synthesized C-S-H sample was immersed in the two types of chloride solution for the ions adsorption study. It indicates that the calcium ions can promote binding ability of the C-S-H sample. Furthermore, the interface model of the chloride solution in C-S-H gel pore of 4.5nm was constructed by the molecular dynamics to unravel chloride ions adsorption mechanism. As the carrier of the chloride ions, the water molecules in the vicinity of C-S-H substrate show ordered layered packing and extremely low diffusion coefficient due to the strong chemical restriction from silicate chains. The structural and dynamic behavior of chloride ions are greatly influenced by the states of the calcium ions near the C-S-H surface. Radial distribution function of Ca-Cl shows strong spatial correlation at 2.7Å, indicating the Ca-Cl cluster formation. The interfacial calcium ions are defined as solid and liquid state to represent the increase of calcium to silicate ratio (C/S) of C-S-H samples and the increase of concentration of calcium ions in the solution. In the former case, the calcium ions penetrate into the silicate channel and transform to the solid state, which contributes to the chlorides adsorption near the surface significantly. In the latter case, the calcium ions, adsorbed by oxygen atoms in the silicate chains, resemble the feature of solvated ions in the bulk solution. Because of the unstable connection of Ca-O-Si, the Ca-Cl cluster cannot remains in the C-S-H surface for long time.

Phosphorus recovery using porous calcium silicate hydrate as seed crystal in form of hydroxyapatite

A phosphorus crystallisation process for recovering phosphate was developed using porous calcium silicate hydrate (PCSH) as seed crystal. Phosphorus can be removed by producing hydroxyapatite (HAP) on the surface of seed crystal, which can be applied to the recovery of phosphates from wastewater. Influence factors were investigated. Results indicated that increasing the initial concentration of phosphorus, intensifying the stirring speed and decreasing the sample dosage were beneficial for the increase in phosphorus content in the resulting samples. The phosphorus content of samples after phosphorus recovery could reach 19·05%. On the basis of Ca2+ and OH− releasing from PCSH and characterisation results from X-ray diffraction, field emission scanning electron microscopy and Brunauer–Emmett–Teller, the mechanism of phosphorus recovery by PCSH as seed crystal in the weak alkaline condition was revealed. A large specific surface area and porous structure of PCSH provided local conditions to form a proper concentration for Ca2+ and OH− releasing. This condition could maintain the pH value at 8·0–9·0 in order to improve the production of HAP on the surface of PCSH.

Low hydrothermal temperature synthesis of porous calcium silicate hydrate with enhanced reactivity SiO2

This study presents a novel approach for the synthesis of porous calcium silicate hydrate (CSH) at a low hydrothermal temperature of 110°C based on enhanced reactivity SiO2 (i.e. silica/polyethylene glycol (PEG2000) composites) as the source silica material. The as-prepared CSH materials exhibited a porous microstructure with a large number of small mesopores. The porosity formation mechanism of CSH was apparent that cavitation, resulting from sonication, enabled PEG2000 (via intercalation on silica) to break apart Si–O–Si structural units, thereby enhancing SiO2 reactivity at a low hydrothermal temperature. In addition, the presence of PEG2000 effectively prevented the aggregation of particles during the formation process of the porous CSH solid. The low temperature synthesis proposed herein represents a viable and effective method for the further development of porous CSH as a functional ceramic material.

Porosity formation and enhanced solubility of calcium silicate hydrate in hydrothermal synthesis

Abstract The objective of this paper was to determine the optimal stirring speed for hydrothermal synthesis of calcium silicate hydrate (CSH) with porous structures. Using Field-emission Scanning Electron Microscopy (FESEM), Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), and Particle size distributions (PSD), coupled with the Improved of Detached Eddy Simulation (IDDES), porous structure formation mechanisms at different stirring speeds were examined. The CSH formation process was a step-by-step pattern during a complex phase condition; however, CSH with porous structure could be obtained effectively under reasonable stirring speed. The established Avrami Dissolution Dynamics Model indicated that the as-prepared CSH exhibited greatly enhanced Ca2+ and OH− release under proper stirring speed. This was likely due to small particle size, low crystallinity, and large specific surface area (SBET). This porous CSH has potential application in phosphorus removal from wastewater via production of HAP, thus reducing the negative environmental effects of continual natural phosphorus resource depletion.

Synthesis and Enhanced Phosphate Recovery Property of Porous Calcium Silicate Hydrate Using Polyethyleneglycol as Pore-Generation Agent.

The primary objective of this paper was to synthesize a porous calcium silicate hydrate (CSH) with enhanced phosphate recovery property using polyethyleneglycol (PEG) as pore-generation agent. The formation mechanism of porous CSH was proposed. PEG molecules were inserted into the void region of oxygen–silicon tetrahedron chains and the layers of CSH. A steric hindrance layer was generated to prevent the aggregation of solid particles. A porous structure was formed due to the residual space caused by the removal of PEG through incineration. This porous CSH exhibited highly enhanced solubility of Ca2+ and OH− due to the decreased particle size, declined crystalline, and increased specific surface area (SBET) and pore volume. Supersaturation was increased in the wastewater with the enhanced solubility, which was beneficial to the formation of hydroxyapatite (HAP) crystallization. Thus, phosphate can be recovered from wastewater by producing HAP using porous CSH as crystal seed. In addition, the regenerated phosphate-containing products (HAP) can be reused to achieve sustainable utilization of phosphate. The present research could provide an effective approach for the synthesis of porous CSH and the enhancement of phosphate recovery properties for environmental applications.

Influence of Hydrothermal Temperature on Phosphorus Recovery Efficiency of Porous Calcium Silicate Hydrate

Porous calcium silicate hydrate (PCSH) was synthesized by carbide residue and white carbon black. The influence of hydrothermal temperature on phosphorus recovery efficiency was investigated by Field Emission Scanning Electron Microscopy (FESEM), Brunauer‐Emmett‐Teller (BET), and X‐Ray Diffraction (XRD). Hydrothermal temperature exerted significant influence on phosphorus recovery performance of PCSH. Hydrothermal temperature 170°C for PCSH was more proper to recover phosphorus. PCSH could recover phosphorus with content of 18.51%. The law of Ca2+ and OH− release was the key of phosphorus recovery efficiency, and this law depended upon the microstructure of PCSH. When the temperature of synthesis reached to 170°C, the reactions between CaO and amorphous SiO2 were more efficient. Solubility of SiO2 was a limiting factor.

A Novel Synthesis Method of Porous Calcium Silicate Hydrate Based on the Calcium Oxide/Polyethylene Glycol Composites

This paper proposed a novel method to prepare porous calcium silicate hydrate (CSH) based on the calcium oxide/polyethylene glycol (CaO/PEG2000) composites as the calcium materials. The porosity formation mechanism was revealed via X‐ray diffraction (XRD), field‐emission scanning electron microscopy (FESEM), Brunauer‐Emmett‐Teller (BET), and Fourier transformed infrared spectroscopy (FT‐IR). The reactivity of silica materials (SiO2) enhanced by increasing pH value. Ca2+ could not sustain release from CaO/PEG2000 and reacted with caused by silica to form CSH until the hydrothermal temperature reached to 170°C, avoiding the hardly dissolved intermediates formation efficiently. The as‐prepared CSH, due to the large specific surface areas, exhibited excellent release capability of Ca2+ and OH−. This porous CSH has potential application in reducing the negative environmental effects of continual natural phosphate resource depletion.

Preparation and phosphorus recovery performance of porous calcium–silicate–hydrate

Porous calcium–silicate–hydrate was synthesized and used to recover phosphorus from wastewater. The principal objective of this study was to explore the phosphorus recovery performance of porous calcium–silicate–hydrate prepared by different Ca/Si molar ratios. Phosphorus recovery mechanism was also investigated via Field Emission Scanning Electron Microscopy (FESEM), Energy Dispersive Spectrum (EDS), Brunauer–Emmett–Teller (BET) and X-ray Diffraction (XRD). The law of Ca2+ release was the key of phosphorus recovery performance. Different Ca/Si molar ratios resulted in the changes of pore structures. The increase of specific surface area and the increase in concentration of Ca2+ release were well agreement together. The Ca/Si molar ratio of 1.6 for porous calcium–silicate–hydrate is more proper to recover phosphorus. The pore structure of porous calcium–silicate–hydrate provided a local condition to maintain a high concentration of Ca2+ release. Porous calcium–silicate–hydrate could release a proper concentration of Ca2+ and OH− to maintain the pH values at 8.5–9.5. This condition was beneficial to the formation of hydroxyapatite. Phosphorus content of porous calcium–silicate–hydrate reached 18.64% after phosphorus recovery.

Amelioration of Cadmium Polluted Paddy Soils by Porous Hydrated Calcium Silicate

Porous hydrated calcium silicate (PS) is a by-product of autoclave light weight concrete and is being used as a silicon fertilizer in Japan. The impacts of this amendment on the yield and cadmium content of rice (Orzya sativa L. var. Kokoromachi), soil pH and the extractability of soil cadmium assessed by 1 M NH4OAc were compared with those of silica gel and CaCO3 by pot experiments. The application of PS at the rate of 2.0% and silica gel at the rate of 1.0% in Andosol and PS at the rate of 0.75% in Alluvial soil significantly increased the grain weight of rice. PS and CaCO3 treatment significantly increased soil pH, decreased the 1 M NH4OAc extractability of cadmium and reduced cadmium content in straw and brown rice in the two soils. However, cadmium content of rice of PS treatments was not significantly different from that of CaCO3 treatments in Andosol, while was significantly lower than that of CaCO3 treatments in Alluvial soil. Soil analysis showed that it was less effective in increasing soil pH and decreasing the extractability of cadmium by 1 M NH4OAc than CaCO3 at the same application rate in Andosol. Soil pH and 1 M NH4OAc extractability of cadmium were not significantly different between 2.0% of PS and CaCO3 treatments in Alluvial soil. The application of silica gel did not improve soil pH nor decrease the extractability of soil cadmium, but resulted in a significant decline of cadmium content in brown rice. These results demonstrate that the supply of silicon together with an increase in soil pH, as obtained by PS application, shows potential to reduce the cadmium content of rice.

Fractionation and solubility of cadmium in paddy soils amended with porous hydrated calcium silicate

Previous studies have shown that porous hydrated calcium silicate (PS) is very effective in decreasing cadmium (Cd) content in brown rice. However, it is unclear whether the PS influences cadmium transformation in soil. The present study examined the effect of PS on pH, cadmium transformation and cadmium solubility in Andosol and Alluvial soil, and also compared its effects with CaCO 3, acidic porous hydrated calcium silicate (APS) and silica gel. Soil cadmium was operationally fractionationed into exchangeable (Exch), bound to carbonates (Carb), bound to iron and manganese oxides (FeMnO x ), bound to organic matters (OM) and residual (Res) fraction. Application of PS and CaCO 3 at hig rates enhanced soil pH, while APS and silica gel did not obviously change soil pH. PS and CaCO 3also increased the FeMnO x -Cd in Andosol and Carb-Cd in Alluvial soil, thus reducing the Exch-Cd in the tested soils. However, PS was less effective than CaCO 3 at the same application rate. Cadmium fractions in the two soils were not changed by the treatments of APS and silica gel. There were no obvious differences in the solubility of cadmium in soils treated with PS, APS, silica gel and CaCO 3except Andosol treated 2.0% CaCO 3 at the same pH of soil-CaCl 2 suspensions. These findings suggested that the decrease of cadmium availability in soil was mainly attributed to the increase of soil pH caused by PS.

Effect of Heavy Application of Porous Hydrated Calcium Silicate on Growth and Silicon Nutrition of Paddy Rice

Effect of Heavy Application of Porous Hydrated Calcium Silicate on Growth and Silicon Nutrition of Paddy Rice

Effect of porous hydrated calcium silicate on sorption and desorption of cadmium by soils

Chemical immobilization of cadmium in soil by alkaline materials is a low-cost and non-destructive method to clean-up soils polluted by cadmium. However, much more attention has been paid to the sorption than desorption of metals from treated soils. Batch experiments were carried out to investigate the effect of porous hydrated calcium silicate (PS) on the sorption and desorption of cadmium from an Andosol and an Alluvial soil, which were collected from paddy fields. Samples of the two soil types were incubated with 0, 0.5, and 2.0% of PS under flooded and cultivated conditions for 2 months, and air-dried. PS treatment increased the soil pH. The sorption experiments showed that the amount of Cd sorbed by the two soil types increased with the increase of the percentage of PS. Sorption isotherms were described by the linear Langmuir equation. The increased q m (sorption maximum) and k values (binding strength) of the PS treated soil samples suggested that the higher sorption capacity and stronger complex between cadmium and soils were caused by the PS treatment. The desorption experiments also showed that less cadmium was released from PS treated soils. There was no obvious difference in the cadmium sorption between the soils with and without PS treatment at the same pH value. It was concluded that PS, which can increase the soil pH, is very effective in stabilizing Cd in soils.

Si amelioration of Al toxicity in Barley (Hordeum vulgare L.) growing in two Andosols

The influence of silicon on aluminium toxicity in barley (Hordeum vulgare L. cv. Shunrai) was studied in two Andosols. Silicon sources were a solution of sodium metasilicate with pH adjusted to 5.0, silica gel, and an industrial waste, porous hydrated calcium silicate. The waste is produced in large amounts in the manufacturing processes of autoclaved light concrete, and has been used as a silicon source for rice plants. The addition of the waste increased the concentration of Si in the soil solution, soil pH and amelioration of aluminium toxicity was observed. The addition of silica gel and sodium metasilicate solution to both soils increased significantly (p<0.05) the Si concentration of the soil solutions, but no amelioration of aluminium toxicity was observed. An amelioration of aluminium toxicity by the waste porous hydrated calcium silicate was probably due to the increase in soil pH rather than to the increase of silicon concentration in the soil solution.

Chemical and morphological changes of porous hydrated calcium silicate in paddy soil

The chemical and morphological changes of porous hydrated calcium silicate material (PS) during the dissolution process in paddy soil were investigated by using both a scanning electron microscope (SEM) and an energy dispersive X-ray (EDX) analysis. The results showed that original PS consisted of agglomerate of various sizes with almost the same elemental composition. The SEM images at a high magnification showed that the original PS consisted of agglomerates of small platelike crystals (≦ μm) of tobermorite [Ca5(Si6O18-H2)·4H2O]. On the other hand, elemental composition showed that the PS agglomerates in paddy soil altered to Si- and Ca-rich agglomerates after incubation for 53 d. The morphological differences between the two types of agglomerates were observed by SEM at a high magnification. The Si-rich agglomerates were similar to the original PS in morphology, whereas the Ca-rich agglomerate appeared as a large crystal. The X-ray diffraction patterns obtained in a previous study suggested the presence of quartz and calcite in PS after incubation for 53 d (Saigusa et al. 2000: Soil Sci. Plant Nutr., 46, 89–95). Si- and Ca-rich agglomerates were identified as silica skeletons of PS and calcite, respectively. It was suggested that the silica skeletons of PS remained as a silicon source for rice plants even after the disappearance of tobermorite revealed by the X-ray diffractogram.

Change of structure of porous hydrated calcium silicate by dissolution in paddy soil

Change of the structure of porous hydrated calcium silicate material (PS) by dissolution both in paddy soil and under laboratory conditions was studied using a X-ray diffractometer and a scanning electron microscope. The main crystalline minerals of PS were 11 Å tobermorite and quartz. The X -ray diffraction peaks of calcite (CaCO3) appeared at 14 d after transplanting (DAT) in paddy soil and their area increased with increase of the incubation time, whereas the area of the peaks of tobermorite decreased. The peaks of tobermorite disappeared at 53 DAT. The shape of PS, however, remained even at 120 DAT. It was suggested that the original shape of tobermorite was replaced by that of the silica skeleton. This silica skeleton with a large surface area was assumed to lead to a continuous supply of silicon to rice plants throughout the growing period. X-ray diffraction patterns of PS after incubation under laboratory conditions indicated that CO2 and moisture are required for the dissolution of tobermorite. Tobermorite reacts with H2C03 to produce calcium carbonate and silica gel.