Cement-based Porous Materials Research Articles

Effects of heat-dry curing temperature on porous silicate cement membranes fabricated by the coupling process of freeze casting and heat-dry curing

Preparation of porous cement-based materials via freeze casting is a process with great potential. However, freeze-drying and steam curing complicate the preparation process and increase the preparation period. To simplify the process, heat-dry curing was used for replacing freeze-drying and steam curing during operation. The new preparation method combined freeze casting and heat-dry curing possesses characteristics of low-energy, simplicity, and time-saving. Compared with the traditional preparation method of porous cement-based materials, the novel preparation process can shorten preparation time by at least 16 % and retrench energy consumption by at least 18 %. As the improvement condition, temperature of heat-drying curing has great influence on the structure and performance of the samples. Therefore, it is of great significance to discuss the relationship between heat-dry temperatures and structural parameters by SEM, N2-adsorption/desorption, TG, and ATR-FTIR. Lamellar pores of membranes can be easier observed at low temperatures than that at 100 °C. The separation performance of oil-water is generally improved with the increase of temperature. Meanwhile, the molecular dynamics simulation was operated to illustrate the reaction mechanism of hydration with three temperatures. This study probably provides more theoretical basis for expanding the application of porous silicate cement membranes.

3D superhydrophobic lightweight foam concrete designed via in-situ foaming and self-assembled silicone microfilms

The poor durability of lightweight foam concrete, stemming from its highly porous and hydrophilic nature, is a major limitation for its wider application. Designing a superhydrophobic structure would be an effective approach to improving the durability of foam concrete. In this study, a facile strategy was developed to produce 3D superhydrophobic foam concrete using Poly(methyl hydrogen)siloxane (PMHS) by means of in-situ foaming and low surface energy characteristic. Results show that PMHS could be hydrolyzed and released hydrogen gas by reacting with the hydration product of Ca(OH)2. The resulting self-foaming lightweight concrete exhibits controllable average pore sizes ranging from 229 to 430 μm, with a density between 610 and 1177 kg/m3. The hydrolyzed PMHS self-assembles on the foam films, and the micrometer pores are carried with gel-like silicone microfilms through PMHS liquid–solid phase transition after 90℃ steam curing. With the synergetic effect of the dense silicone films and the particular pore frame, the superhydrophobic foam concrete demonstrates good fundamental properties, including thermal insulation and sound absorption, mechanical robustness, chemical durability, and weathering resistance. This study provides a practical solution for developing superhydrophobic lightweight concrete, which substantially enhances the durability of porous cement-based materials.

Experimental investigation and numerical modeling on mechanical behavior of highly porous cement-based material: An overview

This paper presents an overview of recent developments in experimental investigation and numerical modeling of highly porous cement-based materials, focusing on traditional Portland cement (PC) and geopolymer foam concrete. It starts by introducing the advantages of using highly porous cement-based materials compared to their dense counterparts and then delves into the properties of PC and geopolymer foam concrete. The first part of the review is dedicated to the experimental investigation, including mixture proportions, foaming methods, mechanical properties and air-void systems. This part also summarizes and discusses various strength prediction models for foam concrete that were drawn from experimental data. The second part of the review is dedicated to the computational methods used to simulate porous materials, and attention is focused on continuum-based and discrete-based approaches. Finally, the failure mechanism of porous cement-based materials is discussed, along with comparing the failure criteria for both dense and porous cement-based materials.

Use of Carbonated Water as Kneading in Mortars Made with Recycled Aggregates

The increased concern about climate change is revolutionising the building materials sector, making sustainability and environmental friendliness increasingly important. This study evaluates the feasibility of incorporating recycled masonry aggregate (construction and demolition waste) in porous cement-based materials using carbonated water in mixing followed (or not) by curing in a CO2 atmosphere. The use of carbonated water can be very revolutionary in cement-based materials, as it allows hydration and carbonation to occur simultaneously. Calcite and portlandite in the recycled masonry aggregate and act as a buffer for the low-pH carbonated water. Carbonated water produced better mechanical properties and increased accessible water porosity and dry bulk density. The same behaviour was observed with natural aggregates. Carbonated water results in an interlaced shape of carbonate ettringite (needles) and fills the microcracks in the recycled masonry aggregate. Curing in CO2 together with the use of carbonated water (concomitantly) is not beneficial. This study provides innovative solutions for a circular economy in the construction sector using carbonated water in mixing (adsorbing CO2), which is very revolutionary as it allows carbonation to be applied to in-situ products.

Microstructure-property relationships in cement mortar with surface treatment of microbial induced carbonate precipitation

Microbial induced calcite precipitation (MICP) can protect the surface of porous cement-based materials and enhance the durability of concrete, the efficiency of which depends on the near surface property. Therefore, this paper is aimed to investigate the effects of bacterial solution to cementation solution ratio (1:2, 1:1 and 2:1) and different calcium sources (calcium chloride, calcium nitrate and calcium acetate) on compressive strength and capillary water absorption coefficient of MICP-treated mortar, and attempted to establish the relationship between microstructure and macroscopic property. The results indicated that the surface treatment by MICP significantly reduced the capillary water absorption coefficient, while it slightly affected the compressive strength of mortar. Supplying calcium source of calcium chloride or calcium nitrate, setting bacterial solution to cementation solution volume ratio of 1:1 were beneficial to improve the compressive strength and reduce the coefficient, in which the CH content of the near surface layer was reduced and the calcite content of that was increased, it showed limited influence on the hydration products of the interior substances. Nanoscale calcite generated in the near surface layer consumed the CH and filled in gel pores to increase the Ca/Si ratio of C–S–H. The increased compressive strength was due to the refined pore structure of the near surface layer, in which the combination effect of improved pore structure of the near surface layer and the precipitation layer of MICP on the surface effectively reduced the coefficients.

The effect of silanes as integral hydrophobic admixture on the physical properties of cement based materials

The paper explores the possibility of using organosilicon compounds (e.g., poly(dimethylsiloxane) and triethoxyoctylsilane) in commercial admixtures as internal hydrophobization agents for porous cement-based materials. The study involved the cement mortar with five different hydrophobic admixtures. Four of them is based on triethoxyoctylsilane, but with various concentration of the main ingredient, and one of them on poly(dimethylsiloxane). Mechanical properties, capillary water absorption, as well as microstructure were investigated. The organosilicon admixtures efficiently decrease the capillary water absorption even by 81% decreasing mechanical strength of cement mortar at the same time even by 55%. Only one admixture, based on poly(dimethylsiloxane) caused significant changes in microstructure of cement mortar.

Kinetics of low radioactive wastewater imbibition and radionuclides sorption in partially saturated ternary-binder mortar

This study seeks to assess the imbibition kinetics of low radioactive wastewater (from the DayaBay nuclear power plant) into a partially saturated ternary-binder mortar, as well as the sorption kinetics of 60Co and 137Cs from the water. Mortar samples with the initial saturation degrees of 0, 0.4, 0.6, 0.8 and 1.0 were prepared for the wastewater treatment. Pore structure of the mortar was characterized using water vapor sorption isotherm and mercury intrusion porosimetry tests interpreted by the Guggenheim-Anderson-de Boer isothermal equilibrium, and volume- and energy-based fractal models. Results show that the mortar has consistent fractal pore structure between the models, and the liquid imbibitions follow the fractal imbibition kinetics, in which the parameters are non-linearly impacted by the initial saturation degrees. The sorption rate and retention capacity of 137Cs are much lower than those of 60Co, and both follow the Brouers–Sotolongo fractional kinetics. The findings uncover the complex liquid imbibition and radionuclides sorption kinetics in cement-based porous materials, and the in-situ data would contribute to the material designs and sorption controls for large scale in-situ treatments of wastewater from nuclear power plant.

Preparation and properties of nanopore-rich cement-based porous materials based on colloidal silica sol

Cement-based porous materials (CPMs) with different dry densities were successfully prepared by using silica sol as pore-forming agent. The rheological behavior of the fresh CPMs was discussed, then pore structure, pore-forming mechanism of silica sol, dry density, compressive strength and thermal conductivity of CPMs were investigated and presented. The results indicate that nanopores in cement paste were formed in-situ by adding silica sol, which led to the optimization of pore structure of CPMs, thereby improving the properties of CPMs. When the substitution of cement matrix by silica sol reached to 95 vol%, CPM with a nanopore volume of 0.675 cc/g, dry density of 488 kg/m3, thermal conductivity of 0.081 W/(m·K) and compressive strength of 0.5 MPa was successfully obtained, showing excellent insulation performance. CPMs with achieved strengths and low thermal conductivities show great potential in the application of semi-structural and structural thermal insulation systems of buildings.

Visualization of mercury percolation in porous hardened cement paste by means of X-ray computed tomography

Mercury intrusion porosimetry (MIP) has been extensively used for the pore structure characterization of porous materials, but the percolation of mercury in porous cement-based materials during MIP has never been comprehensively investigated before. Here we visually probe the mercury percolation in a porous hardened cement paste (HCP) using X-ray computed tomography (X-CT). Novel double surface coatings on the HCP samples before and after MIP tests by an epoxy resin are specifically designed. The first coating enables the one-directional penetration of mercury in the HCP samples during MIP, while the second coating can partially seal the release of the entrapped mercury. MIP tests are operated under different maximum applied pressures (10,000 and 30,000 psi) and equilibrium time (5, 10, and 30 s). The high X-ray attenuation of mercury enables the visual trace of the intrusion mercury percolation pathways and the entrapped mercury clusters in the HCP samples by X-CT. The position-dependent entrapped mercury clusters in the pores account for the gray value changes and gradients in the HCP samples after MIP. The MIP characteristics, such as, total intrusion volume, volume size distribution and mercury entrapment, change with the maximum applied pressure and equilibrium time, but the percolation pore size shows the consistent values. The findings of this work deepen the understandings of MIP for the microstructure characterization of porous materials.

Wood-Inspired Cement with High Strength and Multifunctionality.

Taking lessons from nature offers an increasing promise toward improved performance in man‐made materials. Here new cement materials with unidirectionally porous architectures are developed by replicating the designs of natural wood using a simplified ice‐templating technique in light of the retention of ice‐templated architectures by utilizing the self‐hardening nature of cement. The wood‐like cement exhibits higher strengths at equal densities than other porous cement‐based materials along with unique multifunctional properties, including effective thermal insulation at the transverse profile, controllable water permeability along the vertical direction, and the easy adjustment to be water repulsive by hydrophobic treatment. The strengths are quantitatively interpreted by discerning the effects of differing types of pores using an equivalent element approach. The simultaneous achievement of high strength and multifunctionality makes the wood‐like cement promising for applications as new building materials, and verifies the effectiveness of wood‐mimetic designs in creating new high‐performance materials. The simple fabrication procedure by omitting the freeze‐drying treatment can also promote a better efficiency of ice‐templating technique for the mass production in engineering and may be extended to other material systems.

Reassessment of mercury intrusion porosimetry for characterizing the pore structure of cement-based porous materials by monitoring the mercury entrapments with X-ray computed tomography

Mercury intrusion porosimetry (MIP) is a vital method for assessing the pore structure of cement-based porous materials (CBPMs), but its applicability remains to be clarified. Herein we employed X-ray computed tomography (X-CT) to trace the entrapped mercury in two hardened cement pastes (HCPs) after MIP tests to reassess the method for pore structure characterization. Four different maximum pressures were employed in the MIP tests. Results show that the maximum intrusion pressure had no significant influences on the threshold pore sizes, but impacted the mean pore sizes that were linked to the pore intervals. X-CT can measure the mercury drops entrapped in macro pores. A pore model with the pore structure of thick chamber connected with thin throats was proposed to associate the links between the MIP and X-CT data. The combined uses of MIP and X-CT provide new insights in better understanding MIP results and the possibility for quantitative pore structure characterization of CBPMs.

Microstructural characteristics of sound absorbable porous cement-based materials by incorporating natural fibers and aluminum powder

As transportation on roads and railways has become an essential means enabling the development of cities, residents living near roads and railways face undesirable noise pollution. To reduce this noise pollution, sound absorbable porous cement-based materials have great potential in terms of high sound absorption ability at a wide frequency range. Because sound absorption performance is primarily related to pore characteristics, this study mainly focuses on the development and evaluation of highly porous structures in cement-based materials by incorporating natural fibers and aluminum powder. The fresh and hardened properties of porous cement-based materials are investigated with respect to workability, compressive strength, and water absorption capacity. Furthermore, the porous network in the materials is characterized by microstructural observation using an optical microscope and X-ray computed tomography. A sound absorption test is also conducted to highlight the influences of the material porosity on sound absorption performance. Experimental results indicate that a combination of natural fibers and aluminum powder has a synergistic effect for forming highly porous structures that improve sound absorption performance.

Effects of attapulgite addition on the mechanical behavior and porosity of cement-based porous materials and its adsorption capacity

Cement-based porous materials as the current research hotspots is widely used in various industries because of its high early strength and heat resistance. However, it is difficult to control the reaction rate and the balance between strength and porosity. In this study, attapulgite/magnesium phosphate porous composite materials (AT-MPPCM) with porosity (79.02%), compressive strength (2.87 MPa) and bulk density (0.77 g/cm3) was prepared. The microstructures analysis revealed AT was distributed in the AT-MPPCM framework and coated on the surface to form porous structure. The addition of AT prolonged the casting time to promote the hydration reaction which improved the compressive strength. The AT-MPPCM had the uniformly porous structure with the most probable pore size of about 52 μm and showed selective adsorption performance for ammonia–nitrogen about 73.42% in the simulated solution. Adding AT effectively improved the water resistance of AT-MPPCM. Compared to the traditional adsorption materials, the preparation technology of AT-MPPCM can greatly improve utilization and facilitate collection. As a result, AT-MPPCM has the potential to be used as filtration and adsorption materials to resolve domestic wastewater.

Accelerated carbonation of hardened cement pastes: Influence of porosity

Recently, with the development of accelerating mineral carbonation technology, the preparation of building materials by using carbonation curing cement-based materials has attracted wide attention. The diffusion and transfer mechanism of carbon dioxide in porous cementitious materials was the key point that restricts the improvement of materials carbonation rate. In order to obtain comprehensive information that affects accelerating carbonation rate of porous cement-based materials, this paper talks about the influences of paste porosity controlled by water to cement ratio (w/c) on carbonation and the paste micro-structures evolution. Also, mineral compositions before and after carbonation curing were evaluated. Results show that the carbonation rate, carbonation depth and compressive strength are improved after carbonation, while total porosity of complete carbonation zone decreases with the growth of paste porosity. The pores with diameter larger than 200 nm played a leading role in carbon dioxide diffusion, which were reduced by 84.25% of the total volume after carbonation curing. The carbonization reaction mainly occurred in the first six hours in carbonation curing, while the subsequent carbonation curing were unprofitable to the carbonation effect due to the generation of calcium carbonate prevents the diffusion of CO2, making the subsequent carbonation become difficult. XRD results revealed that during carbonation curing the major reactants were C2S, C3S and some hydration products of Ca(OH)2 after 1 d hydration.

Poroelastic Insights into Stress Dependence of Chloride Penetration into Saturated Cement-Based Porous Materials

Poroelastic Insights into Stress Dependence of Chloride Penetration into Saturated Cement-Based Porous Materials

Tracing mercury entrapment in porous cement paste after mercury intrusion test by X-ray computed tomography and implications for pore structure characterization

Mercury entrapment in a porous material after mercury intrusion porosimetry (MIP) test is widely reported, yet the characterization of the entrapped mercury remains unclear. This study reported an experimental investigation on the entrapped mercury in a porous cement paste with/without surface-conformance control determined by X-ray computed tomography (X-CT). MIP tests were performed under three different maximum intrusion pressures. Results show that the surface-conformance control can substantially depress the first rise of throat size distributions and lower the threshold throat sizes. The entrapped mercury drops can be easily traced by X-CT due to the high mass attenuation coefficient of mercury. Both the entrapped and released mercury drops only occupy the limited volume fraction of the intruded pores. The surface conformance can decrease the mercury volume released from the pores. The findings of this study provide insights in better pore structure characterization of porous cement-based materials.

Oven dying kinetics and status of cement-based porous materials for in-lab microstructure investigation

Oven drying is one of the most commonly used methods to remove water confined in cement-based porous materials (CBPMs) for microstructural investigations. Water removal from porous solids can be governed by moisture diffusion, capillary flow and sorption-based mechanisms, which generate various drying models. Taking into account material types and water to cement ratios, the water removal kinetics and status of hardened cement pastes and mortars were investigated by oven drying at 60°C. Diffusion- and sorption-based drying kinetic models were revisited and employed to capture the drying profiles of CBPMs. The characteristics of drying profiles, model parameters and their relations to the pore structures of CBPMs were analysed. The results show that pore structure is a primary factor impacting drying kinetics and status. For dense porous solids, a long enough drying time is required to obtain a sufficiently dried status and for predicting the equilibrium moisture content by the models. Oven drying at moderate temperature can be an efficient drying method for removing water from a dense porous solid.

Experimental Investigation on Transport Properties of Cement-Based Materials Incorporating 2D Crack Networks

This paper investigates the correlation between the geometry of crack networks and the altered transport properties of cement-based porous materials. Cracks were artificially introduced into slice specimens to obtain bidimensional (2D) crack networks, and the network was characterized by the crack density, orientation, connectivity and crack opening aperture. For the permeability, the water vapor sorption isotherms were measured and an algorithm was established to solve the intrinsic permeability of cracked specimens with the help of moisture transport modeling and the data of drying tests. The electrical conductivity of cracked specimens was measured using an alternative current method. The study on the specimens with percolated cracks shows that: (1) the pertinent geometry parameters for altered transport properties include average-based crack density, crack opening and local crack connectivity; (2) the water permeability of cracked specimens is correlated to the combination $$b^{1.7}\rho f$$ and electrical conductivity to $$b^{0.45}\rho f$$ ; (3) the different exponents on the crack opening/length ratio reflect the resistance of tortuosity of crack paths to the water and current flow and this resistance is stronger for current flow.

Temperature dependency of the thermal conductivity of porous heat storage media

Analyzing the variation of thermal conductivity with temperature is vital in the design and assessment of the efficiency of sensible heat storage systems. In this study, the temperature variation of the thermal conductivity of a commercial cement-based porous heat storage material named – Fullbinder L is analyzed in saturated condition in the temperature range between 20 to 70°C (water based storage) with a steady state thermal conductivity and diffusivity meter. A considerable decrease in the thermal conductivity of the saturated sensible heat storage material upon increase in temperature is obtained, resulting in a significant loss of system efficiency and slower loading/un-loading rates, which when unaccounted for can lead to the under-designing of such systems. Furthermore, a new empirical prediction model for the estimation of thermal conductivity of cement-based porous sensible heat storage materials and naturally occurring crystalline rock formations as a function of temperature is proposed. The results of the model prediction are compared with the experimental results with satisfactory results.

A two-parameter stretched exponential function for dynamic water vapor sorption of cement-based porous materials

Finding a simple analytical model to represent dynamic water-vapor sorption (DWVS) in cement-based porous materials (CBPMs) is a challenging task because the physical–chemical interactions between water molecules and materials are quite complex. This paper presents a two-parameter stretched exponential function called the fractional kinetic (FK) model to represent the DWVS process in CBPMs. The FK model has a flexible and robust formula that is determined by two key parameters: the kinetic order n and the time index $$\alpha$$ . With conditioned n and $$\alpha$$ values, the FK model reduces to the classic exponential model and first- and second-order sorption models. The DWVS data of hardened blended cement-paste samples with different shapes was used to test the applicability of the FK model. The FK model can represent the early sorption data that is generally described by Fick’s diffusion law and the late sorption data that matches the classic exponential model. Sorption of water vapor in CBPMs is a complex time-dependent process that is associated with the interactions between water molecules and the contact surfaces, as well as the microstructures of the cement-based materials.