Imperial College London


Faculty of EngineeringDepartment of Civil and Environmental Engineering

Research Associate



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239Skempton BuildingSouth Kensington Campus





Publication Type

19 results found

L'Hermitte A, Azzan H, Yio MHN, Rajagopalan AK, Danaci D, Hirosawa T, Isobe T, Petit Cet al., 2023, Effect of surface functionalization on the moisture stability and sorption properties of porous boron nitride, Microporous and Mesoporous Materials, Vol: 352, ISSN: 1387-1811

Porous boron nitride (BN) is a promising adsorbent owing to its high surface area and porosity, as well as thermal and oxidative stability. It has been explored in the past decade for applications in gas and liquid separations, such as CO2 capture and water cleaning. However, the material has displayed hydrolytic instability. Owing to the presence of moisture in most industrial settings, whether it is for storage or cyclic adsorption processes, ensuring the moisture stability of an adsorbent is crucial. While this topic has been researched for other adsorbents such as zeolites and metal organic frameworks (MOFs), little is known on controlling the hydrolytic stability of porous BN. In this study, we propose a method to enhance porous BN's hydrolytic stability via surface functionalization using a fluoroalkylsilane. We explored two different routes of functionalization: (i) functionalization of porous BN powder followed by pelletization (route 1) and (ii) coating of porous BN pellets with fluoroalkylsilane (route 2). Spectroscopic, analytical and imaging techniques confirmed the functionalization process qualitatively and quantitatively. We subjected the functionalized samples to moisture exposure at 54% RH (similar to common storage conditions) and 92% RH (similar to flue gas stream conditions with high moisture content), and characterized them to probe their resistance to moisture. We also investigated their equilibrium and kinetic sorption properties in the context of CO2/N2 separation. Both routes produced materials with enhanced moisture stability. However, we noted differences between both functionalization routes. Route 2 produced a sample with a higher grafting yield and hydrophobic nature, and therefore better resistance to moisture exposure than route 1. From a sorption point of view, despite reduced porosity, the functionalized samples maintain reasonable CO2 uptakes. The functionalization led to changes in the textural features of the samples, which cause

Journal article

Wang J, Yio MHN, Zhou T, Wong HS, Davie CT, Masoero Eet al., 2023, Water sorption isotherms and hysteresis of cement paste at moderately high temperature, up to 80 °C, Cement and Concrete Research, Vol: 165, Pages: 1-12, ISSN: 0008-8846

The constitutive models of concrete often consider water desorption isotherms to be near-equilibrium andsignificantly affected by moderately high temperature, 40–80◦C, typically through microstructural changes.However literature data suggest that adsorption, not desorption, is near-equilibrium and moderate temperaturesdo not cause microstructural changes. This work supports the latter theory, through dynamic vapor sorptionexperiments on cement paste at 20–80◦C. Samples were pre-conditioned at 60% relative humidity and 20◦C,and isotherms were measured for several humidity ranges and testing rates. The results, corroborated byclassical DFT simulations, indicate that adsorption is near-equilibrium and mostly unaffected by temperature,whereas desorption is out-of-equilibrium due to the ink-bottle effect at high humidity, and interlayer waterat low humidity. Starting from the second cycle, desorption at higher temperatures features a shift of thecavitation pressure and overall a smaller hysteresis. A conceptual model of pore-specific temperature-dependenthysteresis is proposed to qualitatively explain the results.

Journal article

Yio MHN, Ho YW, Abdul Wahid F, Wong H, Buenfeld NRet al., 2022, Analysis of cement paste and aggregate content of concrete using micro X-ray fluorescence, Magazine of Concrete Research, Vol: 74, Pages: 889-904, ISSN: 0024-9831

A new method for determining the cement paste, fine aggregate and coarse aggregate content of hardened concrete using micro X-ray fluorescence (μXRF) is presented. The method involves mathematical and morphological operations to extract aggregate particles from large-area (100 x 50 mm2) composite element maps acquired with μXRF. The method was tested on five concretes containing different types of aggregates including gravel, limestone, siliceous sand, and sintered lightweight aggregates. The results were compared against point count analysis and data from the actual mix design. The average errors in relation to the mix design for the measured fine aggregate, coarse aggregate, total aggregate, cement paste contents and fine/coarse aggregate ratio were 12%, 12%, 3.4%, 10%, and 27% respectively. All measured values fell within ±4% of those point-counted. An image size of > 2000 mm2 (at least five times maximum aggregate size) was found to be required to obtain representative results. The advantages and limitations of the proposed method are discussed.

Journal article

Zhang K, Yio M, Wong H, Buenfeld Net al., 2022, Optimising confocal Raman microscopy for spectral mapping of cement-based materials, Materials and Structures, Vol: 55, Pages: 1-18, ISSN: 1359-5997

Raman spectroscopy combined with confocal imaging, i.e. confocal Raman microscopy (CRM) is a relatively new technique with huge potential for high-resolution chemical mapping of phase composition and spatial distribution in cement-based materials. However, the effects of sample preparation and various operating parameters on mapping quality has not been systematically studied. This paper optimises CRM for spectral mapping of carbonated and non-carbonated cement-based materials. The effects of sample preparation and scanning parameters on the detection of four main phases (calcite, portlandite, ettringite and unreacted cement) were investigated. Results show that although freshly cut cementitious samples can be analysed as-is, the Raman signals improve with short gentle drying and surface grinding/polishing prior to analysis. Increasing laser power, exposure time and scan accumulation, and short laser wavelength yields higher signal-to-noise (SNR) ratio in the obtained spectrum. The use of a 4.15 mW laser power, 2 s exposure time and scan accumulation of 2 with 532 nm laser represents a good operating condition for Raman analysis of cement-based materials. This produces SNR > 10 for all investigated phases at short testing time and low risk of laser-induced damage. Microcracking caused by localised heating during closely-spaced mapping can be limited by impregnating the sample with epoxy to protect the microstructure. We show for the first time that CRM can be used to quantify the volume fraction of calcium carbonate and portlandite at high resolution when combined with SEM. The advantages and limitations of CRM for mapping cement-based materials are discussed.

Journal article

Zhang K, Yio M, Wong H, Buenfeld Net al., 2022, Real-time monitoring of carbonation of hardened cement pastes using Raman microscopy, Journal of Microscopy, Vol: 286, Pages: 126-133, ISSN: 0022-2720

This study investigated the feasibility of Raman microscopy for monitoring early surface carbonation of hardened cement pastes in real time for up to 7 days. Samples were exposed to natural carbonation (440 ppm CO2) and accelerated carbonation (4% CO2), and the evolution of calcium carbonate (CaCO3) polymorphs, portlandite, ettringite, C-S-H gel and unreacted cement particles was followed. Results showed that calcite is the main polymorph formed under both natural and accelerated carbonation. Under accelerated carbonation, the formation of calcite on the sample surface completed within 1 day whereas under natural carbonation, the formation of calcite is expected to continue beyond 7 days. The contents of portlandite and ettringite decreased rapidly under accelerated carbonation but much more gradually under natural carbonation. However, calcium silicate minerals in unreacted cement particles remained unchanged throughout the carbonation processes. Overall, this study demonstrated that Raman microscopy is a valuable tool for non-destructive real-time imaging of surface carbonation in cement-based materials.

Journal article

Yio MHN, Mac MJ, Yeow YX, Wong H, Buenfeld NRet al., 2021, Effect of autogenous shrinkage on microcracking and mass transport properties of concrete containing supplementary cementitious materials, Cement and Concrete Research, Vol: 150, ISSN: 0008-8846

It is well-known that supplementary cementitious materials (SCMs) and low water-to-binder (w/b) ratio increase autogenous shrinkage, but the impact on microcracking and long-term transport properties is less understood. This paper examines the effect of microcracking induced by autogenous shrinkage on transport properties of concretes cured up to ~3.6 years. Variables include SCM type (9% SF, 70% GGBS), w/b ratio (0.20–0.45), maximum-aggregate-size (MSA: 5–20 mm) and shrinkage reducing admixture (SRA). Oxygen diffusivity, permeability and water sorptivity were correlated with microcracks characterised using laser scanning confocal microscopy and 3D X-ray microtomography. Results show greater microcracking in mixes containing SCMs, low w/b ratio and large MSA. At the same w/b ratio and binder type, strong positive correlations are observed between transport and microcracking with increasing MSA, confirming the negative impact of autogenous shrinkage. SRA was effective in reducing these effects. The significance is compared with drying shrinkage and implications for durability are discussed.

Journal article

Mac M, Yio MHN, Wong H, Buenfeld NRet al., 2021, Analysis of autogenous shrinkage-induced microcracks in concrete from 3D images, Cement and Concrete Research, Vol: 144, Pages: 1-15, ISSN: 0008-8846

A new image analysis procedure for quantifying microcracks from three-dimensional (3D) X-ray microCT images of concrete is presented. The method separates microcracks from air voids and aggregates by combining filtering and morphological operations. It was applied to study the effects of supplementary cementitious materials (SCMs) and curing age on autogenous shrinkage-induced microcracks in low w/b ratio concretes, and to determine the representative elementary volume (REV) for various properties of microcracks and air voids. Results showed that slag and silica fume significantly increased autogenous shrinkage and related microcracking. These SCMs increased volume fraction, width, length, dendritic density, anisotropy, and connectivity of microcracks, but decreased specific surface and tortuosity. Similar trends were observed with age. Comparison between 3D and 2D measurements was made. REV analysis showed that a sampling volume of ~20 × 20 × 25 mm3 is sufficient for characterising most parameters of autogenous shrinkage microcracks and air voids in concrete.

Journal article

Yio M, Yue X, Ji R, Russell M, Cheeseman Cet al., 2021, Production of foamed glass-ceramics using furnace bottom ash and glass, Ceramics International, Vol: 47, Pages: 8697-8706, ISSN: 0272-8842

This research has produced foamed glass-ceramics from coal fired power station furnace bottom ash (FBA) and soda-lime-silica glass. The as-received FBA was wet milled with different additions of glass. The resultant slurry was dried and formed into a powder. The powder was pressed and sintered at a range of temperatures with additions of a fluxing agent (sodium tetraborate decahydrate), a bubble stabilising additive (tri-sodium phosphate) and a bloating agent (calcium carbonate) and this produced foamed FBA-glass-ceramics. The effect of glass content and sintering temperature on the properties of the sintered ceramic foams are reported. A range of potential applications including thermal insulation and biological filters for water and wastewater treatment are discussed. The research demonstrates that it is possible to engineer the properties of FBA derived glass-ceramic foams by careful control of the composition and processing conditions in order to transform a problematic waste into commercially interesting materials.

Journal article

Mac M, Yio M, Desbois G, Casanova I, Wong H, Buenfeld Net al., 2021, 3D imaging techniques for characterising microcracks in cement-based materials, Cement and Concrete Research, Vol: 140, ISSN: 0008-8846

Concrete inherently contains pores and microcracks that can adversely impact its mechanical properties and long-term durability. However, characterising microcracks is difficult due to their complex, multiscale and three-dimensional (3D) nature. This paper presents an evaluation of 3D imaging techniques for characterising microcracks induced by different mechanisms. Seven cement pastes, mortars and concretes subjected to drying shrinkage, autogenous shrinkage and freeze-thaw cycles were investigated using focused ion beam nanotomography (FIB-nt), broad ion beam serial section tomography (BIB-SST), laser scanning confocal microscopy (LSCM) combined with serial sectioning and X-ray microtomography (μCT). The study shows that the characteristics of microcracks vary significantly depending on exposure conditions. Yet there is no single technique that can capture the entire size range of microcracks from sub to tens of μm within a sufficiently representative sampling volume. The achievable image volume and resolution, and the advantages and disadvantages of each technique are compared and discussed.

Journal article

Wong H, Poole AB, Wells B, Eden M, Barnes R, Ferrari J, Fox R, Yio MHN, Copuroglu O, Guðmundsson G, Hardie R, Jakobsen UH, Makoubi K, Mitchinson A, Raybould P, Strongman J, Buenfeld Net al., 2020, Microscopy techniques for determining water-cement (w/c) ratio in hardened concrete: A round-robin assessment, Materials and Structures, Vol: 53, ISSN: 1359-5997

Water to cement (w/c) ratio is usually the most important parameter specified in concrete design and is sometimes the subject of dispute when a shortfall in concrete strength or durability is an issue. However, determination of w/c ratio in hardened concrete by testing is very difficult once the concrete has set. This paper presents the results from an inter-laboratory round-robin study organised by the Applied Petrography Group to evaluate and compare microscopy methods for measuring w/c ratio in hardened concrete. Five concrete prisms with w/c ratios ranging from 0.35 to 0.55, but otherwise identical in mix design were prepared independently and distributed to 11 participating petrographic laboratories across Europe. Participants used a range of methods routine to their laboratory and these are broadly divided into visual assessment, measurement of fluorescent intensity and quantitative backscattered electron microscopy. Some participants determined w/c ratio using more than one method or operator. Consequently, 100 individual w/c ratio determinations were collected, representing the largest study of its type ever undertaken. The majority (81%) of the results are accurate to within ± 0.1 of the target mix w/c ratios, 58% come to within ± 0.05 and 37% are within ± 0.025. The study shows that microscopy-based methods are more accurate and reliable compared to the BS 1881-124 physicochemical method for determining w/c ratio. The practical significance, potential sources of errors and limitations are discussed with the view to inform future applications.

Journal article

Yio MHN, Wong H, Buenfeld N, 2019, 3D pore structure and mass transport properties of blended cementitious materials, Cement and Concrete Research, Vol: 117, Pages: 23-37, ISSN: 0008-8846

The effect of supplementary cementitious materials on three-dimensional pore structure and how this influences mass transport properties are not well understood. This paper examines the effect of silica fume, fly ash and ground granulated blastfurnace slag on 3D structure of capillary pores (>0.24 μm) within 1003 μm3 cement paste for the first time using laser scanning confocal microscopy, combined with backscattered electron imaging and mercury intrusion porosimetry. Pastes containing different binder types, w/b ratios and curing ages were tested. Results show that SF enhances 3D pore structure from early ages whereas PFA and GGBS show improvements at later ages. SCMs not only reduce the volume and size of accessible pores, but also decrease connectivity and increase tortuosity, pore coordination number and formation factor. Measured 3D pore parameters were used as modelling inputs to estimate diffusivity and permeability. Predictions to within a factor of five from measured values were obtained.

Journal article

Yio MHN, Wong HS, Buenfeld NR, 2017, Representative elementary volume (REV) of cementitious materials from three-dimensional pore structure analysis, Cement and Concrete Research, Vol: 102, Pages: 187-202, ISSN: 0008-8846

The representative elementary volume (REV) is a fundamental property of a material, but no direct measurements exist for cementitious materials. In this paper, the REV of cement pastes with supplementary cementitious materials (GGBS, PFA, SF) was determined by analysing the three-dimensional pore structure (> 0.2 μm) using laser scanning confocal microscopy (LSCM). The effect of axial distortion inherent to LSCM on 3D pore structure was also investigated. A range of 3D pore parameters was measured using skeletonisation, maximal ball and random walker algorithms. Results show that axial distortion has insignificant effects on most parameters except Euler connectivity, average pore and throat volumes and directional diffusion tortuosities. Most pore parameters become independent of sampling volume at ≈ 603 μm3 except diffusion tortuosities and formation factor. The REV for porosity calculated based on a statistical approach at eight realisations and 5% relative error was found to be ≈ 1003 μm3.

Journal article

Yio M, 2017, Characterising the microstructure of cement-based materials using laser scanning confocal microscopy

Thesis dissertation

Yio MHN, Wong HS, Buenfeld NR, 2016, 3D Monte Carlo simulation of backscattered electron signal variation across pore-solid boundaries in cement-based materials, Cement and Concrete Research, Vol: 89, Pages: 320-331, ISSN: 0008-8846

Three-dimensional (3D) Monte Carlo simulation was used to study the variation of backscattered electron (BSE) signal across pore-solid boundaries in cement-based materials in order to enhance quantitative analysis of pore structure. The effects of pore size, depth and boundary inclination angle were investigated. It is found that pores down to 1 nm can generate sufficient contrast to be detected. Visibility improves with larger pore size, smaller beam probe size and lower acceleration voltage. However, pixels in shallow pores or near pore boundaries display higher grey values (brightness) than expected due to sampling sub-surface or neighbouring solid material. Thus, cement-based materials may appear less porous or the pores appear smaller than they actually are in BSE images. Simulated BSE images were used to test the accuracy of the Overflow pore segmentation method. Results show the method is generally valid and gives low errors for pores that are 1 μm and greater.

Journal article

Yio MHN, Wong HS, Buenfeld NR, 2016, 3D Pore Structure Characterisation of Blended Cement Pastes using Laser Scanning Confocal Microscopy, Young Researchers’ Forum III, Innovation in Construction Materials

Conference paper

Yio MHN, Mac MJ, Wong HS, Buenfeld NRet al., 2015, 3D imaging of cement-based materials at submicron resolution by combining laser scanning confocal microscopy with serial sectioning, Journal of Microscopy, Vol: 258, Pages: 151-169, ISSN: 0022-2720

In this paper, we present a new method to reconstruct large volumes of nontransparent porous materials at submicron resolution. The proposed method combines fluorescence laser scanning confocal microscopy with serial sectioning to produce a series of overlapping confocal z-stacks, which are then aligned and stitched based on phase correlation. The method can be extended in the XY plane to further increase the overall image volume. Resolution of the reconstructed image volume does not degrade with increase in sample size. We have used the method to image cementitious materials, hardened cement paste and concrete and the results obtained show that the method is reliable. Possible applications of the method such as three-dimensional characterization of the pores and microcracks in hardened concrete, three-dimensional particle shape characterization of cementitious materials and three-dimensional characterization of other porous materials such as rocks and bioceramics are discussed.

Journal article

Yio MHN, Wong HS, Buenfeld NR, 2014, Fluorescence laser scanning confocal microscopy for real-time imaging of early cement hydration, 15th Euroseminar on Microscopy Applied to Building Materials

Conference paper

Yio MHN, Phelan JC, Wong HS, Buenfeld NRet al., 2014, Determining the slag fraction, water/binder ratio and degree of hydration in hardened cement pastes, Cement and Concrete Research, Vol: 56, Pages: 171-181, ISSN: 0008-8846

A method for determining the original mix composition of hardened slag-blended cement-based materials based on analysis of backscattered electron images combined with loss on ignition measurements is presented. The method does not require comparison to reference standards or prior knowledge of the composition of the binders used. Therefore, it is well-suited for application to real structures. The method is also able to calculate the degrees of reaction of slag and cement. Results obtained from an experimental study involving sixty samples with a wide range of water/binder (w/b) ratios (0.30 to 0.50), slag/binder ratios (0 to 0.6) and curing ages (3 days to 1 year) show that the method is very promising. The mean absolute errors for the estimated slag, water and cement contents (kg/m3), w/b and s/b ratios were 9.1%, 1.5%, 2.5%, 4.7% and 8.7%, respectively. 91% of the estimated w/b ratios were within 0.036 of the actual values.

Journal article

Yio MHN, Stovin V, Werdin J, Vesuviano Get al., 2013, Experimental analysis of green roof substrate detention characteristics, Water Science and Technology, Vol: 68, Pages: 1477-1486, ISSN: 0273-1223

Green roofs may make an important contribution to urban stormwater management. Rainfall-runoff models are required to evaluate green roof responses to specific rainfall inputs. The roof's hydrological response is a function of its configuration, with the substrate – or growing media – providing both retention and detention of rainfall. The objective of the research described here is to quantify the detention effects due to green roof substrates, and to propose a suitable hydrological modelling approach. Laboratory results from experimental detention tests on green roof substrates are presented. It is shown that detention increases with substrate depth and as a result of increasing substrate organic content. Model structures based on reservoir routing are evaluated, and it is found that a one-parameter reservoir routing model coupled with a parameter that describes the delay to start of runoff best fits the observed data. Preliminary findings support the hypothesis that the reservoir routing parameter values can be defined from the substrate's physical characteristics.

Journal article

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