Optimising confocal Raman microscopy for spectral mapping of cement-based materials

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

Influence of supplementary cementitious materials on microstructure and transport properties of spacer-concrete interface

Reinforcement spacers are a critical component of concrete structures. Their presence affects microstructure and transport properties of concrete cover though this is not widely appreciated. This paper presents the first study to determine whether the negative effects of spacers can be mitigated through the use of supplementary cementitious materials such as silica fume, fly ash and blast-furnace slag. Concrete samples (>200) with different spacers, binders, curing and drying regimes were prepared and tested for diffusion, permeation, absorption, electrical conductivity, carbonation and microstructure. It was found that spacers increase all transport properties, the extent depending on type of spacer, drying regime and transport mechanism. The spacer-concrete interface is weak, porous and micro-cracked, and this lowers the resistance of concrete to ingress of aggressive agents. The beneficial effects of SCMs (strength enhancement and densification) and prolonged curing (120-day) are insufficient to overcome the negative effects of spacers. Implications for durability are discusse

Improving the Spacer-Concrete Interface for Bond Strength and Durability

Spacers are important devices in reinforced concrete that are used to support reinforcing steel during concreting in order to achieve the required concrete cover. They are placed at every meter length or less of steel reinforcement and left permanently in the structure. However, it has been shown that the weak interface between spacer and concrete is highly porous and microcracked. This lowers the resistance of the concrete cover to ingress of aggressive agents causing degradation. This study aims to address this problem by improving spacer design to enhance bond strength and durability of the spacer-concrete interface. Cementitious spacers with a range of surface textures were produced prior to casting into concrete. Samples were prepared with CEM I Portland cement at a water/cement (w/c) ratio of 0.4 and cured for 3, 7, and 28 days in a fog room and then conditioned at 50°C to equilibrium moisture content. The spacer-concrete interface was then tested for tensile bond strength and mass transport properties including oxygen diffusivity, oxygen permeability, water absorption and electrical conductivity. The results were compared against control samples made of either the entire cementitious spacer or concrete. The measured surface properties were then correlated to the measured bond strength and transport properties to establish the effects of surface texture on the spacer-concrete interface

Estimating transport properties of mortars using image analysis on backscattered electron images

Abstract 8 The pore structure of two ordinary Portland cement mortars at water-cement ratio of 0.35 and 0.70 was 9 characterised using quantitative backscattered electron imaging. The mortars were cured and conditioned to 10 produce a range of pore structure characteristics. Image analysis was used to characterise the pore structure in 11 terms of simple morphological parameters such as resolvable porosity and the specific surface area. These were 12 found to be correlated to measured transport coefficients (diffusivity, permeability and sorptivity), suggesting the 13 feasibility of image analysis to derive valuable quantitative information describing the pore structure that can be 1

Microscopy techniques for determining water-cement (w/c) ratio in hardened concrete: A round-robin assessment

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

Estimating transport properties of mortars using image analysis on backscattered electron images

Abstract 8 The pore structure of two ordinary Portland cement mortars at water-cement ratio of 0.35 and 0.70 was 9 characterised using quantitative backscattered electron imaging. The mortars were cured and conditioned to 10 produce a range of pore structure characteristics. Image analysis was used to characterise the pore structure in 11 terms of simple morphological parameters such as resolvable porosity and the specific surface area. These were 12 found to be correlated to measured transport coefficients (diffusivity, permeability and sorptivity), suggesting the 13 feasibility of image analysis to derive valuable quantitative information describing the pore structure that can be 1

Discussion with end-users regarding cement/waste systems database and predictive models

Introduction of impurities into cement is inherent to recycling of industrial by-products in building materials, and to treatment of industrial wastes by solidification prior to disposal. These impurities can cause problems with materials handling and durability. In this respect, 3-year project is funded under the European Commission's Brite-Euram III programme, on Neural Network Analysis for Prediction of Interactions in Cement/Waste Systems (NNAPICS).This project applies neural networks to predict the physical and environmental properties of cement/waste systems. Existing data concerning solidified wastes and building materials containing industrial by-products are being collected into a database and analysed using neural networks, and supplementary data are being generated in a laboratory study. Thus, the main deliverables from the project will be: (1) a database containing information about cement/waste systems (the “CWS Database”), and (2) predictive models constructed using the information in the database.L'introduction d'impuretés dans les ciments est inhérente au recyclage des sous-produits industriels en matériaux de construction ainsi qu'au traitement de déchets industriels par solidification avant mise en décharge. Ces impuretés peuvent engendrer des problèmes liés à la manipulation et à la pérennité des matériaux. Pour étudier et prévenir ces problèmes, le Programme Brite-Euram III de la Commission Européenne, finance un projet en 3 ans sur l'Analyse par Réseau Neuronal pour la Prévision des Interactions au sein des Systèmes Ciment/Déchet (NNAPICS).Ce projet utilise les réseaux neuronaux afin de prévoir les propriétés physiques et environnementales des systèmes Ciment/Déchet. Des données déjà existantes sur les déchets solidifiés et les matériaux de construction contenant des sous-produits industriels sont collectées afin de constituer une base de données ; elles sont ensuite analysées en utilisant les réseaux neuronaux et complétées par les résultats d'une étude en laboratoire. Ainsi, les principales productions de ce projet seront : (1) une base de données contenant des informations sur les systèmes Ciment/Déchet (Cement/Waste Systems « CWS Database ») et (2) des modèles de prévision construits à partir des informations de la base de données