Revealing the dark side of Portlandite Clusters in cement paste by circular polarization microscopy

Plane and crossed polarization are the two standard light modes in polarized light microscopy that are widely used to characterize crystalline and amorphous phases in cement-based materials. However, the use of the crossed polarized light mode has been found to be restrictive for studying birefringent phases quantitatively due to the extinction phenomenon that arises depending on the crystal orientation. This paper introduces circular polarization microscopy as an alternative technique to overcome the extinction problem during the examination of cementitious materials’ microstructure with optical microscopy. In order to evaluate the feasibility of this technique, selected optical and micromorphological features of portlandite clusters were investigated in cement paste. Image analysis results showed that compared to the conventional crossed polarization technique, circular polarization offers significant advantages when portlandite quantification is of interest, and it stands out as a promising low-cost alternative to backscattered electron microscopy.Structural EngineeringCivil Engineering and Geoscience

The characterisation, improvement and modelling aspects of Frost Salt Scaling of Cement-Based Materials with a High Slag Content

Blast furnace slag cement concrete is used extensively in a number of countries. In comparison with OPC, it is particularly well known for its excellent performance in marine environments. One dis-advantage of slag cement is its vulnerability to scaling under the combined load of freezing-thawing and de-icing salts. The current investigation was triggered by positive observations regard-ing certain grinding agents used in slag cement production to improve frost salt scaling resistance. The investigation was aimed at explaining the cause of this improvement, at finding alternative methods to improve scaling resistance and at developing a model that would be suitable for the simulation of frost salt scaling behaviour. The investigation conclusions are essentially confined to high slag cement, particularly type CEM III 42,5/B which has a 67% granulated slag content. The w/c ratio of the paste, mortar and concrete specimens is generally maintained at 0,45. Carbonation, known as the critical parameter in frost salt scaling, constituted the key area of inter-est. From previous investigations it is known that carbonation increases porosity and coarsens the pore system in slag cement paste while it actually does the reverse in OPC paste. In the light of lit-erature a new hypothesis has been suggested that the transition zones, which are the weakest points in normal-performance cement-based materials, critically determine frost salt scaling resis-tance. These zones are even more indicative in the case of slag cement pastes because of the sig-nificant amount of transition zones that can be weakened by carbonation unlike with low-slag ce-ment or OPC pastes. In the present investigation it was observed that carbonation causes significant slag cement paste shrinkage. It was especially the transition zones between non-reacted slag particles and hydration products that were found to be affected. Consequently this process leads to the paste having a coarser pore structure thus making it prone to greater water uptake when compared to non-carbonated slag cement paste or OPC paste. The new hypothesis was supported by findings emerging from the ESEM study. It was observed that frost salt scaling attack generates cracks in the microstructure which adhere to slag-matrix interfacial zones. This was confirmed by nano-indentation tests which demonstrated that carbonation creates a significant number of weak zones in the slag cement paste. In the case of OPC paste the picture that emerged was quite different. Natural air carbonation influences the mineral characteristics of cement pastes. The XRD study re-vealed that both slag cement paste and OPC paste possess various types of carbonate minerals, namely: calcite, aragonite and vaterite. However, accelerated carbonation creates overwhelmingly stable calcite phases in both types of cement pastes which are subsequently transformed from me-tastable carbonates. This observation draws attention to the role played by Ca(OH)2 in the good scaling resistance of OPC or low-slag cement systems. A curing regime, especially curing in lime water, appears to be favourable for slag cement materials. However, when compared to the effect of carbonation, the influence that the curing water quality has on scaling resistance is minimal. The contribution made by prolonged water curing to scaling resistance could have been greater but, as it was, the curing periods were limited to 5 weeks in the interests of remaining realistic and practi-cal. The main goal of the project was to investigate the improvements in frost salt scaling resistance in-stigated by chemical grinding agents on the basis of the various positive results gained from the preliminary tests. The intention was to study the effects that the chemicals had on the cement paste microstructure in order to understand frost salt scaling resistance in slag cement concrete and so as to contribute to structural improvements in that area. A microstructural comparative study was carried out on slag cement pastes that contain alkanola-mines/hydrocarboxylates (the best performing ones) and diethylene glycolâbased (the worst per-forming example) grinding agents. The most notable difference was in the pore structure of the paste samples. Alkanolamines/hydrocarboxylates-based grinding agents were found to produce smaller pore sizes when compared to the ones containing diethylene glycol. This is consequently likely to give rise to higher carbonation resistance, lower water uptake and, eventually, to higher frost salt scaling resistance. However, the improvement achieved by alkanolamines / hydrocarboxy-lates is not sufficient to enhance the scaling resistance of the slag cement materials investigated in similar detail to OPC pastes. Another technique that was investigated was sodium monofluorophosphate (Na-MFP) surface treatment. Remarkable improvements in frost salt scaling resistance were achieved by applying a 10% Na-MFP solution to the surface of the carbonated slag cement paste and concrete. The scal-ing resistance improved by about 95% after 7 freeze-thawing cycles. Evidence was found pertain-ing to the reaction between Na-MFP and metastable carbonates in the carbonated slag pastes. The application appears to significantly increase the tensile strength of the carbonated slag cement paste which is extremely favourable in terms of scaling resistance. The study finally resulted in the development of a new integrated model. The model mainly takes into account the glue-spall theory and the hypothesis developed in this thesis and it runs on the Delft Lattice Model platform. The model successfully demonstrates the experimental observations and the crack patterns created by the scaling action. The glue-spall theory suggests that cement-based material surface scaling derives from external ice layer cracking due to further cooling. Cooling consequently generates tensile stress due to the shrinkage of ice and causes cracking when the stress exceeds the tensile strength of the ice. This theory can explain many phenomena including the pessimum effect. On the basis of this theory, the new integrated model proved to be capable of simulating two important experimental observa-tions. Under identical conditions the model can predict higher surface scaling at a 3% salt concen-tration level in relation to higher and lower values. The effect of ice layer thickness is furthermore found to be crucial with respect to frost salt scaling. Under identical material and environmental conditions the thicker external ice layer creates more damage than thinner ice layers. This observa-tion was also successfully demonstrated with the new integrated model.Civil Engineering and Geoscience

EMABM 2015: Proceedings of the 15th Euroseminar on Microscopy Applied to Building Materials, Delft, The Netherlands, 17-19 June 2015

Structural EngineeringCivil Engineering and Geoscience

Optimizing self-healing mechanism of slag cement concrete under natural carbonation (poster abstract)

Structural EngineeringCivil Engineering and Geoscience

Modeling of expansion and cracking due to ASR with a 3D lattice model

It is generally possible to consider modeling of ASR damage in concrete in two main groups: modeling of gel formation and its expansion; modeling of ASR related damage. In this paper, authors take an attempt to combine both: simulating the correct crack formation and the connected concrete expansion. It is aimed to simulate ASR damage in a cementitious material bearing reactive aggregates. The model that is used is a 3D lattice type model. It models concrete on a meso-scale in which particles embedded in a cement matrix are taken into account. The particle structure is obtained by CT-scanning of samples. With the model the concrete expansion can be simulated. One of the inputs in the model is the local expansion of the gel. For that the mechanical properties of the gel should be known, which are obtained from an experimental procedure developed by the authors.Structural EngineeringCivil Engineering and Geoscience

The interaction of pH, pore solution composition and solid phase composition of carbonated blast furnace slag cement paste activated with aqueous sodium monofluorophosphate

Blast Furnace Slag (BFS) is a waste product of industrial steel production and a common additive in the cement industry in Northern European countries. However, cementitious materials made from slag-rich cement, particularly CEM III /B, are very susceptible to carbonation. Recent investigations have shown that the surface application of aqueous sodium monofluorophosphate (Na-MFP) as pre- and post-carbonation treatment can improve the surface durability of cementitious materials with a high BFS content. Significant improvements have been observed in the micro-mechanical characteristics of concrete surface and frost salt scaling resistance. On the basis of previous studies we are investigating self-healing of blast furnace slag cement (BFSC) products treated with the inorganic self-healing agent Na-MFP from a mineralogical point of view. In this study we combine results of pore solution pH analyses and main element composition under the influence of Na-MFP with the presence of crystalline phases found in the solid aliquot of the samples. Pore solutions were investigated by inductively coupled optical emission spectrometry (ICP-OES). Solid-material investigation was performed by X-ray powder diffractometry, including Rietveld quantitative phase analyses. Our results show that the element concentration and the pH of the paste pore solutions have direct influence on the formation of crystalline and amorphous phases forming in the solid sample aliquot during carbonation and self-healing by Na-MFP. In this work we focus especially on the influence of sulfur in solution and the formation of ettringite. In addition we discuss, why the formation of the crystalline phosphate apatite does not occur in cementitious products after Na-MFP treatment.Structural EngineeringCivil Engineering and Geoscience

Interactive mathematical model of self-healing in carbonated cementitious materials

A mathematical model for the post-damage recovery of carbonated cement is described. The model is based on a two-dimensional initial-boundary value problem for a system of partial differential equations. The study is embedded within the framework of investigating the effect of using lightweight expanded clay aggregate, which is incorporated into the impregnation of the sodium monofluorophosphate (Na-MFP) solution. The model of the self-healing process is built under the assumption that the position of the carbonation front changes in time. Here the rate of diffusion of Na-MFP into the carbonated cement matrix and the reaction rates of the free phosphate and fluorophosphate with the components of the cement are comparable to the speed of the carbonation front. The main modeling results presented in the paper are related to the autonomous improvement of the structure of carbonated blast furnace slag cement. Nevertheless, the parameters of the model can be set for various types of cement paste and different conditions of the healing process (including both carbonation under natural conditions and accelerated carbonation). The governing diffusion-reaction equations are solved using a Galerkin finite-element method. For usage convenience, the model is completed as an interactive application on the basis of computer algebra system Mathematica

Application of encapsulated lightweight aggregate impregnated with sodium monofluorophosphate as a self-healing agent in blast furnace slag mortar

This paper studies the potential of using expanded clay lightweight aggregate impregnated with sodium monofluorophosphate (Na2FPO3) solution which is eventually encapsulated by a cement paste layer to produce a self-healing system in blast furnace slag cement mortars. It was found that the technique significantly improved the quality of the interfacial transition zone in mortars subjected to carbonation shrinkage. Consequently the frost salt scaling durability of blast furnace slag mortars was evidently promoted. The findings from ESEM and EDS studies showed that the healing mechanism would be due to the combination of treatment by Na2FPO3 solution and calcium hydroxide supplied from the cement paste coating layer

Surface crack self healing behaviour of mortars with expansive additives (abstract)

Structural EngineeringCivil Engineering and Geoscience

Semi- and full quantitative EDS microanalysis of chlorine in reinforced mortars subjected to chloride ingress and carbonation

Energy dispersive X-ray spectrometry (EDS) is a powerful tool for research studies on building materials. Elemental quantification in cementitious phases contained in the concrete microstructure can be performed at an excellent spatial resolution. However, accurate compositional quantification requires a standard-based analysis, whereas currently the most common are standardless microanalysis. Reasons behind this approach lie on the difficulty of finding appropriate microanalysis standards in EDS studies. In standard-less analyses, elemental quantification from an EDS spectrum provides normalized quantification, i.e. an analytical total of 100%. Most EDS detectors cannot provide reliable data for elements with smaller atomic number than sodium, which results in the stoichiometric determination of oxygen. The aim of this paper is raise awareness about the importance of a controlled experimental parameters and the use of mineral standards when performing EDS analysis on cementitious materials. For this purpose, both types of investigation (with and without standards) are performed simultaneously in order to obtain chloride profiles in six reinforced mortar specimens subjected to chloride ingress and carbonation. Results illustrate that differences in element concentrations can be derived by the absence of mineral standards which could lead to inaccurate interpretations of element concentrations such as chlorine.Structural EngineeringCivil Engineering and Geoscience