Long-term mitigating effect of lithium nitrate on delayed ettringite formation and ASR in concrete – microscopic analysis

Abstract: This paper presents long-term results and microscopic analyses for a six-year experimental study on the effectiveness of lithium nitrate in mitigating delayed ettringite formation (DEF) with or without alkali-silica reaction (ASR). In the previous publication [1], interesting findings were found showing that lithium nitrate is effective in controlling DEF or ASR-DEF mechanisms in concretes. In the present study, microstructural features associated with microcracking and ettringite infilling showed relatively reduced intensity of internal distress and damage when lithium nitrate was admixed in DEF expansive concrete. Microanalysis done using plots of Al/Ca and S/Ca shows that, the use of lithium nitrate leads to formation of non-expansive ettringite similar to normal early-age ettringite formed in moist-cured concretes. These observations give some insights into the mechanism responsible for mitigation of DEF by lithium nitrate

MAT-759: PARTICLE SIZE ANALYSIS AS A MEANS TO BETTER UNDERSTAND THE INFLUENCE OF FLY ASH VARIABILITY IN CONCRETE

Fly ash is generated from thermal power stations as an industrial by-product of coal combustion materials. Its particles are generally glassy, spherical in shape, and typically range in size from 0.5-300 µm. Coal fly ash is widely used as a partial cementitious material in concrete, which not only offers economic and environmental benefits but also improves concrete performance. However, variability of the physical description and chemical composition of fly ash has been considered to be a major barrier to its increased use in cement and concrete. In this study the variability and properties of fly ash are characterized with an emphasis on particle size analysis as a means for fly ash producers to better understand material properties in relation to the process of production, classification, and potential modes of utilization. Fly ash samples were collected from different coal-fired power plants from certain Indian and Canadian sources. The particle size analysis results using Laser Diffraction Technique showed a wide variation between the particle size distributions of the studied sources. However, no correlation between the varied size distributions and chemical compositions of fly ash samples was found. Laboratory experiments on the selected fly ash samples are being undertaken to correlate fly ash characteristics and their effects on the performance of concrete mixtures with cementitious replacement level up to 50%

Supplementary Cementitious Materials for Concrete: Characterization Needs

peer reviewedA wide variety of materials are currently used as supplementary cementitious materials (SCMs) for concrete, including natural materials and byproducts from various industries. Historically, natural SCMs, mostly derived from volcanic deposits, were common in concrete. In recent years, the dominant SCMs have been industrial by-products such as fly ash, ground granulated blast furnace slag (GGBFS), and silica fume. There is currently a resurgence of research into historic and natural SCMs, as well as other alternative SCMs for many reasons. The primary benefits of SCM use in improvement of long-term mechanical performance, durability, and sustainability are widely accepted, so local demand for these materials can exceed supply. This paper describes some of the SCMs that are attracting attention in the global research community and the properties and characteristics of these materials that affect their performance. Special attention is paid to the importance and demands of material characterization. Many SCMs do not necessarily lend themselves to characterization methods used in standardized test methods, which sometimes fail to describe the properties that are most important in predicting reactivity

Reactivity tests for supplementary cementitious materials: RILEM TC 267-TRM phase 1

A primary aim of RILEM TC 267-TRM: “Tests for Reactivity of Supplementary Cementitious Materials (SCMs)” is to compare and evaluate the performance of conventional and novel SCM reactivity test methods across a wide range of SCMs. To this purpose, a round robin campaign was organized to investigate 10 different tests for reactivity and 11 SCMs covering the main classes of materials in use, such as granulated blast furnace slag, fly ash, natural pozzolan and calcined clays. The methods were evaluated based on the correlation to the 28 days relative compressive strength of standard mortar bars containing 30% of SCM as cement replacement and the interlaboratory reproducibility of the test results. It was found that only a few test methods showed acceptable correlation to the 28 days relative strength over the whole range of SCMs. The methods that showed the best reproducibility and gave good correlations used the R3 model system of the SCM and Ca(OH)2, supplemented with alkali sulfate/carbonate. The use of this simplified model system isolates the reaction of the SCM and the reactivity can be easily quantified from the heat release or bound water content. Later age (90 days) strength results also correlated well with the results of the IS 1727 (Indian standard) reactivity test, an accelerated strength test using an SCM/Ca(OH)2-based model system. The current standardized tests did not show acceptable correlations across all SCMs, although they performed better when latently hydraulic materials (blast furnace slag) were excluded. However, the Frattini test, Chapelle and modified Chapelle test showed poor interlaboratory reproducibility, demonstrating experimental difficulties. The TC 267-TRM will pursue the development of test protocols based on the R3 model systems. Acceleration and improvement of the reproducibility of the IS 1727 test will be attempted as well

Ternary blends

International audienceTernary binders have become used more frequently for several reasons. In some cases, such as combining a rapidly reactive SCM such as silica fume with a more slowly reactive SCM such as fly ash or slag, the use of ternary binders can provide benefits for both early-age and later-age properties and durability of concrete. In other cases, high-alkali pozzolans have been combined with slag to both accelerate the slag hydration and bind the alkalis from the pozzolan. As well, two SCMs may be combined to improve economy of the concrete mixture. This chapter describes properties of various ternary binders. © RILEM 2018

Test methods for resistance of concrete to sulfate attack : a critical review

Sulfate attack comprises a series of chemical reactions between sulfate ions and the components of hardened concrete. As these reactions may lead to cracking, spalling or strength loss of concrete structures, appropriate test methods are needed to determine the resistance of concrete under sulfate expoures. Accelerated test methods are most suitable since sulfate attack is typically a long term process. The current ASTM C1012 (2004) test method accelerates the attack mechanism by using a solution with a high sulfate concentration in which mortar specimens are immersed. The SVA procedure uses smaller specimens to obtain results earlier. In the Wittekindt method not only smaller specimens are used but also the w/c-ratio is increased. However,tese test still require several months. Test methods such as ASTM C452 (2006) and the Chatelier-anstett test use a mixture of cement and gypsum. Since in this case the sulfate source is located internally, no more time is needed for sulfate ingress. With these test methods results are obtained after 2 weeks, but the attack mechanism no longer represents field conditions in a realistic way

Validation of Mualem's conductivity model and prediction of saturated permeability from sorptivity

Advanced modeling of unsaturated water and contaminant transport in concrete requires knowledge of the unsaturated conductivity (permeability) function and the water retention curve. An assumption of simple Fickian diffusion is not sufficient due to the nonlinear water and contaminant fluxes. The Van Genuchten-Mualem conductivity model, widely used in describing hydraulic properties of soils, is validated by predicting experimental moisture content profiles obtained by nuclear magnetic resonance during simple absorption experiments with concrete cylinders. A new tortuosity parameter is suggested. Analytical methods are extended based on the Van Genuchten-Mualem model, to provide a means of estimating the saturated permeability from the much simpler sorptivity experiment. The predicted permeability is similar to the long-term experimental permeability. This suggests that only the very long-term saturated permeability is suitable for describing unsaturated moisture flux