Mechanical effects of alkali silica reaction in concrete studied by SEM-image analysis

The occurrence of alkali-silica and alkali-silicate reactions causes damage in concrete. Even though the reaction has been known for some time, the progress of reaction in affected structures is difficult to predict. This research programme aims to study the relationship between the progress of the reaction and the mechanical properties of the concrete in order to support better prognosis of the effect of ASR on affected structure. The basic principal of the research programme is to characterise the chemical, microstructural and mechanical state of the concrete, the degree of expansion in the initial and final states and how the rate of change of the chemical reaction is related to the changes in mechanical properties. In practice this is difficult to do because: In the field the induction and reaction periods are very long necessitating accelerated testing in the laboratory The amount of reactive material in the aggregate is usually small, and hence difficult to measure The relation between the degree of reaction, expansion and the change of mechanical properties is not known. The microstructural characteristics of some Swiss aggregates have been studied and quantified using microscopic techniques. Mortar and concretes samples were made with the aggregates and subjected to ASR. Image analysis of SEM-BSE obtained from polished samples was used to quantify the reaction degree. In this project, a series of mechanical tests, in parallel to the chemical reaction were undertaken in order to determine the effect of ASR on the mechanical properties of concrete. The principal objective of this research is the development of a tool relating laboratory results to real structures. Important aspects of the research were: A multidisciplinary approach to chemical, microstructural and mechanical characterisation to increase the chances of finding a combination of techniques which can be used to assess the degree of reaction Use of modelling approach to link between microstructural changes and mechanical properties, which can in turn, be used to extrapolate the evolution of properties in the long term The results show a strong relationship between the observed reactivity and the degrees of expansion in the concretes and mortars. The presented micromechanical modeling is able to correctly reflect the laboratory results and sheds further light on the mechanism of ASR pertinent to field structures

Alkali metal distribution in composite cement pastes and its relation to accelerated ASR tests

Accelerated testing of alkali silica reaction (ASR) in concrete at elevated temperatures of 38 and 60 °C has an unknown impact on the alkali metal distribution in the cement paste. This paper investigates how the alkali metals released from hydrating Portland cement, limestone, and SCMs distribute between non-reactive and unreacted phases, C-A-S-H, and the pore solution. The SCMs investigated were fly ash and a volcanic pozzolan. The hydrate assemblage and pore solution of cement pastes cured at 20, 38 and 60 °C were analysed and related to the expansion of concrete prisms. There is little difference in alkali metal distribution at 20 and 38 °C, whereas curing at 60 °C has a large impact for the SCM containing blends. At alkali metal concentrations in the pore solution below 0.5 mol/L (Na + K) expansion of concrete was suppressed. Pore solution analysis could be used to screen new SCMs for ASR mitigation.publishedVersio

Effect of leaching on the composition of hydration phases during chloride exposure of mortar

Mortar specimens were exposed to either a 3% NaCl solution or a 3% NaCl+KOH solution for up to 180 days. Exposure to the NaCl solution provoked much more leaching than the NaCl+KOH exposure. Leaching strongly impacted the chloride ingress profiles. The extended leaching led to a maximum total chloride content almost three times higher and a deeper chloride penetration than exposure with limited leaching after 180 days. The higher maximum chloride content seems to be linked to the enhanced binding capacity of the C-S-H and AFm phases upon moderate leaching as determined by SEM-EDS. The total chloride profile appears to be governed by multi-ion transport and the interaction of chloride with the hydration phases. Service life prediction and performance testing both rely on total chloride profiles and therefore ought to take these interactions into account.publishedVersio

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

Advances in understanding ye'elimite-rich cements

International audienc

A method to determine the critical moisture level for unsaturated transport of ions

This study describes a method to obtain a certain humidity level by a certain drying treatment. This treatment was applied on cement based materials in order to identify the critical moisture level above which ion transport can occur. The drying is performed in two steps, first in one direction to reach a certain moisture level and then in the opposite direction to identify if ions can move at this lower level. This treatment gives an opportunity to determine ion transport at a well defined humidity, both in terms of moisture content and relative humidity. Ion transport was indicated by analyzing the potassium distribution on slices of mortar by SEM–EDS

CO₂ Mineralization Methods in Cement and Concrete Industry

Production of Portland clinker is inherently associated with CO2 emissions originating from limestone decomposition, the irreplaceable large-scale source of calcium oxide needed. Besides carbon capture and storage, CO2 mineralization is the only lever left to reduce these process emissions. CO2 mineralization is a reversal reaction to clinker production—CO2 is bound into stable carbonates in an exothermic process. It can be applied in several environmentally and economically favorable ways at different stages of clinker, cement and concrete life cycle. These possibilities are assessed and discussed in this contribution. The results demonstrate that when combined with concrete recycling, the complete circularity of all its constituents, including the process CO2 emissions from the clinker, can be achieved and the overall related CO2 intensity significantly reduced

Factors affecting the reactivity of slag at early and late ages

This study investigates the difference between the reactivity of slag in NaOH solution (w/s = 100) and in cement paste (w/b = 0.4). The reactivity of slag is much higher in NaOH solution than in cement paste. When the NaOH solution is saturated with portlandite, the rate of reaction of slag decreases to approach that in cement paste. This suggests that the slower reaction of slag in cement paste might be due to the presence of calcium ions released by the clinker phases. At 28 days, relative humidity measurements indicate that water is only present in pores less than 6 nm. Above 6 nm, hydration cannot take place due to the lack of water. From a thermodynamic point of view, the small size of water filled pores may slow down the slag reaction due to the high concentration of species in solution required for hydrate growth to balance the increase of the surface energy.LM