Chloride-induced corrosion of steel reinforcement in alkali-activated slag/ metakaolin blended concretes

The aim of this study is to understand the corrosion mechanism of steel embedded in alkali-activated slag/metakaolin blended concretes via electrochemical testing after 407 days of exposure to a 3.5 wt.% NaCl solution, with an emphasis on characterising the material prior to the onset of corrosion. In order to accurately depict the material’s state, the importance of timedependent stability of the corrosion potential (Ecorr) is discussed in the context of both twoelectrode cell set-up on a carbon-steel exposed to basic solutions (0.8 M NaOH and 0.8 M NaOH + 1.2 M NaCl) and reinforced alkali-activated concretes. When carbon-steel is exposed to NaOH solutions, the Ecorr values did not attain a logarithmic equilibrium or passive condition prior to 72 hours of exposure. At the same NaOH concentration, and in the presence of chlorides, Ecorr values fluctuated as a consequence of localised corrosion activity at the metal surface. In order to circumvent the instability caused by chlorides, both direct-current (linear polarization resistance) and alternating-current (electrochemical impedance spectroscopy) techniques were utilized to determine and cross-validate the polarization resistance (Rp) in steel-reinforced alkali-activated concretes. With a 0.6% difference in Rp values, results demonstrate excellent agreement by utilizing both techniques. Thus, results demonstrate that these concretes do not initiate corrosion activity before 158 days of aggressive exposure to chloride. These results demonstrate some stability of the oxide films forming in alkali-activated concretes, and highlight the need for long duration testing of these materials to truly understand how corrosion might take place in reinforced concretes

Carbonation rate of alkali-activated concretes and high-volume SCM concretes: a literature data analysis by RILEM TC 281-CCC

The current understanding of the carbonation and the prediction of the carbonation rate of alkali-activated concretes is complicated inter alia by the wide range of binder chemistries used and testing conditions adopted. To overcome some of the limitations of individual studies and to identify general correlations between mix design parameters and carbonation resistance, the RILEM TC 281-CCC ‘Carbonation of Concrete with Supplementary Cementitious Materials’ Working Group 6 compiled and analysed carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥ 66% of the binder) were also included in the database. The analysis indicates that water/CaO ratio and water/binder ratio exert an influence on the carbonation resistance of alkali-activated concretes; however, these parameters are not good indicators of the carbonation resistance when considered individually. A better indicator of the carbonation resistance of alkali-activated concretes under conditions approximating natural carbonation appears to be their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, where the subscript ‘eq’ indicates an equivalent amount based on molar masses. Nevertheless, this ratio can serve as approximate indicator at best, as other parameters also affect the carbonation resistance of alkali-activated concretes. In addition, the analysis of the database points to peculiarities of accelerated tests using elevated CO2 concentrations for low-Ca alkali-activated concretes, indicating that even at the relatively modest concentration of 1% CO2, accelerated testing may lead to inaccurate predictions of the carbonation resistance under natural exposure conditions.Materials and Environmen

Report of RILEM TC 267-TRM phase 2: Optimization and testing of the robustness of the R3 reactivity tests for supplementary cementitious materials

The results of phase 1 of an interlaboratory test, coordinated by the RILEM TC 267-TRM “Tests for Reactivity of Supplementary Cementitious Materials” showed that the R3 (rapid, relevant, reliable) test method, by measurement of heat release or bound water, provided the most reliable and relevant determination of the chemical reactivity of supplementary cementitious materials (SCMs), compared to other test methods. The phase 2 work, described in this paper aimed to improve the robustness of the test procedure and to develop precision statements for the consolidated test procedure. The effect of the pre-mixing and mixing conditions, and the impact of the mix design on the test method robustness were assessed and fixed for optimal conditions to carry out the R3 heat release test. The effect of the drying step was evaluated to define the R3 bound water test procedure in more detail. Finally, the robustness of the consolidated final test methods was determined by an interlaboratory study to define the precision statements.Materials and Environmen

RILEM TC 281-CCC Working Group 6: Carbonation of Alkali Activated Concrete: Preliminary Results of a Literature Survey and Data Analysis

The current understanding of the carbonation of alkali-activated concretes is hampered inter alia by the wide range of binder chemistries used. To overcome some of the limitations of individual studies and to identify general correlations between their mix design parameters and carbonation resistance, the RILEM TC 281-CCC working group 6 compiled carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥ 66% of the binder) were also included in the database. A preliminary analysis of the database indicates that w/CaO ratio and w/b ratio exert an influence on the carbonation resistance of alkali-activated concretes but, contrary to what has been reported for concretes based on (blended) Portland cements, these are not good indicators of their carbonation resistance when considered individually. A better indicator of the carbonation resistance of alkali-activated concretes under conditions approximating natural carbonation appears to be their w/(CaO + Na2O + K2O) ratio. Furthermore, the analysis points to significant shortcomings of tests at elevated CO2 concentrations for low-Ca alkali-activated concretes, indicating that even at a concentration of 1% CO2, the outcomes may lead to inaccurate predictions of the carbonation coefficient under natural exposure conditions.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen

Carbonation Rate of Alkali-Activated Concretes: Effects of Compositional Parameters and Carbonation Conditions

The current ability to predict the carbonation resistance of alkali-activated materials (AAMs) is incomplete, partly because of widely varying AAM chemistries and variable testing conditions. To identify general correlations between mix design parameters and the carbonation rate of AAMs, RILEM TC 281-CCC Working Group 6 compiled and analysed carbonation data for alkali-activated concretes and mortars from the literature. For comparison purposes, data for blended Portland cement-based concretes with a high percentage of SCMs (≥66% of the binder) were also included in the database. The results show that the water/CaO ratio is not a reliable indicator of the carbonation rate of AAMs. A better indicator of the carbonation rate of AAMs under conditions approximating natural carbonation is their water/(CaO + MgOeq + Na2Oeq + K2Oeq) ratio, where the index ‘eq’ indicates an equivalent amount based on molar masses. This finding can be explained by the CO2 binding capacity of alkaline-earth and alkali metal ions; the obtained correlation also indicates an influence of the space-filling capability of the binding phases of AAMs, as for conventional cements. However, this ratio can serve only as an approximate indicator of carbonation resistance, as other parameters also affect the carbonation resistance of alkali-activated concretes. In addition, the analysis of the dataset revealed peculiarities of accelerated tests using elevated CO2 concentrations for low-Ca AAMs, indicating that even at the relatively modest concentration of 1% CO2, accelerated testing may lead to inaccurate predictions of their carbonation resistance under natural exposure conditions.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen

Report of RILEM TC 267-TRM phase 3: validation of the R³ reactivity test across a wide range of materials

RILEM TC 267 TRM– “Tests for Reactivity of Supplementary Cementitious Materials” recommends the Rapid Reliable Relevant (R3) test as a method for determining the chemical reactivity of supplementary cementitious materials (SCMs) in Portland cement blends. In this paper, the R3 test was applied to 52 materials from a wide range of conventional and alternative SCMs with the aim to validate such test. An excellent correlation was found between the cumulative heat release and the bound water determined following the R3 test method. Comparison of the R3 test results to mortar compressive strength development showed that all conventional SCMs (e.g. blast furnace slag and fly ashes) followed the same trend, with the notable exception of very reactive calcined kaolinitic clays. It is discussed, through an in-depth statistical regression analysis of the R3 reactivity test results and the 28 days relative compressive strengths, how reactivity threshold values for classification of the chemical reactivity of SCMs could be proposed based on the R3 test results.Materials and Environmen