Understanding and Measuring the Reactivity of Supplementary Cementitious Materials
Presented By: Karen Scrivener
Affiliation: Ecole Polytechnique Federale De Lausanne
Description: The increasing use of SCMs either pre-blended in cement or added to concrete is a very important strategy to reduce CO2 emissions associated with the production of cement and concrete. In this context we need better methods to understand their reactivity. The classic “strength activity index” has many problems, dependence on cement type, variable water to cement ratio to get mortars at concrete flow and most of all ability of totally inert materials to pass the test. In this presentation the work of the RILEM committee TC267 TRM will be described, which led to the standard ASTM C1897 – “R3” cement to assess reactivity. The work of this committee also revealed problems with other proposed tests, such as the silica activity which is used to classify pozzolans in the European norm. Looking behind the results of the TC 267 TRM, the difference between glassy SCMs (fly ash, slag, etc.) and calcined clays in terms of reactivity will be addressed. Finally, this presentation will look at the implications of all given information for future cement and concrete standards.
High-Alkali Natural Pozzolans and Their Ability to Mitigate ASR
Presented By: Prasad Rangaraju
Affiliation: Clemson University
Description: In light of the growing need to find suitable alternatives to high-quality fly ashes that meet ASTM C618 specifications, this study investigates the ability of the premise that high-alkali natural pozzolans (HANP) and reclaimed fly ashes (RFA) may be ineffective in mitigating ASR. Use of supplementary cementitious materials (SCMs) that are high in their alkali content is generally avoided as a strategy to mitigate Alkali-Silica reaction (ASR) in concrete. The reason being the alkalis in SCMs may supplement the alkali-loading in concrete, which may exacerbate the problem. In this study, six high-alkali natural pozzolans (HANP) and two reclaimed fly ashes (RFA) that have high total alkali content were selected for evaluation at three different dosage levels. Using an established reactive aggregate, these SCMs were not only evaluated for their ability to mitigate ASR using standard test methods (AASHTO T380, ASTM C1293, ASTM C1567, and ASTM C441), but also the evolution of pore solution chemistry and alkali-binding ability of matrices containing these SCMs were investigated. In addition, the pozzolanic reactivity of SCMs was evaluated to track the mechanisms that aid in ASR mitigation to help predict the potential performance of these SCMs. Results to date indicate that vast majority of the HANPs are effective in mitigating ASR, while the performance of RFAs in highly variable. Also, modifications to existing test procedures needed to assess the ASR mitigation performance of high-alkali pozzolans will be presented.
Rapid Innovative Approach Based on Pore Solution Alkalinity Estimation to Determine Optimum Fly Ash Dosage for ASR Mitigation
Presented By: Anol Mukhopadhyay
Affiliation: Texas A&M Transportation Institute
Description: The current tests to determine optimum fly ash (FA) dosage to suppress/mitigate alkali silica reaction (ASR) in concrete require testing for a wide range of replacement levels, which is time-consuming and impractical. Moreover, the alkali boosted test conditions of the current tests (e.g., ASTM C1567 and C1293) make these tests insensitive on detecting the effects of soluble alkalis from certain fly ashes on ASR expansion. The current empirical prediction models (e.g., chemical index and extended chemical index) to determine fly ash dosage for mitigating ASR ignore the role of fly ash soluble alkali contribution on ASR evaluation. Therefore, a rapid screening tool (ST) was developed to determine optimum fly ash dosage to mitigate ASR in concrete mixes in the present work. The ST uses an innovative approach to estimate the concrete pore solution alkalinity (PSA) at different fly ash replacement levels considering the combined effect of soluble alkali contribution from cement & water-soluble alkali (WSA) from fly ashes. A modified ASTM C114 procedure was used to measure WSA from the tested fly ashes. Additionally, a non-linear regression model was also developed to estimate WSA from fly ashes. The ST predicts optimum fly ash dosage for ASR mitigation based on the concrete PSA vs. aggregate threshold alkalinity (THA) relationship, i.e., PSA = THA to suppress ASR. Based on the evaluation of wide variety, type, and composition of fly ashes in the current study, the ST predictions of optimum FA dosage demonstrated a mean absolute error (MAE) (= ± 6-9%) compared with the current prediction models. ST was found to be effective to detect the limitations of ASTM C1567 (e.g., underestimation) for certain fly ashes and short out the ashes that need further validation by reliable concrete testing (e.g., ACCT with 90 days and C1293 with 2 years). Notably, ST demonstrated dosage predictions with higher reliability (= 87%) for the unconventional ashes.
Critical Assessment of Rapid Methods to Qualify Supplementary Cementitious Materials for Use in Concrete
Presented By: Saif Al-Shmaisani
Affiliation: Cowtown Redi Mix, Inc.
Description: In this study, several tests for supplementary cementitious materials (SCMs) were evaluated to find the best methods to rapidly screen out inert materials, measure overall reactivity, and differentiate between pozzolanicity and latent hydraulicity. The R3 matrix and lime reactivity tests were found to be the most effective at quickly screening out inert materials. However, slow-reacting materials may appear to have low reactivity in both tests and extending the test duration better depicts material reactivity. Additionally, SCMs with higher alumina content perform better in the R3 tests due to the higher heat release and more bound water associated with the formation of calcium aluminate hydrates compared to calcium silicate hydrates, creating reactivity bias when compared to SCMs with lower alumina content. Measuring the calcium hydroxide content of R3 pastes, through thermogravimetric analysis or single point mass loss, can also differentiate between pozzolanic and hydraulic materials.
Linking the Reactivity of Pozzolans to the Durability Performance of Blended Cements
Presented By: Mahipal Kasaniya
Affiliation: University of New Brunswick
Description: A wide range of pozzolans including ground glass, natural pozzolan, fly ash, ground bottom ash, silica fume and blended pozzolan were tested for reactivity evaluation. A modified lime-reactivity test, a modified ASTM C618 test and the ASTM C1897 heat release test was performed to quantify the reactivity of pozzolans. Compressive strength and bulk electrical resistivity were measured in the modified tests as reactivity indicators, whereas heat evolution was measured using an isothermal calorimeter in ASTM C1897. Blended cements were produced using portland cements and pozzolans and investigated for durability performance in mortar and concrete. Concrete cylinders containing blended cements were prepared and inspected for resistance to chloride-ion ingress in terms of bulk electrical resistivity and non-steady-state chloride diffusion coefficient. The efficacy of pozzolans in mitigating alkali-silica reaction was studied using the Pyrex Mortar Bar Test or PMBT (ASTM C441). In addition, pastes of blended cements were prepared to determine the effect of pozzolans on the pore solution chemistry. Finally, the resistance of blended cements against sulfate attack was determined using ASTM C1012 by submerging mortar bars in a 50 g/L sodium sulfate solution until 42 months. The outcomes of different reactivity tests are in moderate to well agreement with one another and indicate that the pozzolans studied demonstrate a broad range of reactivity. The reactivity results are useful in predicting the resistance of blended cements against chloride-ions ingress. The role of pozzolans in mitigating alkali-silica reaction and sulfate attack and lowering pore solution concentration is dependent on the reactivity of pozzolans up to a certain extent; however, either alkali, calcium or alumina content of pozzolans is significant in influencing these properties.
Characterization and Quantification of the Pozzolanic Reactivity of Natural and Non-Conventional Pozzolans
Presented By: Farshad Rajabipour
Affiliation: Pennsylvania State University
Description: High quality pozzolans are needed to produce durable and low CO2 concrete. In this talk, eleven natural and non-conventional pozzolans (NNPs) are presented, including: 3 calcined clays, 3 volcanic ashes, 3 ground bottom ashes, and 2 FBC fly ashes. The chemistry, mineralogy, and physical properties of these NNPs are evaluated and their pozzolanic reactivity is quantified using the R3 test (ASTM C1897-20), based on the results of calorimetry, bound water, portlandite consumption, and degree of reaction of each NNP. A comparison among these different metrics of the pozzolanic reactivity will be presented. The reaction products of each pozzolan in the R3 pastes are also identified using QXRD and thermodynamic modeling and a comparison is made between different NNP types.
What’s Happening in the R3 Test?
Presented By: Yujia Min
Affiliation: Ohio State University
Description: Understanding reactivity is an important part of assessing the suitability of potential new pozzolans for use in concrete. Several promising tests have been pushed to the forefront for assessing reactivity – the Rapid, Robust, Reliable (R3) method1, the modified R3 method2, and the modified Chapelle method3. In each of these tests a pozzolan is subjected to a simulated high pH pore solution and a higher-than-ambient temperature in order to accelerate reactions between the material and the solution. These accelerated conditions provide insight into the potential level speed of, and extent of pozzolanic reaction that will occur through use of the material in a cementitious mixture over the long term. However, despite the proliferation of testing that has been performed using the R3 and other methods, how the R3 pore solution interacts with various pozzolanic materials, especially highly heterogenous pozzolans such as fly ash, remains unclear. In this talk we will present the results of recent research in our lab investigating how fly ash dissolution is affected by exposure to simulated pore solutions, such as those used in the R3 method, so that we can begin to understand what is happening to the materials in tests and discern the limits of the test.