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Calcined Natural Pozzolans, Including Calcined Clays, Part 1 of 2

Tuesday, October 19, 2021  10:00 AM - 12:00 PM

RILEM Technical Committee 282 CCL will co-sponsor. This RILEM committee “Calcined Clays as Supplementary Cementitious Materials” formed in 2018 and working toward developing standards for use of calcined clays in concrete.
This session will address both fundamentals of pozzolan activation through calcination, considering relatively pure and impure natural sources, and practical use of these materials in concrete construction. The intended audience includes engineers, contractors, researchers, and students interested in alternative sources of pozzolans.
Session 1 will cover the fundamentals of pozzolan calcination, including effects of composition and processing, on structure and performance.
Session 2 will address practical use of calcined pozzolans at an industrial scale.
Learning Objectives:
(1) Explain influence of source composition on the natural pozzolan reactivity;
(2) Describe the role of processing variables, including calcination, on the natural pozzolan reactivity;
(3) Summarize the influence of calcined pozzolans on microstructure;
(4) Report up-to-date information on any changes required in design and specification of concrete, to facilitate broader use of calcined natural pozzolans.

This session has been approved by AIA and ICC for 2 PDHs (0.2 CEUs). Please note: You must attend the live session for the entire duration to receive credit. On-demand sessions do not qualify for PDH/CEU credit.

The Impact of Clay Quality on Reactivity of Calcined Impure Clays

Presented By: Katelyn O'Quinn
Affiliation: University of Texas
Description: Moderate purity clays have shown promise as supplementary cementitious materials (SCMs) due to their vast geological reserves and low cost. This study investigates the reactivity of calcined clays sourced from across North America, including East, West, and Central locations, with varying mineral compositions and quality. The clays were tested using chemical reactivity methods to assess the impact of clay quality. The results show that composition and quality are important in the assessment of natural SCMs and that the sourced moderate purity calcined clays are pozzolanic and may offer significant benefits as SCMs.

Activation of Na- and Ca- Bentonites: Comparison of Thermal and Mechanical Treatments

Presented By: Alastair Marsh
Affiliation: University Of Leeds
Description: To help realize the global potential of calcined clays as supplementary cementitious materials, there is a move towards valorizing a wider range of common clays exhibiting lower purities and non-kaolinitic clay minerals. 2:1 clay minerals are less favorable for calcination, as their layer structure largely retains its ordering after dihydroxylation. For this reason, mechanical treatment is advocated as an alternative to calcination for increasing the pozzolanic reactivity of 2:1 clays. However, there is still a relatively poor understanding of the comparative effects of thermal and mechanical treatments on the structural ordering of 2:1 clay minerals and their reactivity. In this study, high purity smectites were used as relatively simple systems to help understand processing-structure-properties relations. A Ca-bentonite and a Na-bentonite were subjected to thermal treatment by soak calcination in a furnace, and mechanical treatment by intensive milling in a planetary ball mill. A variety of techniques were used to characterize the changes in hydroxyl and water group distribution (TG-MS, FTIR), structural disorder (XRD, 27Al MAS-NMR) and physical characteristics (laser particle diffraction, SEM) for the different treatments. This information was then used to explain differences in reactivity observed through calorimetry (R3 cumulative heat release test).

Impact of Calcined Clay Particle Size on Rheology and Optimal Water Content

Presented By: Anna Landreville
Affiliation: Imerys Performance Minerals
Description: The influence of calcined clay particle size on the performance of mortars was investigated. Samples of calcined kaolin were milled to produce a variety of particle size distributions, with median particle sizes ranging from 1-10 microns. Finely ground samples had high specific surface areas that resulted in reduced flow and increased water demand in mortar mixes. Coarser calcined clays required less water to achieve flow values that were equal to the control mixes. The milled calcined clay samples were tested in mortars using a range of water-cement ratios. The water-cement ratio is a critical factor that impacts the compressive strength development of mortars. Typically, an inverse relationship is observed between the water-cement ratio and compressive strength. However, some finely ground calcined clays showed improved compressive strengths with increasing water-cement ratios. This effect is not observed in other pozzolans.

Understanding the Role of Calcined Clay on Pore Structure and Transport Properties of Cement Composites

Presented By: Yuvaraj Dhandapani
Affiliation: University of Leeds
Description: The use of calcined clay as a substitute material has multiple effects on the evolving microstructure and significantly alters the durability performance of concrete systems. In this study, concretes prepared with calcined clay were characterized for resistance to chloride and moisture ingress using non-steady state chloride migration and unidirectional capillary absorption experiments, respectively. The studies were performed on a range of concrete prepared with a lower grade calcined clay (50% kaolinite content) as supplementary cementitious materials (SCMs) in binary and ternary forms along with fine limestone powder. Notably, the interaction of calcined clay positively influenced physical structure at an early age, i.e., 3 days, which was captured by mercury porosimetry and formation factor assessment. The chloride resistance of concretes prepared with calcined clay significantly improved, and the dilution of hydrates due to limestone addition was effectively neutralized by the synergistic interaction between calcined clay and limestone. Long term moisture ingress was studied using capillary absorption experiments for an exposure period of 28 days, and the results reveal that both binary and ternary binders containing calcined clay had a lower absorption rate than the OPC system. Unlike chloride ingress, the moisture uptake profiles were able to capture the difference caused due to limestone addition explicitly. The transport parameters were examined using estimates of pore network parameter, i.e., formation factor and tortuosity. Concrete prepared with calcined clay and the calcined clay-limestone combination had a higher tortuosity value, which explains improved transport properties using a physicochemical framework. The applicability of these pore network parameters could serve as a basis to conceptualize the link between reactivity, microstructural evolution, and transport characteristics in cement composites.

Monte-Carlo Based Thermodynamic Analysis on the Performance of Calcined Clays in Cementitious Systems

Presented By: Keshav Bharadwaj Ravi
Affiliation: Oregon State University
Description: Calcined clays (CCs) show a high degree of variability in chemical composition and reactivity. Their high-alumina content also results in additional reactions in systems that contain limestone; therefore, the performance of CC-incorporated mixtures also depends highly on the chemical composition of the base ordinary portland cement (OPC). Considering the wide range of replacement levels that are used, experimental studies alone cannot capture the performance of all possible variations of CCs in cementitious mixtures because virtually an infinite number of these combinations exist. A Monte Carlo analysis framework is developed to randomly generate input data (chemical composition and reactivity of materials) for thermodynamic calculations. Thermodynamic modeling is a proven tool to predict reaction products, pore solution chemistry, and other performance indicators such as porosity. The randomly generated input for each case is created from available statistical data on CCs and OPC. With this approach, millions of realistic simulations can be conducted. The presentation will begin with a brief description of the Monte-Carlo-based thermodynamic modeling framework. This will be followed by the statistical distribution of the composition and reactivity of CCs that will be used to generate the input data for thermodynamic calculations. The impact of the range of compositions and reactivities on cementitious systems' performance will be studied by comparing several indicators such as type and amount of different reaction products, pore solution chemistry, and porosity. The statistical distribution of these properties at various replacement levels of OPC with CC and will be shown. Furthermore, the interplay between CC composition/reactivity and limestone content in the cementitious systems will be discussed.

Pozzolanic Reactivity and Performance of Calcined Byproduct Clays of Various Kaolinite Contents in Concrete Mixtures

Presented By: Khashayar Jafari
Affiliation: Pennsylvania State University
Description: In this study, byproduct clay samples from a commercial sand and gravel pit in United States was investigated. QXRD testing showed that after calcination, the clays contained quartz in the range 29 to 60%, muscovite 10 to 13%, hematite 3 to 7.5%, and amorphous 28 to 49.5%. Additionally, surfactant-assisted sedimentation of the low kaolinite clay prior to calcination resulted in an enriched calcined clay with 2% quartz, 26% muscovite, 6% hematite, and 64% amorphous. These three clay samples were evaluated for their chemical and physical properties (per ASTM C311 and C618) and pozzolanic reactivity (per ASTM C1897, the R3 test). XRD and thermodynamic modeling were used to identify and quantify the pozzolanic reaction products. Further, the performance of these clays in a concrete mixture with w/cm=0.47 was evaluated by measuring the impact on workability, setting, compressive strength, and durability (ASR mitigation, drying shrinkage, chloride penetrability, and scaling resistance). Given that the three clays were from the same source and of similar chemistry, the results allow evaluating the impact of the kaolinite content on the reactivity and performance of such calcined clays in concrete.

Metakaolin in Concrete: The Role of Alumina

Presented By: Michael Thomas
Affiliation: University of New Brunswick
Description: Data are presented from a number of studies on the durability of mortar and concrete containing metakaolin (MK), a calcined high-purity kaolin clay (kaolin: Al2O3·2SiO2·2H2O). Long-term data from concrete specimens exposed to the tidal zone on a marine-exposure site indicate that chloride penetration into concrete containing 12% MK is much less than that predicted based on permeability/diffusion measurements and comparisons with concrete containing silica fume with similar permeability and diffusion coefficients. The increase in chloride resistance of MK concrete is attributed to the significant increase in chloride-binding resulting from the use of MK which is high in alumina (approximately 45% Al2O3) and results in the formation of increased amounts of Friedel’s salt. The high alumina content of MK has also been credited for its efficacy in controlling damage due to delayed ettringite formation (DEF), a form of internal sulfate attack, with elimination of deleterious expansion at relatively low levels of replacement (for example, 10% MK). Unfortunately, the high alumina content of MK has been found to reduce the resistance of mortars exposed to external sulfate attack when the MK is used at moderate levels of replacement (10 to 15% MK) and this has been ascribed to the increased capacity for ettringite formation. At higher levels of replacement (= 20% MK), the sulfate resistance of mortars is increased by the use of MK. Reasons for this paradoxical behavior are discussed. Finally, data are presented from studies on alkali-silica reaction (ASR) which indicate that the alumina content has little impact on the ability of MK to suppress the alkalinity of the concrete pore solution and control expansion with reactive aggregates; these benefits are shown to depend predominantly on the silica content of the MK (approximately 55% SiO2).

Upper Level Sponsors

Brasfield Gorrie
Concrete Sealants, Inc.
Metromont Corporation
Thomas Concrete

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