Link to Latest Report: Coming Soon
Background:
Concrete’s high carbon footprint is an ongoing concern for infrastructure sustainability and environmental stewardship. A primary strategy in the quest for low-carbon concrete is to replace a portion of the portland cement with pozzolanic wastes generated by other industrial or societal activities. Biochar is a product of biowaste combustion. Some sources are pozzolanically active and can therefore enhance concrete’s later-age mechanical and durability properties of concrete. In addition, biochar efficiently absorbs atmospheric CO2, a first step in CO2 sequestration in concrete by carbonate mineralization. But despite this potential multifunctional nature of biochar, its assured use in concrete for bridge elements will not be realized until its influences on freshstate workability and early strength development are better understood and controlled. New materials enabled by this research will significantly decrease anthropogenic GHGs, increase the resilience and sustainability of civilian infrastructure, and provide a sink for biowaste materials that otherwise would be deposited in landfills.
Objective:
The overarching research objective is to develop low-carbon concrete materials infused with biochar that potentially can be implemented in bridge construction and repair. The primary knowledge gaps toward that end are the early-age structure-processing-property relationships. Quantifying these relationships will enable biochar-infused concrete to be used reliably and at higher volume fractions of replacement for portland cement. Within that objective, our specific goal in this project is to map the relationships among biochar dosage and properties to fresh flow properties and early-age mechanical properties and microstructure.
Scope:
The proposed project includes several tasks
- Task 1 – Screen and characterize biochar sources
No more than three biochar candidates will be chosen based on ash content, composition and alkaline reaction rates, porosity, specific surface area, and density. With the target of leveraging pozzolanic activity for later-age strength and durability, the biochars will all be chosen to have a high silica content. Among the high-silica sources we will choose candidates to span the practical range of specific surface areas and densities.Based on that initial screening for materials selection, the chosen biochars will be thoroughly characterized for their true density, particle size distribution, chemical composition, crystalline and amorphous content, and hydraulic activity in alkaline solutions:
• X-ray fluorescence (XRF) spectroscopy will be used to obtain elemental composition on
an equivalent oxide basis;
• Microthermogravimetric analysis (TGA) will be used to determine not only the loss on ignition (LOI), but also to identify the presence of residual hydroxides or carbonates.
• Isothermal microcalorimetry (IC) will be used to characterize the pozzolanic potential of
each biochar material in an alkaline solution to simulate conditions in a concrete binder.
• Nitrogen adsorption BET isotherms will be used to measure specific surface area.Pycnometry will be used to determine true density and water absorption.
• Scanning electron microscopy (SEM) will be used to visualize particle morphology - Task 2 – Map the rheological properties of pastes and mortar
Each of the selected biochars will be integrated with a Type I/II portland cement to make a range of pastes and mortars that will be the subject of a factorial design experiment that will determine the influence on fresh rheological properties of mixture proportions as well as any interactions among the mixture componentsls.
Three mixture characteristics will be chosen as factors in a balanced designed experiment. Temperature will be held constant at 23 °C.
• Factor I: Biochar replacement level. The initial plan is to replace, by volume, 5 % or 15 % of the portland cement with biochar. These targets may have to be modified depending on the feasibility of making the mixtures with existing equipment.
• Factor II: Water-solids volume ratio. We will target volume ratios of 1.10 and 1.42. In a neat portland cement, these ratios would correspond to water/cement mass ratios of 0.35 and 0.45, respectively. We will use the moisture content and absorption capacity of the biochar to determine the total volume of water that must be added to achieve these volume ratios of free water in the mixture.
• Factor III: Superplasticizer dosage. We plan to use a polycarboxylate ether superplasticizer for these experiments. The dosage will be 1 % or 3 % by volume of the total solids volume. A midpoint mixture will also be made, bringing to nine the total number of mixtures per measurement type.Task 2.2. Rheological property measurements. The important fresh properties of the binder that characterize its flow behavior under arbitrary conditions are the viscosity as a function of shear rate, the yield stress, and thixotropy. First the paste viscosity as a function of shear rate will be measured in terms of flow curves. These data will be gathered with an Anton Paar MC 3023 rheometer, using a serrated parallel plate geometry which provides absolute measurements. In addition, thixotropy and yield stress will be measured by oscillatory shear methods with the same rheometer at three different times after mixing to characterize the contributions of agglomeration and initial hydration on those rheological properties (39 experiments).We therefore anticipate a minimum of 52 rheological experiments, the order of which will be randomized according to standard factorial design principles. The results will be analyzed by Analysis of Variance (ANOVA) formalisms to create a response surface that should highlight main effects as well as second- and third-order effects among the variables in terms of their influences on viscosity, yield stress, and thixotropy. The data and response surfaces acquired from this work will form the basis for developing and optimizing 3D printing methods for bridge components using these materials in future years.
Task 3 – Determine biochar influence on early-age hydration, carbonation, and mechanical
properties.
We will use a subset of the combinations described above of mixture formulations to determine the influence of different biochars on overall reaction rates, setting times, microstructure evolution, and 3-d and 7-d mechanical properties.
Task 3.1. Early-age hydration and microstructure
Early hydration of the mixtures will be monitored continuously by isothermal microcalorimetry, from which we will be able to observe, relative to neat paste control samples, the influence of biochar on the main alite hydration event and the aluminate renewal event. The rates will be characterized in terms of (a) slope of the inflection point of the rate curve prior to the maximum heat release, (b) maximum heat release rate, and (c) inverse time to the maximum heat release rate. As in prior studies, tracking these three characteristics of the calorimetry curve can also reveal filler effects provided by the biochar additions [Oey 2013]. Besides microcalorimetry, we intend to track the phase development in the pastes with X-ray powder diffraction. If sealed yet X-ray transparent specimen holders can be procured, the primary peaks of target phases will be monitored at short time intervals, about once every ten minutes, to track changes in these phases during the acceleration period. Target phases could be ettringite, portlandite, or calcite. These short-time phase change rates are interesting and can help illuminate hydration mechanism modifications induced by biochar. Nevertheless, these measurements are unlikely to be critical in developing a practical understanding, so if adequate specimen holders cannot be procured, the phases can still be monitored ex situ by XRD at 12-h or 24-h intervals by normal sampling procedures, arresting hydration at prescribed times by solvent exchange prior to the measurement. In total, approximately 12 to 15 XRD measurements are anticipated in this project
Task 3.2. Initial and final setting times
The standard test method for initial and final setting times by the Vicat apparatus (ASTM C191) will be employed on each of the mixtures. The most efficient means for this is to make enough paste of each mixture to measure both a flow curve and setting times at the same time, especially since the flow curve can be obtained within about 30 minutes of mixing, leaving plenty of time for the determination of setting times. In total, 13 setting time measurements will be made, which is one for each mixture made in Subtask 2.1, plus four additional replicates of the midpoint to assess variability.
Task 3.3 Carbonation
Rates of carbonation will be characterized as a function of CO2 pressure using all of several CO2 curing chambers available in our labs. Samples will be exposed to a constant CO2 pressure and the material will be subjected to thermogravimetric analysis at prescribed times to determine the amount of CO2 absorption and chemical conversion. The data will be analyzed as a function of CO2 pressure and time. We intend to attempt 3D microstructure characterization of the hydrated microstructures using X-ray computed microtomography (XCT). The primary cement particle sizes are too small to be resolved by lab-scale XCT, but we expect that larger scale biochar components will be large enough and have enough contrast in their X-ray absorption to be detected. These data can provide insights on the dispersion and distribution of biochar throughout the material.
Task 3.4. Early-age mechanical properties
The compressive strengths of mortars made from the biochar-infused pastes will be measured according to the ASTM C109 standard test method. Strength measurements will be made at 3 d and 7 d, for a total of 26 strength measurements. From these measurements we expect to regress the Powers gel-space ratio model of compressive strength to forecast the strength out to 28 d. These forecasts will be tested on no more than five of the mixtures, bringing the total number of strength measurements to no more than 31. In addition to compressive strength, we will apply nanoindentation on cut-and-polished sections of the material to determine the mechanical properties of the bulk paste and the interfacial transition zone that may develop between the biochar and the paste.
Research Team:
Principal Investigator: Jeffrey W. Bullard
Co-Principal Investigator: Yong-Rak Kim