Characterization of supplementary cementitious materials by thermal analysis

Working Group 1 of RILEM TC 238-SCM ‘Hydration and microstructure of concrete with supplementary cementitious materials (SCMs)’ is defining best practices for the physical and chemical characterization of SCMs, and this paper focusses on their thermal analysis. Thermogravimetric analysis (TGA) can provide valuable data on the chemical and mineralogical composition of SCMs. Loss-on-ignition (LOI) testing is a commonly used, standardized, but less sophisticated version of TGA that measures mass at endpoints only, with heating generally in air. In this paper we describe the use of TGA and LOI to characterize Portland cement with limestone additions, coal combustion fly ashes, ground-granulated blast furnace slag, calcined clays, and natural pozzolans. This paper outlines the value and limitations of TGA and LOI (in the formats defined in different standards regimes) for material characterization, and describes testing methods and analysis. TGA testing parameters affect the mass loss recorded at temperatures relevant for LOI measurements (700–1000 °C) of slags and fly ashes, mainly associated with oxidation reactions taking place upon heating. TGA of clays and natural pozzolans is utilized to identify optimal calcination conditions leading to dehydroxylation and consequent structural amorphization, particularly for kaolinite. However, dehydroxylation and amorphization do not occur at similar temperatures for all clays, limiting the applicability of TGA for this purpose. Although TGA is widely utilized for characterization of SCMs, the testing parameters significantly affect the results obtained, and TGA results require careful interpretation. Therefore, standardization of TGA testing conditions, particularly for LOI determination of slags and fly ashes, is required

Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

Lettuce discolouration is a key post-harvest trait. The major enzyme controlling oxidative discolouration has long been considered to be polyphenol oxidase (PPO) however, levels of PPO and subsequent development of discolouration symptoms have not always correlated. The predominance of a latent state of the enzyme in plant tissues combined with substrate activation and contemporaneous suicide inactivation mechanisms are considered as potential explanations for this phenomenon. Leaf tissue physical properties have been associated with subsequent discolouration and these may be influenced by variation in nutrient availability, especially excess nitrogen and head maturity at harvest. Mild calcium and irrigation stress has also been associated with a reduction in subsequent discolouration, although excess irrigation has been linked to increased discolouration potentially through leaf physical properties. These environmental factors, including high temperature and UV light intensities, often have impacts on levels of phenolic compounds linking the environmental responses to the biochemistry of the PPO pathway. Breeding strategies targeting the PALand PPOpathway biochemistry and environmental response genes are discussed as a more cost-effective method of mitigating oxidative discolouration then either modified atmosphere packaging or post-harvest treatments, although current understanding of the biochemistry means that such programs are likely to be limited in nature and it is likely that they will need to be deployed alongside other methods for the foreseeable future

Modeling hydration of cementitious systems

Concrete performance, including strength, susceptibility to delayed ettringite formation, and residual stress development are dependent on early-age temperature development. Concrete temperature prediction during hydration requires an accurate characterization of the concrete adiabatic temperature rise. This study presents the development of a model for predicting the adiabatic temperature development of concrete mixtures based on material properties (for example, cement chemistry and fineness and supplementary cementitious materials (SCM) chemistry), mixture proportions, and chemical admixture types and dosages. The model was developed from 204 semi-adiabatic calorimetry results and validated from a separate set of 58 semi-adiabatic tests. The final model provides a useful tool to assess the temperature development of concrete mixtures and thereby facilitate the prevention of thermal cracking and delayed ettringite formation in concrete structures

Characterizing minus no. 200 fine aggregate for performance in concrete

ASTM C 33 limits the amount of microfine aggregate smaller than 75 µm (No. 200 sieve) to be used in concrete. In the past, it was believed that this fraction was clay and, therefore, a poor performer. This is not necessarily the case with manufactured fine aggregates. While work continues toward altering ASTM C 33 to allow a higher percentage of microfine aggregates, there is need for a method of determining whether these microfines will have deleterious effects or not. Fourteen aggregates were collected for evaluation in this study. Methods of characterizing the microfines to determine their effects on concrete properties were developed in this study. In addition to fully characterizing the aggregates using advanced techniques, simple tests for microfines that can be used as a criterion for their exclusion or inclusion were evaluated. For such a test to be meaningful there must be a strong correlation between its results and concrete performance. Mortar and concrete mixes incorporating microfines from fourteen different aggregates are tested in this project for a variety of performance criteria. This project fully characterizes microfines and evaluates simple tests for predicting performance in concrete.Aggregates Foundation for Technology, Research, and Education (AFTRE)Civil, Architectural, and Environmental Engineerin

Effects of Construction Time and Coarse Aggregate on Bridge Deck Cracking

Bridge deck temperature changes in the first few days after placement due to the concrete heat of hydration and changes in ambient conditions have long been identified as a significant contributor to early-age cracking. The goal of this project was to develop a method of quantifying how materials and construction methods can influence the thermal stresses in bridge decks. A series of tests on concrete mixtures were then performed to quantify the concrete material thermal stress behavior in bridge decks with different placement times and coefficients of thermal expansion. Concrete with a high coefficient of thermal expansion placed in the morning led to the development of thermal stresses equal to 75% of the stress at cracking. It was also found that the thermal stresses could be reduced by up to 50% by using concrete with a lower coefficient of thermal expansion and placing at night

Quantification of Effects of Fly Ash Type on Concrete Early-Age Cracking

The mechanisms that contribute to early-age cracking are complex. Determining the relative importance of each mechanism as well as the combined cracking potential for a given concrete material is essential for the concrete industry to construct structures with a long service life. A method for quantifying the cracking risk of a concrete mixture is presented. The method involves testing for the concrete heat of hydration, setting time, free thermal and autogenous movement, restrained stress, and mechanical property development. The concrete uniaxial stress under restrained conditions is measured using a rigid cracking frame. This test setup was used to quantify the effects of using fly ash on the concrete cracking risk using four different fly ashes with varying calcium oxide contents. All fly ashes reduced the cracking risk because of the decrease in the heat of hydration of the cementitious materials and, to a lesser extent, the increased early-age creep

Methods for Calculating Activation Energy for Portland Cement

The accurate prediction of thermal gradients in concrete calls for models that characterize the temperature sensitivity of the hydration of cementitious materials. The most common method used for this purpose is the Arrhenius equation, which requires the selection of an activation energy Ea to define the temperature sensitivity of the reaction. For cementitious materials, Ea is typically computed using either isothermal calorimetry or compressive strength data. There is disagreement in literature as to the proper method to determine Ea. The Ea of different cementitious pastes was determined from isothermal calorimeter results using three different computational methods. The results were used to develop a systematic computational method for characterizing Ea to account for the effect of temperature on the overall rate of hydration of cementitious materials. This work lays the groundwork for more extensive studies to determine the effect of a wide variety of variables on Ea

Temperature Boundary Condition Models for Concrete Bridge Members

The temperature development of mass concrete elements is strongly dependent on constituent materials and mixture proportions, as well as the formwork type, geometry, and environmental conditions. This paper presents a method to account for the effects of convection, radiation, and shading on the surface temperature of mass concrete. Solar radiation, atmospheric radiation, surface-emitted radiation, and formwork radiation exchange were considered. Wind speed, ambient temperature, and surface roughness were included in the convection model. The model described was incorporated into a mass concrete temperature prediction model. The predicted temperatures were then compared with measured near-surface concrete temperatures. The ability of the model to predict the maximum temperature and maximum temperature difference were also examined. The results show that the model accurately estimates the near-surface concrete temperatures, the maximum temperature, and maximum temperature difference of the 12 concrete members instrumented