Laurent Barcelo

Effect of Fly Ash on Optimum Sulfate Levels Measured Using Heat and Strength at Early Ages

Calcium sulfate is typically added to cement clinker to prevent flash set and to improve early age strength development without causing additional volume instabilities. Recent changes to ASTM C150, “Standard Specification for Portland Cement,” have enabled greater flexibility in determining optimum sulfate levels in Portland cement by not requiring ASTM C563, “Approximation of Optimum SO3 in Hydraulic Cement Using Compressive Strength,” to set sulfate target levels. ASTM C563 requires strength testing using only hydraulic cement at 23°C, which is not always indicative of the optimum sulfate level for field use, because supplementary materials (e.g., fly ash) might be used and the optimum sulfate level might be sensitive to temperature changes. Adding additional sulfate to account for the sulfate demand of fly ashes can enable an improvement in the early age strength for cement–fly ash systems and decrease the number of problems that may be attributed to cement–admixture–fly ash incompatibility, such as abnormal setting and strength gain. This research provides experimental data on the strength development and heat release during early hydration for cement–fly ash systems with different sulfate levels. It is demonstrated that some fly ashes have their own sulfate demand, and when these ashes are used in cement–fly ash blends there is effectively an increase in the optimal sulfate level that should be used for the Portland cement. It is also shown that the optimum sulfate level determined according to the heat of hydration measured with isothermal calorimetry is similar to the optimum sulfate level determined based on compressive strength at 1 day.

Cement and CO 2 , a victim of success!

Concrete is by far the most widely used building material in the world. The cement industry, which supplies the binder for concrete, is one of the larger industrial sources of CO2 emissions. This is due more to the extensive use of cement and concrete and less to the energy required to produce the cement and concrete. The International Energy Agency (IEA) has issued technology roadmaps for reducing CO2 emissions in different sectors, including the cement industry, through the year 2050. This paper will explain how CO2 is emitted during the cement manufacturing process, the traditional levers to reduce CO2 emissions and what else will need to be done to comply with the IEA roadmap. It will be seen that the traditional levers of energy efficiency, alternate fuels, and clinker substitution will only take the industry about half way to the 2050 targets. This paper points out why the industry should collaborate on the following: (1) Develop and promote a common approach to carbon footprint analysis; (2) Actively promote the overall carbon efficiency of concrete; (3) Push for a common understanding and accounting for traditional levers; (4) Support the IEA roadmap and their sectorial approach, and; (5) Further promote and educate others on life cycle analysis. The cement industrys story is one of tremendous success in overall carbon footprint management that needs to be told in the proper perspective.

Influence of Slag Aggregate Production on Its Potential for Use in Internal Curing

Internal curing is effective at reducing shrinkage and early-age cracking in cementitious systems with low water-to-cementitious materials ratios. In the United States, internal curing is typically accomplished using prewetted lightweight aggregate made by expanding slate, clay, or shale. This research focused on the use of porous slag aggregate, a byproduct of the iron and steel industry, for the internal curing of concrete. Five aggregates were evaluated for use in internal curing. The aggregates were produced from different manufacturing processes. Expanded, pelletized, and air-cooled slag aggregates were chosen for advanced testing. The research began by measuring the absorption and desorption properties of the aggregates. Laboratory testing of concrete mixtures containing select aggregates was performed to evaluate mechanical and durability properties. Full-scale testing was carried out with concrete produced at a ready-mix plant. A conventional department of transportation bridge deck mixture was compared with a similar concrete that was internally cured with prewetted expanded slag aggregate. Internally cured concrete made with expanded slag aggregate was shown to reduce shrinkage cracking with similar or improved overall mechanical and durability properties when compared with the conventional mixture.

Lowering the Carbon Footprint of Concrete by Reducing Clinker Content of Cement

Significant efforts have been made to reduce carbon dioxide (CO2) emissions associated with the manufacture of portland cement, primarily by making the process more energy efficient and increasing the use of alternative fuels. Further reductions in CO2 can be achieved by lowering the clinker component of the cement because the pyroprocessing used to manufacture clinker produces approximately 1 tonne of CO2 for every tonne of clinker. Traditionally reductions in the clinker content of cement have been achieved by producing blended cement consisting of portland cement combined with a supplementary cementing material (SCM). In Canada, it is now permitted to intergrind up to 15% limestone with cement clinker to produce portland limestone cement or blended portland limestone cement. Recent trials were conducted at the Brookfield cement plant in Nova Scotia to evaluate the performance of a blended cement containing 15% ground, granulated blast furnace slag (an SCM) with that of a blended portland limestone cement containing the same amount of slag plus 12% interground limestone. Performance was evaluated by the construction of a section of concrete pavement using concrete mixtures produced with the two cements and various amounts of fly ash (another SCM). A wide range of laboratory tests were performed on the concrete specimens cast on site during the placement of the concrete pavement. The results indicated that the cements were of equivalent performance.