Concrete Products covers the issues that attract producers of ready mixed and manufactured concrete focusing on equipment and material technology, market development and management topics.
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FEATURE BY TOM KUENNEN PORTLAND-LIMESTONE CEMENT MIXES PERFORMANCE, SUSTAINABILITY Portland-Limestone Blends, Self-Consolidating Concrete, Nanomaterials on 92nd TRB Meeting Agenda Portland-limestone (PL) cement blends, self-consolidating concrete for drilled shafts, benefits of nanosilica materials in concrete, and interaction of admixtures were among the ready mixed concrete and cast-in-place practice themes presented at the 92nd annual meeting of the Transportation Research Board (TRB) in January. Convening in Washington, D.C., nearly 12,000 transportation professionals from around the world were offered 4,000-plus presentations in upwards of 750 sessions and workshops. Here are some of the most significant papers involving ready mixed concrete; next month we will look at precast concrete at TRB. For more information, visit www.trb.org. PL CEMENT SYNERGIES There are profound benefits in performance and sustainability of concrete structures made with portland cement blended with limestone, but careful attention must be given to the fineness of the limestone portion, say V. Tim Cost, P.E., Holcim (US) Inc., and Isaac L. Howard, Ph.D. and Jay Shannon, Mississippi State University, in their paper, Integrated Improving Concrete Sustainability and Performance Using Portland-Limestone Cement (PLC) Synergies. Concrete Products has covered the drift toward portland-limestone cement (PLC) blends in recent years, but the recent inclusion of PLC—a cement blend of portland cement—in the ASTM and AASHTO specification canons mean PLC is going PHOTOS: Cost, Howard and Shannon Cement paste thermal testing for portland-limestone cement blends 32 | MARCH 2013 mainstream. PLC is similar to ordinary portland cement, but it also contains a specific minor fraction of finely ground limestone particles, the authors say, and can be produced at any portland cement plant. For this, crushed limestone is metered along with clinker and gypsum to the finish grinding mill. "The sources of limestone typically used in cement making as a raw material are ideal for the limestone used in PLCs, thus no additional special materials must be acquired," say Cost, Howard and Shannon. "Limestone is softer than clinker and grinds more easily, so the limestone particles in finished PLC are generally finer than the ground clinker, which favorably effects the overall particle size distribution of the cement." Compared to conventional portland cement, PLC measurably improves concrete sustainability through the reductions of greenhouse gas emissions associated with cement production (by roughly the percentage of limestone used) and by the lower embodied energy of the cement, they add. "Increased use of portland-limestone cements in the U.S. is anticipated, as a new provision for PLCs containing up to 15 percent limestone has been added to blended cement specifications," the authors write. Specifications for blended cements under ASTM C595 and AASHTO M 240 now include provisions for Type IL cements (PLCs) containing up to 15 percent limestone as well as Type IT cements containing combinations of limestone and pozzolan or slag. Published research has documented performance synergies of cementitious mixtures with finely ground limestone (particle sizes generally smaller than for cement), especially in combination with WWW.CONCRETEPRODUCTS.COM