Concrete Products

MAR 2018

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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Page 8 of 75 March 2018 • 7 GUEST EDITORIAL SIX SIGMA CONCRETE the concrete surface. All things considered, BCSA cement exhibits about half or a third of the shrinkage found in portland cement. Sigma 4 – Sustainability It is estimated that portland cement pro- duction worldwide contributes to about 5-10 percent of the anthropogenic carbon emissions. The production of BCSA cement is estimated to emit 25-35 percent lower carbon emissions when compared to the production of portland cement. There are two main rea- sons for this reduction. One, the cement mix design requires less limestone. The calcina- tion of limestone is the primary contributor of CO 2 emissions, and so less limestone results in a lower carbon factor. Second, the produc- tion of BCSA cement requires lower burning temperatures (about 200° C). The lower com- bustion temperature requirements come from the absence of alite, which requires higher burning temperatures. With portland cement netting about one ton of carbon dioxide per ton of finished product, the use of BCSA cement addresses some of the environmen- tal concerns often associated with cement production and is a much more sustainable product. Sigma 5 – Silica Reaction Alkali silica reaction represents a severe form of concrete deterioration. This topic has been a hot issue more recently, and more govern- ment entities are requiring ASR testing in concrete specifications. Unfortunately, the fundamental mechanisms of ASR are not very well understood. There are a few industry standard tests that have been helpful, but even they do not always perfectly predict ASR behavior. However, the use of BCSA cement has shown better mitigation than portland cement. This is likely due to a combina- tion of lower alkalis, lower pH, and reduced porosity in BCSA cement, but it has yet to be confirmed. Still, the use of BCSA cement in confronting the problem of ASR is quite promising. Sigma 6 – Sulfate Resistance It has been well documented that sulfate resistance is directly tied to the amount of celite in the cement. Each type of portland cement has varying levels of celite, and the less you have, the more sulfate resistant the cement. For example, Type V is the most sul- fate resistant portland cement in the ASTM C150 family, and it typically has 4-5 percent celite. In BCSA cement there are no detectible amounts of celite, and therefore it exhibits the greatest sulfate resistance, even better than Type V portland cement. This fact has long been established in academia as well as in the field. Solid track record Belitic calcium sulfoaluminate cement is an innovative solution to many of the short- comings of traditional portland cement. While it may not be the only solution, BCSA cement is definitely capable of producing a six sigma concrete. Since the 1970s, BCSA cement has a proven track record of suc- cess. It is used worldwide by airport and department of transportation officials to achieve high quality, long lasting concrete results. ASTM C150 VS. BELITIC CSA CEMENTS Cement type Alite, Belite Celite Calcium sulfoaluminate C 3 S (%) C 2 S (%) C 3 A (%) C 4 S 3 A (%) Type I portland 59 15 12 0 General purpose Type II portland 46 29 6-8 0 Moderate sulfate resistance Type III portland 60 12 12-15 0 High early strength Type IV portland 30 46 5-7 0 Low heat of hydration Type V portland 43 36 4-5 0 High sulfate resistance Belitic CSA 0 45 0 25 Rapid Set Cement Alite or tricalcium silicate is responsible for early strength and begins hydrating at around 24 hours. Belite or dicalcium silicate imparts late age strength and begins hydrating much later than alite for an extended, indefinite period. Hydrating alite and belite form CSH or calcium silicate hydrate gel, prone to shrinkage due its dimensionally unstable character- istics. Celite or tricalcium aluminate reacts with sulfates and causes undesirable sulfate attack in concrete. CSA or calcium sulfoaluminate hydrates immediately and is practically complete within 24 hours, hence its early strength and rapid hardening properties.

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