Variations in carbon dioxide absorption in new cement

Robby Lee Borden

The cement industry contributes about 5% of global carbon dioxide (CO2) emissions and is an important component in concrete applications, such as buildings, superstructures and pavements. These hydrated cementitious structures have varying levels of carbonation (re-absorbed CO2) that occur during placement, the primary life of the structure, and later in secondary or disposal applications. Understanding CO2 sequestration in various concrete applications is crucial in determining the carbon footprint and performing life cycle assessments with concrete. Many previous studies have addressed carbon absorption in buildings and other vertical structures, but few have addressed horizontal applications such as pavements. The purpose of this study is to review carbonation in pavement type applications. The cementitious specimens were formulated with ordinary Portland cement and water; some included fly ash. In order to more closely simulate the actual pavement environment with respect to precipitation; weekly drops of de-ionized water (0.06 grams/drop) were applied to each specimen. The specimens were monitored during the aging process and the masses, temperature, and relative humidity were recorded periodically throughout this preparation period. The samples were analyzed for carbonation levels using a Thermogravimetric analyzer (TGA). This research focused on maximum carbonation levels in thin cementitious paste specimens of various thicknesses, cementitious contents, water-to-cement (w/c) ratios, water availability and exposure time (18 to 30 months). This research also examined methodologies to evaluate the carbonation levels from calcium hydroxide (C-H) and calcium silicate hydrate (C-S-H) depending upon the assorted specimen preparation conditions. The results of this research were that it is possible to achieve surface level carbonation for cementitious materials above 70% based on stoichiometric calculations in the first 30 months. These results were highly dependent on thickness and water availability. The thicker specimens, which more closely represented pavement conditions, achieved higher surface levels of carbonation than thinner specimens. This research also found that including fly ash in the cementitious mixture significantly increased the average carbonation levels higher than specimens that were 100% ordinary Portland cement.

Variations in carbon dioxide absorption in new cement

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