Advances in Sustainable Cements
Concrete is crucial for infrastructure development and is the most widely used construction material in the world by mass. Concrete's use will only continue to increase as rapidly developing countries like India and China invest in their infrastructure and developed countries like the United States have to repair an aging infrastructure. However, the concrete industry accounts for 7-8% of the anthropogenic CO2 emissions worldwide and the production of portland cement account for 5% of the anthropogenic CO2 emissions. Anyone trying to improve the sustainability of concrete would do well to try and lower cement's environmental impact. Calcium sulfoaluminate cement is promoted as a sustainable alternative to portland cement because of its lower energy demand and CO2 emissions during production. However, calcium sulfoaluminate cement is not as well studied as portland cement and it could potentially be made even more sustainable if more fundamental knowledge was known about it. For instance the expansive mechanism for calcium sulfoaluminate cement which can cause deleterious cracking and shortened life spans for structures is still debated and not fully understood.
This study utilized a wide variety of analytical, microscopy, and physical techniques to study calcium sulfoaluminate cement and its hydration reactions in order to more fundamentally understand the hydration reactions taking place and to determine calcium sulfoaluminate's elastic properties. High pressure X-ray diffraction was performed at the Advanced Light Source in the Lawrence Berkeley National Laboratory to determine calcium sulfoaluminate's bulk modulus and crystal structure. In-situ hydration reactions were followed using transmission X-ray microscopy at the Advanced Light Source. The hydration reactions were also monitored ex-situ with scanning electron microscopy, X-ray diffraction, and thermogravimetric analysis. The mechanical properties of the cement were studied with compression tests and dimensional stability bars.
The best fitting crystal structure for calcium sulfoaluminate was determined to be orthorhombic and its bulk modulus was calculated to be 69(6) GPa. Calcium sulfoaluminate was found to produce stellated structures during its hydration in dilute suspensions and it is hypothesized that the mechanical interlocking of adjacent stellated structures could contribute significantly to the strength of calcium sulfoaluminate cements. Calcium hydroxide was found to promote the formation of a poorly crystalline solid solution (SO42-/OH-) AFm on the surface of the hydrating calcium sulfoaluminate grains. This coating delays the formation of ettringite and is believed to play an important role in the expansion mechanism of calcium sulfoaluminate cement. Although calcium carbonate's low solubility prevent them from taking part in the early-age hydration reactions, calcite and vaterite did react with monosulfate to yield ettringite and monocarboaluminate. Vaterite was found to be approximately three times faster at converting monosulfate than calcite. Strength increases correlated in timing to the conversion of monosulfate to ettringite and monocarboaluminate. Both calcite and vaterite were shown to decrease the set times of calcium sulfoaluminate cement and decrease the magnitude of expansion. Incorporating calcium carbonates into calcium sulfoaluminate cements appears to be very promising from both an environmental and performance standpoint. Particularly, if a cement plant could capture its CO2 emissions and utilize them to make vaterite, the sustainability of the cement could be greatly advanced.