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Nuclear Magnetic Resonance (NMR) is used to study the early-age hydration as well as autogenous shrinkage and drying shrinkage of cement paste with the addition of coir fibers. The dosage and water content of coir fibers are evaluated. The results show that added coir fibers have a negligible influence on cement hydration kinetics. However, the internal curing effect of saturated coir fibers and water competition induced by dry fibers are visualized. As the saturated fiber content increases, autogenous shrinkage and drying shrinkage decrease due to the internal curing and reinforcing effects of fibers. Dry fibers generate more autogenous shrinkage but less drying shrinkage than saturated fibers of the same dosage due to the introduced water competition. NMR is proven able to track water transportation and shrinkage in cement composites.

The ingress of water-soluble aggressive species is a major cause to the reduced service life of concrete structures. Bulk modification by adding hydrophobic agents in the cementitious materials is commonly applied to reduce water penetration into the concrete matrix. However, conventional hydrophobic modifications of concrete are associated with several unresolved major drawbacks such as high cost and toxicity. Here, we investigate the impact of safe and easy-to-culture bacteria on the hydrophobic properties of concrete. We demonstrate that hydrophobic concrete can be conveniently created by using the liquid culture of bacteria without the need of growing biofilms as suggested by previous research, even with a strain that does not present innate hydrophobicity. These key findings greatly improve the feasibility for the large-scale application of water resistance cementitious materials. Scanning electron microscopy and X-ray diffraction analyses indicated higher levels of micro-calcite formations with the increased addition of bacteria. These micro-calcite structures account for the dramatic increase in surface roughness which leads to elevated hydrophobic properties of the hybrid concrete. This phenomenon is reminiscent of the “lotus-leaf effect”, with ultrahydrophobicity arising from the nano- and microscopic topologies of surfaces. Bacteria-modified cement presents distinct heat-flow profiles during hydration consistent with the increased calcite formation. Taken together, this research gives unprecedented mechanistic insight into the design of microbial-modified concrete with hydrophobic properties particularly useful in floating and offshore structures. Given the large repertoire of bacteria still to be used, our work sets the stage for the future design of microbial-based hydrophobic concrete that can be tailored with additional functions such as self-healing and odor removal.