Biological Self-Healing of Cement Paste and Mortar by Non-Ureolytic Bacteria Encapsulated in Alginate Hydrogel Capsules.

Mohammad Fahimizadeh,Lee Sui Mae,Ayesha Diane Abeyratne,Pooria Pasbakhsh,R K Raman Singh

doi:10.3390/ma13173711

Mohammad Fahimizadeh, Lee Sui Mae + Show 3 more

Open Access

https://doi.org/10.3390/ma13173711

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Abstract

Crack formation in concrete is one of the main reasons for concrete degradation. Calcium alginate capsules containing biological self-healing agents for cementitious materials were studied for the self-healing of cement paste and mortars through in vitro characterizations such as healing agent survivability and retention, material stability, and biomineralization, followed by in situ self-healing observation in pre-cracked cement paste and mortar specimens. Our results showed that bacterial spores fully survived the encapsulation process and would not leach out during cement mixing. Encapsulated bacteria precipitated CaCO3 when exposed to water, oxygen, and calcium under alkaline conditions by releasing CO32− ions into the cement environment. Capsule rupture is not required for the initiation of the healing process, but exposure to the right conditions are. After 56 days of wet–dry cycles, the capsules resulted in flexural strength regain as high as 39.6% for the cement mortar and 32.5% for the cement paste specimens. Full crack closure was observed at 28 days for cement mortars with the healing agents. The self-healing system acted as a biological CO32− pump that can keep the bio-agents retained, protected, and active for up to 56 days of wet-dry incubation. This promising self-healing strategy requires further research and optimization.

Highlights

Concrete is the most popular construction material globally with 30 billion tons used annually in 80% of the construction cases [1,2]
Calcium alginate (CaAlg) capsules were incorporated into cement paste and mortars, and the regain of flexural strength and crack healing by CaCO3 precipitation under wet–dry cycles were observed as self-healing hallmarks
While CaAlg capsules prepared with either calcium lactate (CaL) or CaCl2 displayed signs of degradation after exposure to cement filtrate (CF), this was not observed in capsules placed inside cement samples

Summary

Introduction

Concrete is the most popular construction material globally with 30 billion tons used annually in 80% of the construction cases [1,2]. Concrete can withstand compression but is vulnerable in tensile stress, which can lead to micro-cracks appearing as early as the setting process. Such cracks allow water, gases, and corrosive substances from the surrounding environment to ingress into the structure from an early stage and further diffusion through the interconnected porous network of the concrete matrix, causing structural degradation [3]. The degradation of concrete structures poses serious socioeconomic and environmental burdens, since cement production and distribution are responsible for 7% of global CO2 emissions [2], while the maintenance and repair of aging structures account for 30–50% of all spending in the construction sector [5].

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