Polydopamine@carbon nanotube reinforced and calcium sulphoaluminate coated hydrogels encapsulating bacterial spores for self-healing cementitious composites

Abstract

Bacterial spores have been applied to develop self-healing cementitious composites, while protections of endospores in concrete are required. Herein, one reinforced calcium alginate hydrogel with a protective shell was designed to encapsulate bacterial spores. The hydrogels were reinforced by highly dispersed carbon nanotubes (CNTs) coated with polydopamine, which led to more spherical shaped hydrogels, hence calcium sulphoaluminate (CSA) cement can be uniformly coated on the hydrogels to form the protective shell. With the reinforcement of CNTs and the coating of CSA cement, the hydrogels were conducive to germination, growth and activities of bacteria even after the hydrogels endured long-term exposure to strongly alkaline and calcium concentrated solution. Due to these positive effects of the modified hydrogels on spores, concrete with spores encapsulated in the modified hydrogels obtained enhanced self-healing performance in terms of crack width and permeability reductions as more biological mediated calcium carbonate was precipitated within cracks.

Introduction

Global warming has been an urgent issue for the whole world, which is mainly driven by the greenhouse gas emissions [1], especially CO₂ emissions [2]. To address this challenge, the objective of carbon neutrality by 2050 has been put forward in 2020 by United Nations [3]. The achievement of this aim is inseparable from the efforts of the building and construction sector. The CO₂ emissions associated with buildings contributed 50% of total emissions in Europe [4,5], in which production of cementitious materials led to 7% of the carbon footprint [[6], [7], [8]]. Hence, developing sustainable cement-based materials with low CO₂ emissions is critical.

While concrete structures can have a long service life of over 50 years, cracking of concrete due to overloading and volume instability [9] caused high consumptions of raw materials and labor burden for repairing, which resulted in massive CO₂ emissions [10] and high cost. For example, UK spent £40 billion/year on repair and maintenance of old concrete structures [11], whilst $27.7 billion/year and $53 billion/year were required to rehabilitate bridges and roads respectively in US [12,13]. To lower the cost and carbon footprint resulted from concrete repairing, self-healing cementitious composites have been extensively researched in recent decades.

Self-healing of cementitious composites can be categorized into autogenous healing and autonomous healing. Autogenous healing mainly relies on the further hydration of cementitious materials and carbonation [14,15], which can contribute to the recovery of durability and mechanical properties. As it is only effective when the crack size is restricted [16], autonomous healing by incorporating tailored constituents including shape memory alloy [17], superabsorbent polymers [18,19], minerals [20,21] and bacteria [22] for self-healing has been explored.

Among these constituents, bacteria are very effective in promoting self-healing of cracked concrete as they are capable of inducing calcium carbonate precipitation. The bacteria with negative surface charges can act as nucleation sites [22,23] by attracting calcium ions. Moreover, carbonates can be supplied by metabolic pathways of bacteria in cracks [22,24,25], including catalysis on urea hydrolysis [26,27], oxidation of organic carbon by aerobic respiration [28,29] and denitrification under anaerobic respiration [30,31]. These metabolic activities could elevate local pH, thereby establishing a favorable microenvironment for calcium carbonate precipitation.

Nevertheless, concrete is harsh to the bacteria since the pH of the concrete pore solution can be higher than 13 [32], which is critical to the survival of bacteria, and densification of pore structure may crush bacteria [33].To maintain the viability of bacteria in concrete, protections of bacteria or bacterial spores before concrete cracking by encapsulation [34] or immobilization [35,36] with carriers are required. Among the carriers to encapsulate bacteria in concrete, hydrogels which can provide a moderate pH environment with rich moisture are promising [33]. Specifically, calcium alginate hydrogels with good biocompatibility [37,38] and mild gelation conditions can be applied to encapsulate bacteria. Nonetheless, the application of the calcium alginate hydrogels in concrete is restricted due to the relatively weak crosslinking [33] and susceptibility to environmental factors [39], which could lead to drastic deformation of the hydrogels with densification of cementitious composites and inadequate protections of encapsulated bacterial spores against transport of alkalis.

To address these issues, polydopamine@carbon nanotube (PDA@CNT) reinforced calcium alginate hydrogels with low-alkaline calcium sulphoaluminate (CSA) cement coated as a shell were designed to encapsulate bacterial spores for self-healing concrete herein. Due to the reinforcement of well-dispersed PDA@CNT with high mechanical properties [40,41] in hydrogels, the soft calcium alginate hydrogel can endure the shrinkage stress and maintain its spherical shape. Afterwards the reinforced hydrogels were further coated by CSA cement paste as it has relatively low cost, adequate strength and a pH of around 10–11, which is much lower than that of Portland cement [42], thereby establishing a more moderate environment for hydrogels than Portland cement paste. The schematic of the capsule is shown in Fig. 1. Therefore, it can be anticipated that the reinforced hydrogels with CSA cement shell can protect the encapsulated bacterial spores in Portland cementitious composites and subsequently the encapsulated bacterial spores can contribute to self-healing of cracks effectively. The properties of capsules encapsulated with bacterial spores were investigated before its incorporation into Portland cementitious composites for self-healing study.