Recovered cellulosic fibres as a novel carrier for self-healing cementitious materials

Abstract

Cementitious materials are the most widely used construction material due to their ability to be used in a wide range of applications. However, cementitious materials place environmental stresses by OPC alone, accounting for 7% of global man-made CO2 emissions. Concrete is susceptible to microcracking, which puts a significant financial burden on the industry to repair. The use of self-healing technology has been an effective method in managing microcracking autonomously. This thesis aims to develop a sustainable selfhealing technology utilising recovered cellulose waste material from wastewater treatment as a novel carrier material for the application of self-healing cementitious materials. The approach used in this thesis involves a comprehensive experimental regime in which L. Pakistanensis was selected from a number of soil isolated microorganisms to be the most suitable for self-healing cementitious applications. The media was optimised for this particular bacterium to improve ureolytic activity to maximise CaCO3 production. The bacterial spores were then immobilised and coated to compare leakage, protective capabilities, and survival after immobilisation. Finally, the immobilised recovered cellulosic fibres were then used as additives in mortar to promote self-healing, in which the mechanical properties were measured, along with self-healing capabilities, monitoring the crack width and regained strength of the self-healing material. Finally, the healed products were then characterised using SEM, EDS, FTIR, and XRD to identify the material being produced. This study demonstrates the healing capabilities of a novel wild strain of bacterium, in which the optimised nutrients were able to show CaCO3 production of 0.43 g/100 mL after 72 hours. It was also able to demonstrate that upon investigating the microstructure of the immobilised bacteria and nutrients using oven drying, CaCO3 was produced, which was absent in the freeze-dried specimens. Furthermore, by utilising RCF as a carrier, it was able to heal a crack width of 0.60 mm within 28 days. Additionally, exposure to various healing conditions presented novel brown coloured healing products when exposed to 5% CO2. This study also uncovered the challenges associated with the requirement of liquid water for healing to occur, in which healing was observed with the use of RCF. Signs of healing were observed within 28 days in relative humidity conditions (60% and 95%). Finally, coating immobilised RCF with sodium silicate (30%) was able to demonstrate a regained strength of 47.26% within 28 days. This study shows the valorisation of a waste material which provides excellent self-healing capabilities due to its unique internal structure system, moisture absorption capabilities, and crack width controlling ability. It can therefore be selected as a suitable additive to cementitious material to promote effective healing. Although current challenges limit specific applications of self-healing technology. However, this novel carrier provides an alternative sustainable carrier option, which improves the application options of self-healing technology. There are limited studies in the literature using L. Pakistanensis and RCF for selfhealing applications, along with showing the ability of this novel self-healing agent to work without the presence of liquid water. These findings in this thesis contribute to a circular economy approach in the use of recovered material at the end of their service life and widen the application for self-healing, not limited to the presence of liquid water.