• Expansion of studies with fibres to include use of hemp. • Production of columns of biocement to facilitate injection of cementation medium and to

test for self-healing following unconfined compression testing (physical damage).

6. CURRENT EXPERIMENTS: COLUMN STUDIES

3. SELF-HEALING BIOCEMENT CONCEPT

Development of Self-healing Biocement1Christine A Spencer, 2Henrik Sass

1School of Engineering, 2School of Ocean and Earth Sciences, Cardiff University, Cardiff, UK

Recent studies by Montoya and Dejong (2013), and Botusharova et al. (2017), demonstratedthe ability of microbially induced calcium carbonate precipitation (MICP) treated soilstructures to self-heal, in principle. Shear strength regain was achieved by injecting thenutrients and precursor chemicals required for MICP into a degraded biocement.

To enable a truly autonomous healing process, the nutrients and precursor chemicals (alsoreferred to as the cementation medium), will need to be readily available within the soilmatrix. This research explores the use of carrier materials to facilitate the storage and releaseof cementation medium from within the biocement.

3. MATERIALS AND METHODOLOGY

Preliminary studies:1) Culture of Sporosarcina ureae, and induction of sporulation.3) Production of a concentrated cementation medium consisting of the nutrients and precursor chemicals (Oxoid CM001 nutrient broth, urea, ammonium chloride, sodium bicarbonate and calcium chloride dihydrate) required for MICP.2) Immobilising the concentrated cementation medium within selected carriers including; diatomaceous earth (1), expanded perlite (2), and natural fibres such as coir (3) and jute (4), in addition to encapsulation within sodium alginate beads (5).

3) Quantifying and comparing the immobilisation capacity of the carrier materials through repeated soaking and drying cycles. 4) Quantifying and comparing cementation medium release from carriers, through immersion of loaded carriers in triplicate in deionised water for set periods, draining and drying. 5) Investigating effects of carriers on the MICP process, within aqueous solutions consisting of cementation medium obtained by soaking loaded carriers in deionised water, and inoculating this with Sporosarcina ureae. Use of ICP-OES to measure calcium concentration of samples. 6) Production of mini columns of biocement in triplicates (6), with and without a selected carrier material. Ten treatments of cementation medium over fourteen days via vacuum assisted surface percolation (7). Testing of effluent to monitor pH, dissolved oxygen and aqueous calcium.

Immobilisation and release of cementation medium

7. POTENTIAL APPLICATION

Rail embankment stabilisation • Current method of stabilisation via cementation – mixing soil

with Portland cement and water• Problems – expansion, not sustainable as subject to

deterioration from vibrations and weathering• Potential advantages of Self-healing MICP: ✓ Sustainable eco-friendly approach✓ Improved mechanical properties of the soil structure -

increased strength, stiffness and mitigation against dilation

REFERENCESBotusharova, S. 2017. Self-healing Geotechnical Structures Via Microbial Action. PhD Thesis, Cardiff UniversityMontoya, B.M. and Dejong, J.T. 2013. Healing of biologically induced cemented sands. Geotechnique Letters 3, pp. 147–151De Belie, N. 2010. Microorganisms versus stony materials: a love-hate relationship. Materials and Structures 43(9), pp. 1191–1202Image 9: https://www.51m.co.uk/wp-content/uploads/2014/01/staffs-viaduct-250x180.jpg

ACKNOWLEDGEMENTSCardiff University School of Engineering are providing the stipend for the first author’s PhD research.SfAM are providing funding to support attendance of the first author at FEMS2019.

This project aims to enhance biocemented soil structures so that they may autonomously self-heal when damaged.

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Key findings and further work required:• Viability of carriers for cementation medium storage and release, with exception of Coir.• Potential beneficial effect on MICP of use of diatomaceous earth.• pH increase and calcium ion depletion indicating MICP.• Loss of immobilised cementation medium during biocementation to be reduced.• Further testing required with fibres.

51 3 42

2. MICROBIALLY INDUCED CALCIUM CARBONATE PRECIPITATION

The production of biocement for this work relies upon microbially induced calcium carbonate precipitation (MICP), via the ureolytic pathway:1) Active ureolytic bacteria produce urease, which catalyses the hydrolysis of urea, resulting in

the production of carbamate (NH2COOH) and ammonia (NH3).2) Carbamate hydrolyses spontaneously to produce ammonia and carbonic acid (H2CO3).3) Simultaneously, in the presence of water, the ammonia and carbonic acid equilibrate, to

produce ammonium (NH4+) and hydroxide (OH-) carbonate (CO3

2-) ions.The global reaction is as follows, (De Belie 2010):

𝐶𝑂(𝑁𝐻2)2 + 2𝐻2𝑂 → 2𝑁𝐻4+ + 𝐶𝑂3

2− (1-3)4) In the presence of calcium, calcium carbonate is precipitated.

𝐶𝑂32− + 𝐶𝑎2+ ↔ 𝐶𝑎𝐶03 (4)

S. ureae (8) spores (stained using malachite green), as observed under an optical microscope, with vegetative cells stained red with safranin, 24 hours after immersion in sporulation medium.

67

Cementation medium immobilised by coir (C) , Jute (J), Expanded Perlite (EP) and Diatomaceous Earth (DE), following repeated soaking of carriers in concentrated

cementation medium for 24 hrs at 21oC.

Cementation medium released into deionised water, as a percentage of medium immobilised,

over 50 hrs, at 21oC.

Effect of carriers on MICP process

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Biocement incorporating expanded perlite preloaded with cementation medium

1. INTRODUCTION

2. MICROBIALLY INDUCED CALCIUM CARBONATE PRECIPITATION

5. PRELIMINARY RESULTS

4. SELF-HEALING BIOCEMENT CONCEPT

3. MATERIALS AND METHODOLOGY

7. POTENTIAL APPLICATION

MICP

9

Calcium ion concentration in S. ureae inoculated solutions containing cementation medium released from carriers and

standard medium as the control, following incubation.

pH of solutions containing cementation medium of equal concentration released from carriers, compared to standard

cementation medium (control), before inoculation with S. ureae and after inoculation and incubation at 30oC.

pH of effluent extracted during biocementation, compared to the cementation medium treatment (pH approx. 6.2).

Calcium ion concentration of effluent extracted during biocementation.

Biocement columns produced by injecting cementation medium daily over 10 consecutive days.

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1st Loading 2nd Loading 3rd Loading (J, EP, DE)