Microbial-Induced Calcite Precipitation
Figure 1: (a) Schematic drawing of the batch reactor and (b) image of specimens after the reactions but before strength testing.
1b shows an example of MICP-treated soil specimens aſter 7 days of reaction. Unconfined compression strength tests. Mechanical
compression tests were conducted on unconfined cylinder- shaped specimens with a 2H:1D ratio (Figure 1b) to determine the unconfined compression strength (UCS). Te tests were conducted under strain-controlled conditions at a uniform loading rate of 1.5%/min in accordance with ASTM D2166. SEM specimen preparation. MICP-treated samples were
removed from the reactor tank for UCS testing to failure. Failed specimens were prepared for microscopy by oven-drying over- night at 105 °C before mounting on SEM stubs with adhesive carbon-conductive tabs. Te fracture surfaces were sputter- coated with gold for SEM analysis and FIB/SEM analysis. SEM instrumentation. Specimens were imaged and ion-
beam-milled in a TESCAN LYRA3 FIB/SEM. Secondary elec- tron images were acquired at 5 kV. Certain fracture surfaces were milled with a Ga+
ion beam current of 10 nA at 30 keV
to expose cross sections of sand particle bonding. Te cross- section surface was then polished with a low ion beam current of 100 pA at 25 keV for 1 hour prior to SEM imaging. Elemental analysis was obtained with a Termo Scientific NORAN (Sys- tem 7) energy-dispersive spectrometer (EDS).
Results Compression tests. Te unconfined compression strength
(UCS) tests were carried out to investigate the mechanical per- formance of MICP treated samples. Table 1 shows the UCS and dry density results for MICP-treated samples over differ- ent numbers of treatment cycles. Te UCS and dry density of MICP-treated samples increases with treatment cycles, indicat- ing that more calcium carbonate precipitation was generated through repeated treatment cycles. Te UCS of a MICP double- treated sample is more than twice that of a MICP single-treated
UCS (kPa) Dry density (g/cm3 26 )
Single-treated 1200±130 1.72±0.12
sample, which indicates that the increase in treatment cycles not only induces more calcium carbonate precipitation but also increases the effective bonding. However, the relative increase in UCS was less for triple- and quadruple-treated samples. Tis was due to the reduction in void space with each treatment, which would reduce the surface area available for bonding. Carbonate bonding of sand particles. Figure 2a shows
an SEM image of untreated Ottawa sand with individual sand particles exhibiting smooth surfaces. Aſter each unconfined compression strength test, the fracture surface of a crushed MICP-treated sample was observed by SEM to analyze the microstructure of CaCO3
. Figure 2b shows sand particles
bonded by calcium carbonate precipitation aſter a single MICP treatment. Tere were many dome-like calcium carbonate precipitates attached to the surface of sand particles that did not participate in bonding. However, Table 1 shows that the strength and density of multiply MICP-treated sand increased with each treatment. Non-treated samples (loose sand) have little strength and are not shown in Table 1. Multiple treatments increase calcium carbonate precipi-
tation and increase the bond area between sand particles, thereby increasing the strength. Figure 3a shows that aſter four processing treatments, the calcium carbonate precipitation wrapped around sand particles so that the sand particles could be better bonded together. Te failure mode of the MICP- treated sand was bond-particle failure. Figure 3a shows that some sand grains were detached from the calcium carbonate bond, leaving smooth concave regions, which were impres- sions of missing sand grains. X-ray emission spectrometry. An EDS X-ray spectrum,
acquired while scanning the entire image frame shown in Figure 3a, is shown in Figure 3b. Te EDS analysis confirms the presence of carbon, oxygen, and calcium, the elements that made up CaCO3
in the MICP-treated sand. Te silicon peak
Table 1: Unconfined compression strength (UCS) of MICP-treated samples over different numbers of treatment cycles for a cementation media concentration of 0.5 M Ca.
Double-treated 2700±250 1.88±0.08
Triple -treated 3500±280 1.96±0.10
Quadruple -treated 4300±670 1.98±0.05
www.microscopy-today.com • 2019 January
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