3.1 Compressive strength test results
The compressive strength of concrete specimens with mineral admixtures and Bacillus licheniformis BSKNAU are given in Table 4.
It can be observed from the Table 4, that the strength of concrete specimens with Bacillus licheniformis BSKNAU is higher than that of the concrete specimens without bacteria. The greatest improvement of 18.72% in compressive strength was obtained for bacterial concrete specimens with 15 % SF and 15 % MK when compared to concrete specimens without bacteria and with the same replacement level. The improvement in compressive strength of 10.61 % was obtained for bacterial concrete specimens without mineral admixtures. For bacterial concrete specimens with the replacement level of 10 % SF and 20 % MK, the improvement in compressive strength was found to be 17.01 %. For replacement level of 20 % SF and 10 % MK, the bacterial concrete specimens showed an improvement of 16.37%.
The strength of bacterial concrete specimens increased for all the replacement levels of cement with silica fume and metakaolin. The improvement in strength is due to the healing of pores and micro cracks in concrete matrix with calcite precipitated by bacteria and due to filler effect contributed by silica fume and metakaolin. Micro cracks and pores open the way for ingress of water and air which creates a favourable environment for germination of endospores and for precipitation of calcite. The greatest enhancement has been recorded for the bacterial concrete specimens for replacement level of 15 % SF and 15 % MK. Similar increase in strength was reported by Chahal et al. (2012b) who concluded that the bacteria S. pasteurii increased the 28 days compressive strength of fly ash concrete by 22% . It can be concluded that the replacement level of 15 % SF and 15 % MK will be effective for use along with the bacterial strain in concrete specimens.
3.2 Water absorption test results
The water absorption values of concrete specimens with mineral admixtures and Bacillus licheniformis BSKNAU are given in Table 5.
From Table 5, it was observed that the lowest water absorption of 0.4 % was obtained for bacterial concrete specimens (Bacillus licheniformis BSKNAU) with 15% SF and 15% MK. The water absorption reduced from 1.2% to 0.8% for bacterial concrete specimens without admixtures. For the replacement level of 10% SF and 20% MK, the water absorption reduced from 0. 9% to 0.6%. For the replacement level of 20% SF and 10% MK, the water absorption reduced from 1.0% to 0.7%.
The lowest water absorption for bacterial concrete specimens with mineral admixtures was obtained at the replacement level of 15% SF and 15% MK. The reduced water absorption of bacterial concrete specimens when compared to non-bacterial concrete specimens indicates the reduction in porosity of bacterial concrete specimens. It also means lower permeability to the flow of water.
3.3 Acid attack test
The percentage weight loss of concrete specimens with mineral admixtures and Bacillus licheniformis BSKNAU, due to immersion in H2SO4 are given in the Table 6.
From the Table 6, it can be seen that the minimum weight losses of 0.24% and 0.44% were obtained for bacterial concrete specimens with Bacillus licheniformis BSKNAU at replacement level of 15% SF and 15% MK, after 2 weeks and 4 weeks of immersion in H2SO4 respectively.
The percentage weight loss of concrete specimens with mineral admixtures and Bacillus licheniformis BSKNAU, due to immersion in HCl are given in the Table 7.
For 2 weeks and 4 weeks of immersion in HCl, the minimum weight losses of 0.28% and 0.40% were obtained for bacterial concrete specimens with Bacillus licheniformis BSKNAU at replacement level of of 15% SF and 15% MK respectively (Table 7).
The results of the acid attack test exhibited a similar trend to the results of water absorption test and good performance was obtained at the replacement level of 15% SF and 15% MK. The minimum weight losses of 0.24% and 0.44% were obtained for Bacillus licheniformis BSKNAU after 2 weeks and 4 weeks of immersion in H2SO4 respectively, at the replacement level of 15% SF and 15% MK. Similarly, for 2 weeks and 4 weeks of immersion in HCl, the minimum weight losses of 0.28% and 0.40% respectively were obtained for concrete specimens with Bacillus licheniformis BSKNAU at the replacement level of 15% SF and 15% MK.
The lower weight loss due to immersion in acids (H2SO4 and HCl) indicates the lower permeability of bacterial concrete specimens to the ingress of harmful sulphur and chloride ions which would affect the properties of concrete and lead to corrosion of reinforcement.
3.7 Rapid chloride permeability test (RCPT)
The RCPT results of the concrete specimens with mineral admixtures and Bacillus licheniformis BSKNAU are given in Table 8.
From the Table 8, it can be seen that the rapid chloride permeability of bacterial concrete specimens incorporating Bacillus licheniformis reduced to low and very low values from moderate and low values of concrete specimens with 0 cells/ml of mixing water, respectively.
The rapid chloride permeability of bacterial concrete specimens incorporating Bacillus licheniformis BSKNAU reduced to very low values, at the replacement level of 15% SF and 15% MK. The reduced rapid chloride permeability also indicates greater resistance to the penetration of chloride ions in bacterial concrete specimens.
3.8 Microstructure analysis
Figure 1 shows the scanning electron micrograph, energy dispersive X-ray spectrum and X-ray diffraction pattern of control concrete specimens.
The scanning electron micrograph of control concrete sample shown in Figure 1 a indicates the presence of limited individual crystals. The EDX spectrum of control concrete sample shown in Figure 1 b indicates the presence of calcium (Ca) and the weight% of calcium is 7.31%. The ratio of Ca/CSH content is 0.66. The X-ray diffraction pattern of control concrete sample shown in Figure 1 c indicates the presence of calcite and CSH peaks.
Figure 2 shows the scanning electron micrograph, energy dispersive X-ray spectrum and X-ray diffraction pattern of bacterial concrete specimens incorporating Bacillus licheniformis BSKNAU with mineral admixtures for the replacement level of 15% SF and 15% MK.
The scanning electron micrograph of concrete samples with Bacillus licheniformis BSKNAU shown in Figure 2 a indicates the presence of individual crystals. The EDX spectrum of concrete samples with Bacillus licheniformis BSKNAU shown in Figure 2 b indicates the presence of calcium (Ca) whose intensity is more than that of the control concrete sample shown in Figure 2 b. The weight% of calcium is 13.50%. The ratio of Ca/CSH content is 0.78. The X-ray diffraction pattern of concrete samples with Bacillus licheniformis BSKNAU shown in Figure 2 c indicates the presence of calcite and CSH peaks.
The individual crystals observed in the concrete samples at replacement level of 15% SF and 15% MK can be interpreted as calcite crystals precipitated by Bacillus licheniformis BSKNAU. The high intensity of calcium in EDX spectra of concrete samples with Bacillus licheniformis BSKNAU at replacement level of 15% SF and 15% MK confirmed the presence of high amounts of calcite in them. The weight% of calcium in the control concrete sample obtained from EDX analysis is only 7.31%. The weight% of calcium obtained for the concrete samples with Bacillus licheniformis BSKNAU (at replacement level of 15% SF and 15% MK) amount to 13.5% which is more than that of the control concrete sample. This confirms the presence of high amounts of calcite in bacterial concrete samples when compared to the less amount of calcite in control concrete samples.
The ratio of Ca/CSH for the concrete samples with Bacillus licheniformis BSKNAU (at replacement level of 15% SF and 15% MK) is 0.78. The increased ratio of Ca/CSH in bacterial concrete samples when compared to the control concrete samples confirms the presence of high amounts of calcite and CSH which is the reason for increased strength and durability of bacterial concrete specimens.
The XRD spectrum of the concrete samples Bacillus licheniformis BSKNAU (at replacement level of 15% SF and 15% MK) indicated the presence of calcite and CSH peaks.
The strength and durability of bacterial concrete specimens increased with respect to non-bacterial concrete specimens. The reason for this increased strength and durability of bacterial concrete specimens with respect to the non-bacterial concrete specimens is due to the plugging of micro cracks and pores in concrete with calcite crystals precipitated by bacteria. The presence of calcite was visualised in the scanning electron micrographs of bacterial concrete samples incorporating Bacillus licheniformis BSKNAU at replacement level of 15% SF and 15% MK. The high intensity and weight% of calcium in these bacterial concrete samples also proved that high amounts of calcite (Ca) were present in them. The increase in CSH content as evident from the high ratio of Ca/CSH and the presence of CSH peaks in XRD spectra was the reason for increased strength of the bacterial concrete specimens.
Dhami et al. (2013) reported that the CaCO3 (calcite) content in control specimens was only 6%. But the CaCO3 content on the surface layer of bacterial blocks was 32%. The high amounts of calcium in bacterial treated cylinders were confirmed by EDX analysis. Vahabi et al. (2015) concluded that EDX analysis confirmed that the crystals formed in the cracks of mortar surfaces by bacteria were predominantly composed of calcium whose weight% was 48.13.