New Tech for Acid Attack — Part 4
by Whitney B. Belkowitz and Jon S. Belkowitz, PhD
Results and Discussion
Control
Figure 2 illustrates the progress of compressive strength development over 56 days. The control developed 77% and 95% of its ultimate strength (56 day) within seven days and 28 days of curing, respectively. The electrical resistivity of the control shown in Figure 3, progressively increases through 56 days but does not reach a chloride penetrability greater than 1.6 kOhms.cm. This means that the grout sample would have a high chloride penetrability and permeability. Due to the high permeability in the control sample shown in Figure 4, it experienced a mass loss of almost 9%, 14%, and 17% at 7, 28 and 56 days, respectively. The total porosity, illustrated in Figure 5 reduces from 29.0% of the HCM at seven days to 22.5% at 56 days. The hardened properties measured and calculated allude to a grout specimen that would have a significant loss due to acid attack. From the image shown in Figure 6 it can be seen that the 17% mass loss at 56 days has a significant impact on the change in the grout sample composite structure. There is a significant reduction in cement paste with large amounts of exposed aggregate. Most of the exposed aggregate is weakly bonded to the remaining cement paste.
Change in hydrated cement matrix
The development of concrete compressive strength is an important property that helps distinguish the durability of concrete to resist acid attack. Normally, a stronger concrete alludes to a denser, higher quality and more durable HCM, grout, and concrete. The higher quality HCM is developed through a densified HCM by pore reduction and additional C-S-H development. Lowering the w/c from 0.45 (control) to 0.30 had the greatest impact on increasing compressive strengths. Increasing the paste content of the grout mixture also increased the compressive strengths. There is a strength reduction with increased water as well as increased cement to sand ratio.
The bulk electrical resistivity is a measure of the specimens permeability. The greater the bulk electrical resistivity; the lower the pore connectivity within the specimen. A greater bulk electrical resistivity and therefore lower permeability suggests a HCM is more durable to acid attack. Lowering the w/c to 0.30 had the greatest impact on increasing bulk electrical resistivity and therefore reducing permeability similar to the compressive strength. Reducing the paste content also had a positive impact on permeability. As with strength and permeability, mass loss is a procedure used to simulate conditions found on mining and wastewater sites. The lowest mass loss was realized by the 0.30 w/c, as seen in Figure 4, this was expected due a higher strength development and low permeability.
Impact of Metakaolin
When the Metakaolin was used to replace OPC there was an increase in compressive strength when compared to control as shown in Figure 7. The greater replacement of OPC with Metakaolin had a greater impact on the compressive strength samples. An OPC replacement of 5.5% and 30% Metakaolin proved to have the greatest impact on compressive strength increase. The increase in compressive strength was expected due to the pozzolanic reactions induced by the Metakaolin. Similarly to the compressive strength, there was an increase in electrical resistivity readings when Metakaolin was added to the mix. As seen in Figure 8, the highest readings were present in the 7.5% replacement of OPC with Metakaolin. In general, we expect our electrical resistivity to increase (when compared to the control) over time with Metakaolin content. Like the compressive strength, this increase in electrical resistivity and reduction in permeability is a direct result of HCM densification from the pozzolanic reaction promoted by the addition of Metakaolin. While the highest replacement of OPC with Metakaolin showed the greatest reduction in mass loss at 7 days, it was the lower dosages that proved to have long-term significant impact on mass loss reduction, illustrated in Figure 9. From the data provided there seems to be a correlation between a change in compressive strength, permeability and an associated mass loss. Furthermore, the grouts with Metakaolin were shown to have a significant reduction (except for the highest dosage of Metakaolin, M4) in total porosity, illustrated in Figure 10, over 56 days. From the data presented it seems that there needs to be an optimum dosage of Metakaolin to balance the increase of compressive strength and an increase in electrical resistivity in order to effectively enhance the HCM to reduce acid attack. The grout mixture that had 5.5% and 15% OPC replacement illustrates this phenomenon. The samples made from these grout mixtures had the highest combined compressive strength and the bulk electrical resistivities. These grout samples also showed the lowest mass loss of the Metakaolin mixtures.
Impact of nano silica
The development of compressive strength over 56 days with addition of nano silica is illustrated in Figure 11. The greatest increase in compressive strength is with the addition of the largest and unmodified nano silica, NS1, at its highest dosage. Similarly, the NS2 shows some moderate compressive strength increases but not as significant as those found with NS1. When combining the nano silica and the Metakaolin, an increase in compressive strength was observed over the grout mixture that had the same amount of Metakaolin without nano silica. This increase in compressive strength is believed to have been caused by the high surface area from the nano silica that promotes instantaneous pozzolanic reaction, accelerated cement dissolution and heterogeneous nucleation. Like the compressive strength, electrical resistivity was found to increase the most with NS1 at its highest dosage, shown in Figure 12. The NS1 with the lower dosage had lower electrical resistivity readings than the control. Despite the fact that the NS1 at its lowest dosage and the NS2 at both dosages increase compressive strength, these dosages do not seem to be high enough to have a significant impact on reducing permeability. This could be due to the lower amount of reactive surface area of the large NS1 particle and the surface modified NS2 particle. All mixes with nano silica additions experienced a decrease in mass loss by 28 days when compared to the control with significant decreases at 56 days, as seen in Figure 13. Finally, the total porosity presented in Figure 14 suggests that the nano silica alone and combined nano silica and Metakaolin would develop reduction in total porosity. The lowest mass loss was achieved by the NS1 particle with the higher dosage, with a loss of only 2% at 56 days. This is likely due to this nano silica particle creating a highly densified microstructure.
