1. Objective
The goal of this project is to investigate if partially replacing Portland cement with fly ash or pumice affects the uniaxial compressive strength (UCS) of mine backfill.
2. Introduction
Pozzolans are siliceous or siliceous and aluminous materials that react with the calcium hydroxide (CaOH) released during cement hydration to produce compounds with cementing properties. Pozzolans are widely used in civil and mining applications to reduce costs and improve the mechanical and physical properties of concrete. Cement can account for up to 75% of mine backfill costs; therefore, it is common practice in the mining industry to partially replace relatively expensive cement with relatively inexpensive pozzolans (Edraki et al., Reference Edraki, Baumgartl, Manlapig, Bradshaw, Franks and Moran2014). Examples of artificial pozzolans are slag and fly ash, and examples of natural pozzolans are volcanic ash and the pumice formed during lava solidification. Artificial pozzolans are widely used, but natural pozzolans have not been fully exploited, although they are abundant in many countries. For example, Saudi Arabia is fortunate to have a large resource of natural pozzolans, and research is ongoing to investigate their use in the concrete industry (Al-Amoudi et al., Reference Al-Amoudi, Ahmad, Khan and Maslehuddin2019; Celik et al., Reference Celik, Jackson, Mancio, Meral, Emwas, Mehta and Monteiro2014; Kupwade-Patil et al., Reference Kupwade-Patil, Al-Aibani, Abdulsalam, Mao, Bumajdad, Palkovic and Büyüköztürk2016; Moufti et al., Reference Moufti, Sabtan, El-Mahdy and Shehata2000). In this study, pumice was investigated for its potential use in mine backfill compared with fly ash and cement.
3. Methods
Backfill samples were prepared using tailings from a nickel mine in Canada. Figure 1 shows the particle size distribution and Table 1 summarizes the physical properties of the tailings. Control samples were prepared with general use Portland cement. Experimental samples were prepared with class F fly ash or pumice, which comprises mainly silicon dioxide (amorphous aluminum silicate), some aluminum oxide, and small amounts of other oxides. The chemical and physical properties of pumice can strongly influence the durability and strength properties of concrete (Çolak, Reference Çolak2003; Lemougna et al., Reference Lemougna, Wang, Tang, Nzeukou, Billong, Melo and Cui2018; Pekmezci & Akyüz, Reference Pekmezci and Akyüz2004).
Particle size distribution of tailings from a nickel mine in Canada
Physical properties of tailings from a nickel mine in Canada
Five treatments were tested (Table 2): reference samples made with 100% Portland cement; experimental samples made with Portland cement and 10 or 20 wt.% fly ash; and experimental samples made with Portland cement and 10 or 20 wt.% pumice. Binder dosage and pulp density were maintained at 5 and 80 wt.%, respectively.
Five backfill treatments tested
Backfill samples were cured for 7, 14, or 28 days in a moist cabinet, where temperature and relative humidity were maintained at 25 ± 2 °C and 90 ± 2%, respectively, to simulate underground conditions. At each curing age, two samples from each batch were tested for UCS at a loading rate of 1 mm/min until failure (Figure 2). Duplicate means are reported in the results.
Broken backfill samples after performing uniaxial compressive strength test
4. Results
The mean UCS of backfill samples increased with curing time (Figure 3a). Samples prepared with 20% pumice had the lowest UCS at all curing times and reference samples had the highest UCS at 14 days curing time (Figure 3a). By 28 days curing time, the reference, fly ash, and 10% pumice samples had almost similar UCS values (0.51–0.55 MPa), whereas backfill prepared with 20% pumice was approximately 19% lower (0.42 MPa).
a) Uniaxial compressive strength (UCS) for each cemented mine backfill mixture after 7, 14, and 28 days curing time; heat map for curing times of b) 7 days, c) 14 days, and d) 28 days
5. Discussion
The results indicate that after 7 and 14 days of curing, poorer strength was achieved for fly ash and pumice samples than reference samples, which is expected since these pozzolans have a slower hydration rate than Portland cement. However, after 28 days of curing, backfill containing 10 or 20% fly ash or 10% pumice can achieve almost a similar UCS as backfill prepared with Portland cement. Thus, partially replacing cement with 10% pumice appears to be a viable option to achieve a similar UCS at a lower cost. These results agree with those of Zeyad et al. (Reference Zeyad, Tayeh and Yusuf2019), who studied the use of up to 30% pumice in high-strength concrete and found that replacement with 10% pumice improved the mechanical properties of the concrete. Although samples with 20% pumice had the lowest UCS values in the present study, Zeyad et al. (Reference Zeyad, Tayeh and Yusuf2019) found that samples containing 20% pumice have the highest strength development after 180 days of curing. Therefore, future work will investigate UCS development in cemented mine backfill prepared with 20% pumice and cured for at least 180 days.
6. Conclusion
This investigation showed that there is a strong potential for the mining industry to exploit abundant natural pozzolans like pumice in backfilling operations. Cement can be replaced by 10% pumice without compromising UCS and potentially reduce the cost of cement in mine backfilling. Although this study investigated strength development over the short term, the literature suggests that at a higher pumice content, the backfill requires a longer curing time to achieve maximum strength. Future studies will evaluate other types of natural pozzolans cured for 180 days to longer than one year to fully understand the pozzolanic effect on strength development. The use of natural pozzolans to minimize cement consumption in mine backfill will help promote a sustainable and eco-friendly approach to mining.
Acknowledgement
The author would like to thank the technical teams from McGill University and King Abdulaziz University.
Funding
No Funding received.
Conflicts of interest declaration
The author declares no conflict of interest.
Data availability
The data are presented in the manuscript.
Supplementary Materials
To view supplementary material for this article, please visit http://dx.doi.org/10.1017/exp.2020.61.
Open access
Comments
Comments to the Author: In this article, Pumice has been investigated to know whether it can be used as a mine backfill material. The results show that it can replace cement by 10%. The manuscript can be further improved by revising it based on the following suggestions.
(1) Include “Pumice” in the title.
(2) The cited literature in the “Introduction” section is quite old. It is suggested to include some recent and more relevant literature.
(3) The samples are prepared using tailings from Nickel mine. Please add details to the study area.
(4) The author claims that the use of Pumice is compared to cement and class F fly ash. However, it is not well elaborated and discussed in the “Results and Discussion” section. Present detailed results and provide a thorough discussion of your findings.
(5) The first two sentences in the “Conclusion” section should be rewritten to make them completely convincing.
(6) Figure 2 and Table 2 are not cited in the text.
(7) Please follow the journal guidelines and recheck the entire manuscript for grammar, spelling, and punctuation errors. Be cautious when writing chemical formulas and equations