Introduction
This research paper focuses on durability issues based on sulfate attack on concrete. It presents a brief literature review and how durability issues related to sulfate attack are generally tested according to specific ASTM standards. The research includes synthesis of the used literature and suggestions on the improvement of the current test methods.
Background / Literature Review
It is imperative to recognize that multiple factors influence durability of the concrete, and the sulfate attack is among them (Merida & Kharchi, 2015). Sulfate attack is a chemical reaction that involves the attack of elements of the cement paste by sulfate ions. In this context, researchers have focused on i. Sulfate enters into the concrete via the capillary pores and moves through osmosis as the concentration gradient between the inner concrete sections and the sulfate source increases. The reaction occurs between sulfate ions and unhydrated elements, such as calcium hydroxide and calcium aluminate hydrate of the hardened concrete paste forming mainly portlandite (Rozière, Loukili, Hachem, & Grondin, 2009). They further react to form ettringite and gypsum, leading to ultimate expansion, cracks, disruption, and deterioration of a concrete structure (Al-Akhras, 2006). The attack causes calcium silicate hydrate (C-S-H) degradation because of leaching of calcium components. This reaction eventually leads to gradual weakness of the concrete structure alongside surface erosion, loss of outer layer, and chipping (Merida & Kharchi, 2015).
Internal sources of sulfate attack are rare, but they emanate from materials used to make cement, including admixture, natural gypsum, and Portland cement. Conversely, external sources of sulfate are mainly “ground water, soil with high sulfate contents, or high contents from industrial wastes and atmospheric pollution” (Merida & Kharchi, 2015, p. 832).
Nie et al. (2015) had observed that Portland cement concrete could only experience such sulfate attack in sulfate-rich conditions. This attack is a complicated process based on several chemical and physical parameters that could facilitate the reaction (Irassar, Maio, & Batic, 1996). These parameters include the level of concentration of sulfate ions, favorable temperature, the type of cement and its composition, cement to water ratio, permeability and porosity of the concrete, and the availability of additional cementitious materials (Al-Akhras, 2006). Hence, it is most unlikely for sulfate attack to occur when properties of the concrete are improved or when the conditions are not favorable.
Available literature appreciates the relevance of sulfate attack on the concrete and subsequent impacts on its durability (Atahan & Arslan, 2016; Merida & Kharchi, 2015). External and internal sulfate attacks, for instance, are issues of concern that could become difficult with regard to the durability and service life of the concrete structure.
As a result, studies have focused on the best methods to reduce effects of sulfate attack on concrete structures. For instance, industrial by-products such as fly ash, slag, ground granulated blast-furnace slag, nano (colloidal), and micro silica (MS) are often used in cementitious materials to enhance long—term durability of the concrete by offering protection against external sulfate attack (Atahan & Arslan, 2016; Al-Akhras, 2006; Nie et al., 2015; Rozière et al., 2009; Irassar et al., 1996).
The Current Test Method
The current durability tests of concrete against sulfate attack are based on the American Society for Testing and Materials (ASTM) standards (Irassar et al., 1996). The test standard used was ASTM Type 1 (Al-Akhras, 2006). This test is used to determine strength reduction, such as physical changes in length of the concrete cube, comprehensive strength of the concrete cube, weight variations of the concrete and visual identification of cracks, of some concrete cubes exposed to water containing sodium sulfate solution (Rozière et al., 2009).
Improvement for the Test Methods
ASTM standards do not predict future stability of concrete in different environments. As such, a predictive method driven by experimental asymptotic analysis should be adopted alongside ASTM standards to determine future resistance of concrete under sulfate attack. In fact, the standards should provide long-term data to promote further studies on the essence of accelerated tests on future durability of a concrete structure in a sulfate-rich environment.
Literature Synthesis
Authors’ contributions to sulfate attack
Atahan and Arslan (2016), based on their micro-structural study, demonstrated that appropriate dosage of nano and micro additives could improve concrete resistance against sulfate attack. In addition, the researchers established that gypsum formation was responsible for sulfate attack in case mineral additives were not included in the samples.
Rozière et al. (2009) used accelerated test in a high sulfate concentration environment to demonstrate that mortars were sensitive to such environments, leading to expansion. The study showed how long-term data could be obtained through accelerated tests to indicate both resistance to leaching and external sulfate attack on concrete.
Irassar et al. (1996) also showed that admixtures enhanced sulfate resistance for concrete buried in the soil because of crystallization of sulfate salt. Capillary suction was relatively enhanced in the atmospheric zone leading to increased water and salt movement. Overall, about 20% of fly ash was effective for half-buried concrete structures.
The author noted that autoclaved MK concrete had significantly greater sulfate resistance compared to moist cured concrete because of smaller pores, creating air trap for sulfate attack resistance (Al-Akhras, 2006). Conversely, air-trapped plain concrete had low sulfate attack resistance relative to non-air trapped concrete.
A study by Nie et al. (2015) was important because it showed how numerical simulation could be used by relying on data obtained from USBR field observation to determine sulfate attack resistance of concrete. The authors noted that pozzolanic reaction had a significant reduction on sulfate attack on concrete resulting into an increased service life.
Concurrence views
While these studies were conducted by various researches, during different periods, using various processes and samples, all researchers agreed on the action of sulfate attack on a concrete structure. Besides, they also showed that the standard test method was ASTM. In addition, most researchers sought to go beyond the sulfate attack to find solutions, including mineral admixtures, slag and fly ash and other nano and micro minerals, which reduced sulfate attack by enhancing resistance properties of the cement.
Nie et al. (2015) and Rozière et al. (2009) agreed on the relevance and application of field observation data obtained from the USBR for simulation and accelerated studies.
Further, Al-Akhras (2006) and Rozière et al. (2009) agreed on ASTM standards as the standard test for determining strength reduction of some concrete cubes exposed to water containing sodium sulfate solution.
Authors’ opposing findings
There were no conflicting results observed across these studies. However, some of the researchers did not demonstrate the extent to which mineral additives became detrimental. For instance, only Irassar et al. (1996) showed that 20% of fly ash offered a fundamental solution for half-buried structures. Other authors simply stated that additives were effective for reducing sulfate attack on concrete and mortars.
Areas for further research
Studies have not dwelt much on the concrete durability issue. That is, the long-term durability of the concrete and mortar based on the expected service life under different sulfate concentration environments is yet to be determined effectively. In fact, researchers noted that concrete durability assessment is a prescriptive specification when it is exposed to a sulfate-rich environment (Rozière et al., 2009). Further simulated studies, based on data obtained from the USBR after 40 years of study, are required to overcome limitations associated with longer durations of field studies and laboratory tests.
The articles convincingly presented sulfate attack findings
These studies were based on scientific procedures, which met scientific rigor expected from such technical studies. Collectively, the authors obtained nearly similar findings based on the scope of their studies. These findings also reflect what other earlier researchers had determined between concrete and sulfate attack.
The findings are based on sufficient evidence to support studies. That is, they do not leave out any crucial information pertinent to the study. Further, one may also consider evidence as relevant to sulfate attack and durability of concrete. Finally, sources used by different authors are representative of other studies in the field.
The articles are consistent with sulfate attack studies
Based on the evidence presented by authors, these findings make absolute sense and, they were peer reviewed. That is, the researchers have developed their studies on scientific principles and rigor while adhering to different standards such as ASTM.
Conclusion
Researchers generally agree on sulfate attack on concrete structures, and they now focus on the best methods to control attack and enhance durability of concrete mortar and cements. Specifically, these reviewed studies have demonstrated that MK replacement of cement, fly ash, slag, admixtures, natural pozzolan, and other different micro and nano cementitious materials could significantly enhance cement resistance against sulfate attack by slowing down the rate of chemical reactions and, therefore, increase long-term durability. The ASTM standards are highly relevant for sulfate attack studies. However, the tests do not demonstrate future durability and, therefore, USBR studies on predictive method driven by experimental asymptotic analysis should be adopted to support outcomes on resistance and durability of mortar and concrete against sulfate attack.
References
Al-Akhras, N. M. (2006). Durability of Metakaolin Concrete to Sulfate Attack. Cement and Concrete Research 36(9), 1727-1734.
Atahan, H. N., & Arslan, K. M. (2016). Improved Durability of Cement Mortars Exposed to External Sulfate Attack: The role of nano & micro additives. Sustainable Cities and Society 22(2016), 40-48.
Irassar, E. F., Maio, A. D., & Batic, O. R. (1996). Sulfate Attacks on Concrete with Mineral Admixtures. Cement and Concrete Research 26(1), 113–123.
Merida, A., & Kharchi, F. (2015). Pozzolan Concrete Durability on Sulphate Attack. Procedia Engineering 114(2015), 832 – 837.
Nie, Q., Zhou, C., Li, H., Shu, X., Gong, H., & Huang, B. (2015). Numerical Simulation of Fly Ash Concrete Under Sulfate Attack. Construction and Building Materials 84(1), 261–268.
Rozière, E., Loukili, A., Hachem, R. E., & Grondin, F. (2009). Durability of Concrete Exposed to Leaching and External Sulphate Attacks. Article in Cement and Concrete Research 39(12), 1188-1198.