New Tech for Concrete Acid Attack — Part 1

Introduction to the Mechanisms

Concrete is a popular construction material often used in mining and waste water industries. In both of these industries, the concrete environment is often high moisture with a pH between 2.4 and 2.6. Both moisture and high pH create an acidic environment that can cause the concrete to break down. This phenomenon is referred to as acid attack. The hydrated cementitious matrix (HCM) that makes up the backbone of concrete strength and durability has a stable microstructure at a pH of 9.0 to 10.0 (Taylor 1997). Specifically, the calcium-silicate-hydrate (C-S-H) is stable in this pH range. The C-S-H is the amorphous gel that makes up a large portion of the HCM (Richardson 2007). Acidic environments with a pH between 2.0 to 3.0 can cause catastrophic damage to concrete (Attiogbe 1988). Acids are electron acceptors and have the capacity to break down the C-S-H gel of the HCM by removing the calcium ion from the gel; the broken down gel can have a higher volume, lower strength and lower durability than the former, unattacked C-S-H (Bertron 2005). While the original HCM (not deteriorated by acid attack) had a porous matrix, the HCM that was deteriorated by acid attack would absorb an acidic solution to higher degree than the original HCM. As an acidic solution breaks down the HCM (hydrochloric acid in the case of this research), the acid is neutralized to a degree (Hewayde 2007). This weaker form of the acidic solution would have less of a deleterious impact on the HCM than the original acidic solution. This assumes that the acidic solution is not replenished or replaced.

Methods of Mitigation

While the chemical break down of the C-S-H structure is not totally unavoidable with lower pHs, certain secondary cementitious and pozzolanic materials have been used to increase the durability of grout and concrete to resist the mechanisms of acid attack (Beulah M. 2012). Two different types of pozzolanic materials were investigated as a means to reduce acid attack on grout samples. The first material analyzed was Metakaolin. Research has shown that Metakaolin has the capacity to increase durability to acid attack (Hewayde 2007, Beulah M. 2012). The Metakaolin accomplishes this by reducing the permeability of the HCM, and thus reduces the capacity for an acidic solution to migrate into the body of the HCM. It has also been suggested that the addition of Metakaolin, an alumino-silicate amorphous material, improves on the HCM by producing more of the C-S-H in the cured cementitious microstructure (Ding 2002). This improvement is the addition of the alumina into the hydrated phase creating a calcium-aluminate-silicate-hydrate (C-A-S-H) which has been shown to have a greater resistance to breaking down in the presence of acidic solutions (Mlinarik 2013).

The second material used was a nanoscaled version of the pozzolanic material. Nano silica for concrete and grout is recognized as a colloidal silica dispersion of nano silica particles (J. A. Belkowitz 2010). The addition of nano silica to cementitious composites has been found to enhance both strength and durability (J. B. Belkowitz 2011). An alumina modified nano silica particle was added to compare the change in the hardened properties and acid attack resistance of the cementitious grouts with a nano silica particle that was not treated. Both the Metakaolin and the nano silica alter the HCM through pozzolanic reaction (Taylor 1997). The nano silica has the capacity to accelerate cement dissolution due to its smaller size and surface potential (Bjornstrom, et al. 2004). This reduced surface potential has been shown by Land and Stephan (2012) to be a C-S-H seeding site (Land 2012). Both of the nano silica mechanisms ultimately reduce the permeability and increase the resistance to acid attack in ways that the Metakaolin cannot (J. B. Belkowitz 2014).

The unique aspect of this research, presented in the sections to follow, was to identify and compare the impact of current and novel technologies for acid attack resistance. Where Metakaolin has been shown to reduce mass loss due to acid attack through pozzolanic reaction, the use of nano silica particles would improve on the enhancements that lead to a more durable grout specimen. The objective of this research was to determine how current and novel concrete technologies could be used to impact the durability of cementitious grouts to acid attack. To get a better understanding how these factors influenced the grout mixtures different dosages and combinations of these technologies were used. Compressive strength and bulk electrical resistivity were used to determine how the addition of these technologies changed the density and permeability of the grout specimens. Mass loss due to acid was also used to identify which technologies created a HCM more resilient to the deleterious effects of HCl acid attack.

References

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