Sewer systems are among the most critical assets of the urban infrastructure of modern societies. They serve as a collector and transporter of industrial and domestic wastewater and, at times, runoff from precipitation in urban areas, to wastewater treatment plants. A significant part of sewer systems such as sewer outfalls, manholes, sump pits, and pump stations are made of concrete. Concrete in the sewer environment under certain conditions suffers severe deterioration that may lead to structural and functional failure during the service life of the structure. In general, about 60% of failures are associated with improper structural design, construction, installation, and inadequate control and maintenance,  while  40% are caused by biogenic acid corrosion [1]. The former 60% of failures can be eliminated with proper engineering design and construction. The challenge remains in dealing with biogenic acid corrosion of concrete.

Alice Titus Bakera, Mark Alexander and Hans Beushausen, Department of Civil Engineering, University of Cape Town, South Africa

Biogenic acid corrosion is a biological activity of several microorganisms in a sewer environment that leads to concrete sewer corrosion.  The corrosion occurs after a succession of several complex processes within the wastewater, in the sewer atmosphere, and on the concrete surfaces. Absence of oxygen within the wastewater when flowing in sewer pipes creates the anaerobic conditions that facilitate the growth of Sulphate Reducing Bacteria (SRB). These bacteria exist in the submerged slime layer (biofilm) between the concrete and wastewater, and decompose organic nutrients and sulphate compounds present in sewage to produce hydrogen sulphide (H2S) and carbon dioxide (CO2) gases, see Fig. 1.

The mechanisms of attack

The sewer hydraulics (i.e. turbulence and flow velocity) influence stripping of the gases to the sewer atmosphere. If these gases, especially H2S, ventilate, they cause the noxious odours which pollute the environment, and if inhaled by living organisms might cause fatality. When the gases diffuse into the sewer wall, CO2 carbonates the concrete, while H2S is oxidised to intermediate sulphate compounds (elemental sulphur, sulphite and thiosulphate) which are necessary or Sulphur Oxidising Bacteria (SOB) growth. The SOBs feed on and convert the intermediate sulphates to sulphuric acid (H2SO4) which attacks concrete matrix to form gypsum and ettringite. Gypsum (CS̅) and ettringite are expansive products which cause internal cracking and spalling of concrete, thereby providing sites for further penetration of acid. Therefore, concrete loses its structural integrity and exposes the underlying steel reinforcement to corrosion.

The conventional binder used for most sewer pipe systems is plain Portland Cement (PC), but the poor performance of concrete sewer pipes made with plain PC in very aggressive conditions has indicated that this is not a sound choice. It has been known for several decades that binder systems such as Calcium Aluminate Cement (CAC) have high potential in withstanding acid attack in a sewer environment. Recent developments have confirmed this, but also indicate that Alkali Activated Cement/binder (AAC) systems might also have similar potential to resist acid attack in sewers. These cement-based materials can, therefore, be adopted by industry for sewer construction, especially for parts such as manholes and sump pits.

Calcium Aluminate Cement (CAC)

Calcium Aluminate Cement is produced by sintering or melting limestone with bauxite or aluminium hydroxide mixtures, depending on the acceptable impurity level in the product. Thus, it consists mainly of two oxides; Al2O3 and CaO. The hydration of CAC leads to the formation of two calcium aluminate hydrates (CAH10 and C2AH8) and alumina gel (AH3). CAH10 is formed at lower temperatures below 20°C, while C2AH8 is formed with an increase in temperature. Above 30°C, both hydrates convert to a stable cubic C3AH6 phase. The conversion of these hydrates leads to additional quantities of alumina gel [2], which play a significant role in resisting biogenic acid attack (Fig. 2).

Under biogenic acid attack, CAC is widely known as an excellent sulphuric acid resistance binder. This is due to its long chain of dissolution and neutralisation (Fig. 2) by its hydrates as well the stability of alumina gel at a pH of between 3 and 4. Besides, the alumina gel produced reacts with H2SO4 to form aluminium sulphate of which ions such as Al3+ are deemed to provide a bacteriostatic effect in the microbial ecosystem.

In South Africa, sewer pipes are often constructed with a combination of two binder systems, where concrete pipes made from PC receive an internal corrosion resisting CAC lining. Considering the relatively high cost of CAC, this provides economical yet structurally sound and durable sewer pipe systems. This technology was adopted following long-term experimental studies in live sewer systems, particularly at the Virginia Experimental Sewer (VES) in the Free State province of South Africa. The VES  comprises different sections with two binder systems (PC and CAC) and three types of aggregates; calcareous (dolomitic), siliceous, and aluminate aggregates (Fig. 3). After 14 years of monitoring the performance of the sewer, it was observed that the sections made of CAC with dolomitic and aluminate aggregates outperformed other sections, with an average corrosion rate four times lower than those with PC binder. Therefore, it was recommended that in designing sewer system, the combination of CAC with dolomitic aggregate is a preferred economical and durable option. If added durability is considered more important than cost, aluminate aggregate can be used instead of dolomitic aggregates [3].

Alkali-Activated Cement/Binders (AAC)

Alkali-Activated Cement - also known as Geopolymer cement - is a class of inorganic polymers formed by an activated polycondensation reaction between an alkaline reagent solution and an aluminosilicate material, which results in a hardened concrete-like texture with similar or better structural properties as the conventional PC [4]. Aluminosilicate materials used include calcined clays, volcanic rocks, blast furnace slag, metakaolin, and fly ash, and the alkaline reagent solutions are alkali metal (sodium or potassium) hydroxide/silicate solutions [5].

Concrete made with AAC may have a comparatively higher acid resistance compared to PC [6,8]. This is because its acid-induced corrosion process differs from that of PC concrete. AAC generally consists of lower calcium content compared to PC concrete, which leads to the absence of cement hydrates (i.e. portlandite) that are vulnerable to acid attack. Its resistance against attack is mainly governed by; i) ion-exchange reaction between the penetrating acid protons (H+) and the cations (Na+, Ca2+ or K+) of the aluminosilicate framework, i.e.  by absorption and leaching, ii) depolymerisation due to the breakage of aluminosilicate chains to Si-OH and Al-OH groups, leading an increase in solution pH (neutralization capacity) as well as iii) impermeability of the AAC binders in general [9,10].

However, some studies [9-12] conclude that AAC performs well under biogenic acid attack based on its experimental performance under direct mineral sulphuric acid attack, without considering that these two types of attack contrast [13]. The behaviour of AAC concrete in a sewer environment is as yet uncertain due to a range of contributory factors, including the nature of the specific AAC concrete. Nevertheless, Khan et al [5] compared the resistance of Fly Ash and Slag-based AAC concretes after exposure to a natural aggressive sewer environment for 12 months. They observed that both binders experience biogenic deterioration, with greater deterioration in the Fly Ash-based geopolymer concrete. They rationalized that the cause of deterioration was associated with the depolymerisation of aluminosilicate, which leads to the formation of highly expansive products such as gypsum.

In comparing the performance of AAC with CAC and PC, the literature is unclear on comparative performance. This might be associated with the limited number of studies on the subject, especially in live sewers. In [14], the performance of AAC, PC and CAC systems was compared using the Fraunhofer UMSICHT Accelerated Test (an accelerated biogenic acid test developed by Fraunhofer Institute UMSICHT in Sulzbach-Rosenberg Germany, to evaluate the potential durability of construction materials for sewer infrastructures). It was observed that the AAC concrete failed after turning into a soft white paste during exposure, while CAC based concrete had superior performance. On the other hand, in [15], the performance of AAC, PC and CAC concretes was compared in a sewer environment. After 12 months of exposure in aggressive sewer, AAC concrete had no sign of structural damage, while initial structural surface degradation and strongly visible deterioration were observed on the CAC and OPC concretes, respectively. Drugă et al [16] studied the nature of biofilm formation on AAC-based mortars in comparison to PC and CAC mortars after being exposed to real municipal wastewater. They observed that AAC developed less biofilm on its surface, followed by CAC, while PC had the highest biofilm-affinity. Likewise, Abdel-Gawwad et al [17] observed that AAC had higher resistivity against the biodegrading effect of SOBs in comparison to  PC. However, [16,17] investigated only the potential of AAC to resist the growth of biofilm, and not actual biogenic acid corrosion.

In general, the implementation of AAC in sewer systems to resist biogenic acid corrosion has been  limited to date, although it has been used for spray lining for sewer pipe rehabilitation or for precast concrete sewer pipes [18,19]. This indicates the need for more long-term intensive studies related to sewer environments, similar to those conducted on PC and CAC, to acquire definite evidence that can influence its adoption in industry.   

Closure

The precast concrete industry has long been investigating the selection of suitable cement-based materials for use in sewer construction. This is attributed to the poor performance of concrete sewer pipes made with plain PC. Among the two possible alternatives to PC are CAC and AAC, with industry tending to favour CAC as the best choice. Considerable uncertainty remains on the choice of AAC, despite it being recommended and used in some countries. It is clear that certain AAC performs well under chemical acid attack, but it should not be generalized that it will also perform similarly under biogenic acid attack, since the two attacks differ. Therefore, intensive studies similar to those conducted on PC and CAC are still required to justify the competence of AAC in sewer environments.

References
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