Calcium Aluminate Technology and Biogenic Corrosion
If you are reading this magazine and have any involvement with manhole rehabilitation, you are likely familiar with the term “calcium aluminate.” There are numerous manufacturers and suppliers that market calcium aluminate based repair materials for use in wastewater applications. It is generally understood that calcium aluminate cement-based products are more corrosion resistant than ordinary Portland cement-based materials. This article will help to explain how and why.
There are two general manufacturing processes used to produce cement: sintering and fusion. The calcium aluminate cements used in the wastewater industry are typically manufactured with the fusion process. The raw materials for this process are calcium and alumina. The calcium source is limestone and the alumina source is bauxite (also a raw material for the primary manufacture of aluminum). The limestone and bauxite are proportionately fed into a specialized furnace where the raw materials are eventually melted into a liquid phase through a highly controlled process. The purity of the finished calcium aluminate is dependent upon the purity and proportions of the raw materials. The output of the fusion process is a very dense and hard calcium aluminate “clinker.” In the case of calcium aluminate cement, the clinker is then processed through a rotary mill and ground to cement fineness. There are also products available that contain a manufactured calcium aluminate aggregate system. In the case of aggregate, the same fused clinker is processed through a sophisticated crushing system and is screened and divided into different aggregate gradations.
Calcium aluminate-based repair materials are typically associated with manhole rehabilitation, but there are numerous other wastewater applications where calcium aluminate linings work well and have a history of successful use. These additional applications include lift stations, wet wells, treatment plant structures, junction boxes, and piping systems. Calcium aluminates have actually been used for more than 65 years in extreme wastewater applications worldwide. The first U.S. application was in 1959 located at the Hyperion Treatment Plant in Southern California.
Biogenic Corrosion and Its Effect on Portland Cement
The hydrogen sulfide (H2S) corrosion mechanism is a well-known phenomenon but the specifics of the process are sometimes misunderstood. Surprisingly, wastewater itself is rarely corrosive. The corrosion begins with H2S created by the decomposition of the organic materials within the wastewater. This H2S builds in concentration in the areas of laminar flow. The H2S is then released into the sewerage network in areas of turbulent flow (outfall and force main type situations). Turbulent flow can occur in numerous areas of the system, including piping systems, manholes, pumping situations, treatment plants, etc. This turbulent flow causes the dissolved H2S to become an airborne H2S gas. The H2S gas is heavier than air and initially exists above the effluent level, dissolving in the moisture on the concrete surfaces above the flow level. As water is formed by the oxidation of the hydrogen, the H2S gas deposits elemental sulfur onto these surfaces.
In some instances, a pronounced yellowish build up of the elemental sulfur can actually be seen on the interior surfaces of manholes. This elemental sulfur is a food source for naturally occurring bacteria present in the sewerage system. These bacteria, present generally in the slime layer, actually “eat” the elemental sulfur (as a source of oxygen). The byproduct of the bacteria’s digestion process is sulfuric acid. It is this sulfuric acid that is corrosive to wastewater structures, not the H2S gas itself. Factors that can enhance this biogenic corrosion cycle include long retention times, high ambient temperatures, flat terrain, and low flow values. With the current growth of outlying suburban areas, feeding into the existing infrastructure of larger metropolitan areas, these factors are becoming increasingly prevalent throughout the United States as treatment plants are commonly several miles from the city center, requiring very long distances to transport the effluent.
One should note that biogenic corrosion is often confused with chemical corrosion. Chemical corrosion is commonly tested by immersing specimens in a dilute acid solution (pickle jar test). The diluted acid attack only involves a sudden chemical reaction and is not relevant to biogenic corrosion associated with wastewater degradation.
Today, wastewater structures are typically constructed with Portland cement concrete. Portland cement is a calcium silicate and its hydration inescapably liberates calcium hydroxide [Ca(OH)2]. The sulfuric acid (H2SO4) excreted by the sewer bacterium will react with the liberated calcium hydroxide. The reaction is as follows:
This reaction produces gypsum and water. In a humid sewerage environment, gypsum is dissolved. This ongoing disruptive phenomenon continually leaves a fresh layer of portland cement for attack. In high-strength Portland-based concrete, porosity is not a primary issue. The acid does not penetrate the concrete through the pores, but acts directly on the surface. In that respect, increasing the concrete density does not significantly enhance the corrosion resistance of normal concrete in a severe sewer environment.
How Calcium Aluminates Work
Contrary to the chemistry of Portland cement, the hydration process of calcium aluminate cement does not produce calcium hydroxide but liberates only CA hydrates and Al2OH3 (“gibbsite”). The Al2OH3 liberated from calcium aluminate cement hydration is not susceptible to an attack of this nature. At pH levels above 3.5 Al2OH3 is insoluble and blocks the pores of the concrete, protecting it from the ingress of acid. Below a pH of 3.5 the Al2OH3 contributes to neutralizing the acid at the surface by the consumption of hydrogen ions:
The measure of an acidic pH is actually a measurement of the molecular concentration of hydrogen ions (H+). Therefore, the more H+ there are in solution the lower the measured surface pH will be. Each Al2OH3 hydrate will remove 3 H+ ions from solution making them neutral. This is the “neutralization capacity” of a calcium aluminate. The result of this neutralization reaction is the release of alumina ions (Al3+) which have an inhibitory effect on the metabolism of the bacteria creating the acid. By removing hydrogen ions from solution, the surface pH is locally raised. The released alumina ions react with the bacteria present to slow their activity. In this way, calcium aluminates act as a Protective – Reactive Barrier, greatly reducing the corrosion of the concrete.
It stands to reason that the more Al2OH3 available, the more corrosion resistant a calcium aluminate-based product will be. In general, a mortar will contain approximately 20 to 35 percent cement, with the remaining 65 to 80 percent being aggregate system. Typical natural aggregates include silica sand, limestone, granite, etc. These types of aggregates are relatively inert. While a calcium aluminate/natural aggregate material will perform better than a Portland cement-based material, only the 20 to 30 percent cement portion will have the ability to neutralize acid and inhibit bacterial activity. As mentioned earlier, there are products available that contain both fused calcium aluminate cement and fused calcium aluminate aggregate. In the case of a 100 percent calcium aluminate mortar (both cement and aggregate system), 100 percent of the product has the ability to neutralize acid and inhibit bacterial activity. Simply put, a 100 percent calcium aluminate material will neutralize acid and inhibit bacterial activity significantly longer than a 20 to 30 calcium aluminate material.
Joseph Talley is the SewperCoat market manager for Kerneos Inc. He has been with Kerneos for 17 years, working with calcium aluminate based construction materials. Gregory Wallace is the manager-Western Region for Kerneos Inc.
