What’s Happening in this Photo? Concrete Material Alteration via Chemical Attack
This photo shows a reinforced concrete sulfur pit wall within an oil refinery that is experiencing concrete material alteration. With civil and structural infrastructure, particularly reinforced concrete structures, a proper condition assessment is the best way to identify the root cause of deterioration.
In a sulfur pit environment, concrete deterioration is characterized by the concrete cement paste matrix being chemically modified and no longer exhibiting properties consistent with structural support/containment. Essentially, the concrete mass, altered by exposure to sulfurous products, undergoes chemical reactions that expand the concrete mortar fraction. This resulting expansion always proves fatal to long-term concrete durability. The reactions include:
- Combination of sulfate with calcium ions liberated during the hydration of the cement to form gypsum. (CaSO4 • 32 H2O)
- Combination of the sulfate ion and the hydrated calcium aluminate to form calcium sulfoaluminate (ettringite). (3CaO • Al2O3 • 3CaSO4 • 3H2O)
Both of these reactions1 result in an increase in solid volume. The formation of ettringite is the cause of most of the expansion and disruption of concrete caused by sulfate solutions. Although expanding in volume during chemical attack and as a result of thermal growth due to elevated temperatures associated with keeping elemental sulfur molten, reductions in volume also occur concurrently. High temperature exposure desiccates the concrete removing any free moisture within pores and capillaries within the concrete – ultimately reducing the volume of the exposed structural element. Unfortunately, this reduction in size is not uniform and various areas experience restraint which then stress-relieve themselves by cracking.
Another process that occurs but that is less well understood, is that of polymerization of cemetitious products in contact with molten sulfur. In regions below the vapor zone, concrete construction materials actually change into a harder, more dense product. When drilling or coring concrete in these regions, diamond and tungsten tipped drill tools can dull and blue flames emerge from drill/core locations. Compressive strength testing of collected core specimens has verified that the concrete can double and in some cases triple in compressive strength. When viewed under magnification, the altered concrete is dark in color and voids filled with sulfur crystalline structures. Although not truly a Sulfur Concrete as described in the American Concrete Institute Committee 548 (“Guide for Mixing and Placing Sulfur Concrete in Construction”), characteristics such as high compressive strengths are shared by both types of concrete.References:
1. “Guide to Durable Concrete,” ACI Manual of Practice, Part 1, ACI 201.2R-92, American Concrete Institute, Detroit, MI, 1998.