Assessment of inorganic constituent release from a portland cement matrix as a result of intermittent wetting, drying and carbonation.

摘要

Structural cements and concretes are common construction materials in many countries. It is also now common for potentially hazardous chemical wastes to be immobilized within cement-based materials. Accurate prediction of constituent release from cementitious matrices can reduce liabilities associated with leaching (e.g., failure, corrosion, environmental impact) and can facilitate improvements in cement durability and treatment efficacy. Current characterization and leaching assessment approaches estimate the release of “species of potential concern” (SOPCs) by assuming continuously water-saturated conditions; however, these conditions are unrealistic for many applications of cementitious materials. Thus, it is important to incorporate the effects of intermittent wetting, drying and carbonation into these assessment approaches to better predict long-term release.; The objectives of this research include: (i) evaluation of SOPC release under intermittent wetting as a function of relative humidity and carbon dioxide content, (ii) measurement of moisture, pH and carbonation fronts as a result of “drying” in controlled atmospheres, (iii) characterization of pore water pH, ionic strength, and SOPC concentration in carbonated and non-carbonated materials, and (iv) development of a modeling framework to predict SOPC release from intermittently wetted cement-based materials.; The conditions of the storage period of an intermittent wetting cycle have a significant effect on matrix conditions and release in subsequent leaching intervals. During storage in an unsaturated environment, carbonation had the most significant affect on SOPC chemistry. When storage occurs without carbonation, relaxation of pH and concentration gradients due to continued mass transport within water-filled pores significantly impacts subsequent release behavior.; The Intermittent Mass Transport (IMT) model was developed to describe the release of matrix components (Ca, OH), highly soluble species (Na, K, Cl), and sparingly soluble species (As, Pb, Cd) from intermittently wetted porous materials. The IMT model accounts for phenomena associated with storage in an unsaturated atmosphere including drying of the matrix and relaxation of concentration and pH gradients within the matrix. Projected long-term constituent release was compared amongst common assessment models for two intermittent wetting release scenarios without carbonation. A chosen assessment approach must consider (i) the conservative nature of the required estimate, (ii) characteristic SOPC behavior, (iii) release scenario, and (iv) intermittent wetting conditions.