Leaching model for a cement mortar exposed to intermittent wetting and drying

Abstract

IntroductionOver the past thirty years, the role of cementitious materi-Ž als has expanded from structural building materials cements and concretes used in the current infrastructure of roads, . bridges, and buildings to include cement-based matrices providing environmental control of hazardous contaminant release. It is now common for potentially hazardous species, including radioactive and chemical wastes, to be immobilized within a cementitious matrix by means of a class of treatment Ž . processes known as solidificatonrstabilization SrS . Typically, the treated materials are disposed in regulated landfills where exposure to the environment can be monitored and Ž . controlled Conner and Hoeffner, 1998 . However, a recent shift toward recycling has led to the incorporation of waste Ž and secondary materials into roadbed Berendsen, 1997;. Ž Eighmy et al., 2002 or building materials Jansegers, 1997;. Nishigaki, 2000 . The long-term efficacy of the SrS treatment process largely depends on the durability of the treated material in its environmental scenario. Thus, it has become increasingly important to understand and predict the relationship between constituent release and physical durability of the cement matrix. In addition, an understanding of the aging and failure processes that compromise the durability of Correspondence concerning this article should be addressed to D. S. Kosson. these materials can be used to improve the performance of future materials.Long-term durability of cementitious materials is a complex issue that must account for the effect of both internal and external stresses on the cement matrix. Such stresses affect the performance of both structural cements and SrS Ž . treated materials in much the same way Klich et al., 1999 . Internal stresses include phenomena originating from within the matrix that affect the long-term morphology of the cement composition. Changes in chemical composition and structure due to internal stresses, for example, migration of metal species from waste aggregates to the cement paste Ž . Klich et al., 1999 , usually In undersaturated matrices, the reaction of carbon dioxide with components in the pore water leads to conversion of hydroxides to carbonates and overall neutralization of the ce-Ž ment matrix Lange et al., 1996; Macias et al., 1997; Pa-. padakis et al., 1989 Pa-. padakis et al., , 1991 Mollah et al., 1993 . Over very long release intervals, decalcification due to leaching of calcium from the matrix has been observed to increase porosity and reduce Ž compressive strength Carde et al., 1996; Carde and François, . 1997a,b;Carde and François, 1999 . Matrix alteration resulting from carbonation and constituent release are often-overlooked issues affecting both cement durability and the retention of SrS treatment.Recent studies indicate that constituent release and carbonation can be enhanced by cycles of intermittent wetting Ž . IW in which the release from a saturated matrix is inter-Ž . spersed with periods of storage Garrabrants et al., 2002 . Du...

Influence of curing on pore properties and strength of alkali activated mortars

“…The less solid gel products produced in AACM 2 mortar due to the higher activator dilution may be insufficient to block the interconnecting pores. Another reason for pore continuity could be the leaching of alkali cations into the curing solution thereby resulting in loss of alkali concentration needed for geopolymerisation reaction [28]. A slight degree of hydration reactions may also be a likely contributor to the interconnection of pores under wet curing in the AACM 2 due to the high degree of moist curing.…”

Using PHREEQC for modelling and simulation of dynamic leaching tests and scenarios

“…The chemical model is almost always limited at several species and precipitation reactions. The examples encountered in the literature concern the leaching of cement based monolithic materials in a sequential dynamic leaching test [1][2][3].…”

Compositional changes in cement-stabilized waste during leach tests—comparison of SEM/EDX data with predictions from geochemical speciation modeling

“…The second approach (Approach B) assumes that the equilibrium activity of contaminant metals in solution is determined by the local pore water pH, which is itself controlled by the dissolution of portlandite. The coupled dissolution-diffusion (CDD) model [10,11] and the intermittent mass-transport (IMT) model [12] are both examples of this approach. Application of these models requires empirical equations relating the liquid-solid partitioning of individual contaminant metals to the solution pH.…”