Showing papers on "Portlandite published in 1999"

Chemoporoplasticity of Calcium Leaching in Concrete

Abstract: This paper presents a macroscopic material model for calcium leaching in concrete, for the quantitative assessment, in time and space, of the aging kinetics and load bearing capacity of concrete structures subjected to severe chemical degradation (such as radioactive waste disposal applications). Set within the framework of chemically reactive porous continua, the model accounts explicitly for the leaching of calcium of portlandite crystals and C-S-H, and its cross-effects with the elastic deformation (chemical damage) and irreversible skeleton deformations (chemical softening) treated within the theory of chemoplasticity. In the first part of this paper the governing equations are derived focusing on the chemomechanical couplings between calcium dissolution, increase in porosity, and deformation and (micro-) cracking of concrete. Without any a priori assumption concerning local equilibrium between the solid calcium concentration s and the interstitial calcium concentration c the well-known calcium leachi...

Characterization of High-Calcium Fly Ashes and Their Potential Influence on Ettringite Formation in Cementitious Systems

Abstract: High-calcium Class C fly ashes derived from Powder River Basin coal are currently used in many parts of the U. S. as supplementary cementing materials in portland cement concrete. These fly ashes tend to contain significant amounts of sulfur, calcium, and aluminum, thus they are potential sources of ettringite. Detailed mineralogical characterizations of six high-calcium fly ashes originating from Powder River Basin coal have been carried out. The hydration products formed in pastes made from fly ash and water were investigated. The principal phases produced at room temperature were found to be ettringite (C6AS¯3H32), monosulfate (C4AS¯H12), and stratlingite (C2ASH8). The relative amounts formed varied with the specific fly ash. Three fly ashes were selected for further study. Portland cement /fly ash pastes made with the selected fly ashes were investigated to evaluate ettringite and monosulfate formation. Each of the three fly ashes were mixed with five different Type I portland cements exhibiting a range of C3A and sulfate contents. The pastes had 25% fly ash by total weight of solids and a water: cement + fly ash ratio of 0.45. After mixing, the samples were sealed and placed in a curing room (R.H. = 100%, 23°C) for 28 days and were then analyzed by X-ray diffraction (XRD) and differential scanning calorimetry ( DSC) to determine the principal hydration products. The hydration products identified by XRD were portlandite, ettringite (an AFt phase), monosulfate, and generally smaller amounts of hemicarboaluminate and monocarboaluminate (all AFm phases). Although the amount of ettringite formed varied with the individual cement, only a modest correlation with cement sulfate content and no correlation with cement C3A content was observed. DSC analyses showed that the cement/fly ash pastes generally formed less ettringite than the cement control pastes, but they formed more of the AFm phases (mainly monosulfate). It appears that the addition of high-calcium fly ash reduces the SO4/Al2O3 ratio in the system thus favoring Afm formation.

The partitioning of uranium and neptunium onto hydrothermally altered concrete

Abstract: Cementitious materials that are used to construct the ground support for high-level repositories have a high probability of interacting with radionuclide-bearing fluids derived from failed waste packages. Cementitious materials provide a highly alkaline environment; pore fluids in concrete can have pH {gt} 10 for thousands to hundreds of thousands of years. Studies have shown that fresh concrete and cement phases strongly retard or immobilize certain actinides. Consequently, cementitious materials may serve as a barrier to the release of the radionuclides to the far field. However, the effect of thermal alteration of these materials, which may occur in high-level repositories, on their interaction with radionuclides has not been addressed. In contrast to retardation, colloidal silica-enriched particles that are abundant in the pore fluids of cementitious materials may facilitate radionuclide migration through the near-field into the adjacent geological environment. Due to the uncertainties of these two opposite effects, it is important to investigate the interaction of actinides with cementitious materials under varying conditions. It is expected that cementitious materials in high-level waste repositories will be subjected to and altered by hot dry and/or humid conditions forhundreds to thousands of years by the time they interact with radionuclide-bearing fluids. After alteration, the chemical and mineralogical properties of these materials will be significantly different from that of the as-placed or fresh concrete. To assess the effect that this alteration would have on radionuclide interactions, samples of hardened concrete (untreated concrete) were hydrothermally heated at 200 C for 8 months (treated concrete). The concrete used in the experiments consisted of portland cement with an aggregate of dolomitic limestone. X-ray diffraction analysis has shown that portlandite and amorphous calcium silicate hydrate gels were converted to the crystalline calcium silicate hydrate minerals tobermorite, xonotlite, and scawtite, and clay minerals by the hydrothermal treatment. Calcite, dolomite, and quartz in the aggregate were unchanged by the treatment. This paper presents the results of batch experiments to obtain partition coefficients for U(VI) and Np(V) on untreated and treated concrete in 0.01 M NaCl and 0.01 M NaHCO{sub 3} solutions as functions of the concentration of the radionuclides, pH and time.