Permeability and Pore Structure of Cement Pastes Containing Fly Ash, Slag, and Silica Fume

Abstract

As part of research to develop a highly durable concrete container for radioactive waste disposal in chloride and sulfate bearing granite groundwaters, a variety of cement pastes were studied. A sulfate resisting portland cement was used with various replacement levels of Class F fly ash and pelletized blast furnace slag at a water to solids ratio (W/S) = 0.36. Blends with fly ash, slag, and silica fume were also combined with a super water reducer at W/S = 0.25. Results are presented for strength development, permeability to water, and pore size distribution after 7, 28, 91, and 182 days moist curing. As a direct measure of durability, after 91 days moist curing, paste prisms were immersed in both de-ionized water and a synthetic chloride and sulfate bearing groundwater at 70°C. While all three supplementary cementing materials (mineral admixtures) reduced ultimate permeabilities, silica fume was more effective in reducing permeability at early ages. Silica fume was also the most effective in reducing calcium hydroxide contents of the pastes while slag was the least effective; only reducing calcium hydroxide levels by dilution of the portland cement. From preliminary analysis, there does not appear to be a way of accurately predicting permeability from porosity or pore size parameters alone.

Cost-Effective Cementitious Material Compatible with Yucca Mountain Repository Geochemistry

“…Blending selected slags, Class F fly ash, and other high-silica pozzolans with OPC to form a composition that prevents the formation of a strong base calcium hydroxide (Roy, Scheetz 1992). The potential sources of fine glassy silica are American Society for Testing and Materials (ASTM) Class F fly ashes (ASTM C 618-99), blast furnace slags (Daube, Bakker 1986), silica fume (Malhota et al 1987, Hooton 1986), volcanic pumice a , and other high-silica glassy powders (Glasser et al 1987). …”

“…High-silica contents of blended cements develop finer pore structures and give the final concrete matrices enhanced chemical resistances (Hooton 1986, Malhota et al 1987, Gaal et al 2003. Also, these highsilica binders can seal small stress cracks in structures by forming alteration-minerals that close microcracks caused by localized stresses.…”

“…In addition, these silica additives greatly reduce the pore sizes and the pore volumes. Adding >30% by weight of pozzolan to OPC at a water-to-cement weight ratio (W/C) approaching 0.25 results in a matrix with a critical pore radius a is between 0.04 and 0.01 µm (Hooton 1986) and a porosity approaching 6 vol % (Massazza 2004). At 28 days of curing, OPCs have critical pore radii between 1 and 0.1 µm.…”

“…So while pozzolans minimize the need for superplasticizers by lowering the mix viscosity and the water requirements for workability (Malhota et al 1987), the pozzolans also develop an impermeable matrix (Hooton 1986). This impermeable matrix also retards the release of any organic hydrolysis products.…”

Influence of Mineral Admixtures on the Mechanical Properties of Fresh and Hardened Concrete

“…The mixing of mineral and chemical admixtures in concrete can bring improvement in the engineering properties of hardened and fresh concrete such as tensile and flexural strengths, elastic modulus, workability and durability [2]. Cement is generally replaced partially by mineral admixtures, and the specific effects of admixtures generally vary with the mix proportion, ambient conditions and the dosage that is added.…”