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The effects of aliphatic sugar alcohols (e.g., threitol, xylitol, sorbitol) on the hydration of tricalcium silicate (C 3 S) and ordinary portland cement (OPC) were investigated and compared with those of sucrose, a well-established cement set retarder. Only sugar alcohols which contain threo diol functionality retarded the setting of C 3 S and OPC, their efficacy increasing with the number of threo hydroxy pairs and, to a smaller extent, with the overall population of hydroxy groups. None, however, were as effective as sucrose. The initial and final setting times increased exponentially with the concentration of saccharide, although the hydration of OPC was less inhibited than that of C 3 S. Saccharides function as ''delayed accelerators,'' that is, cement hydration is first inhibited and then proceeds faster than in saccharide-free cement. This behavior is consistent with the theory that the induction period is controlled by slow formation and/or poisoning of the stable calcium silicate hydrate (CSH) nuclei. The early inhibiting influence of saccharides on CSH precipitation is apparently stronger than on the growth of crystalline calcium hydroxide. Saccharides did not negatively affect the degree of hydration and compressive strength of fully set OPC paste; on the contrary, sorbitol yielded modest increases. L. Struble-contributing editorBased in part on the thesis submitted by L. Zhang for the M.Sc. degree in Environmental Engineering, Lakehead

Crushed samples of stabilized/solidified (s/s) waste were leached at constant leachate pH in the pH range 4-7 with nitric acid solutions to evaluate the influence of remineralization on metal release. The s/s waste consisted of synthetic heavy-metal sludge containing 0.1 mol L(-1) copper nitrate, 0.1 mol L(-1) zinc nitrate, and 0.1 mol L(-1) lead nitrate mixed with ordinary Portland cement. Unleached and leached particles were characterized by scanning electron microscopy and energy-dispersive X-ray spectrometry. Two consecutive leaching fronts advancing from the surface of the particles toward the center were identified: the first front was associated with the dissolution of portlandite and partial reaction of the calcium silicate hydrate gel, while the second front was associated with the dissolution of calcium-aluminum hydroxy sulfates such as ettringite and monosulfate. At pH 4 and 5, a remineralization zone rich in heavy metals formed immediately behind the second leaching front. The shell extending from the remineralization zone to the surface of the particles was depleted in calcium, sulfate, and heavy metals. As a result of remineralization, heavy-metal releases to the leachate were reduced by factors ranging between 3.2 and 6.2 at pH 4 and between 74 and 193 at pH 5. At pH 6 and 7, remineralization of Pb and Zn occurred further behind the second leaching front and closer to the surface of the particles. The amount of heavy-metal release depended on both the leachate pH and the remineralization factor.

Experimental results for sandstone cores, glass bead packs, and glass micromodels are presented. Summary It is shown that in the laboratory a very large percentage of low viscosity waterflood residual oil can be recovered with low pressure inert gas injection, assisted by gravity drainage, from both water-wet and oil-wet samples. A semipermeable membrane made with a mixed-wet paste assures the production of both oil and water, but pre"rents gas production and eliminates capillary end effects. Stacks of short core plugs can he produced equally as a single core if a coarse version of the mixed-wet paste is used between adjacent plugs. In water-wet cores, displacement of waterflood residual oil requires a positive spreading Coefficient of oil on water in the presence of gas. Parallel-type macroscopic heterogeneities in the formation may have relatively little effect on the course of oil recovery, but they prevent the establishment of a uniform saturation in the core cross-section. Because the oil is redistributed in smaller pores during gravity drainage, microscopic pore scale heterogeneities decrease the recovery efficiency. Introduction The efficiency in recovering a large fraction of the waterflood residual oil from water-wet porous media by low pressure inert gas injection assisted by gravity drainage, also called tertiary gravity drainage, has been demonstrated in the past. A limitation of all the previous studies has been the presence of capillary end effects in the bottom part of the cores, which made the recovery dependent on the core length. Precise determination of residual oil Saturations has therefore required the use of radioactive tracer or CAT scanner techniques to measure in-situ saturations above the zone affected by capillary end effects. This paper shows how to eliminate capillary end effects successfully by placing a mixed-wet Semipermeable membrane against the core outlet face. The ability of oil to spread as a film on water in the presence of gas is known to be essential for the mobilization and recovery of waterflood residual oil in water-wet media. Nevertheless, it is also of great practical importance to study tertiary gravity drainage in cores of various wettabilities. Experimental results obtained with oil-wet consolidated media are reported here for the first time. When tertiary gravity drainage is tested on composite cores consisting of stacks of short reservoir plugs, Capillary continuity for the oil is not established at the junctions if the plugs are contacted directly. In this study, it is shown that inserting high permeability mixed-wet paste between plugs establishes capillary continuity for all three phases. Since porous media constituting oil reservoirs are seldom homogeneous, the effect of macroscopic and microscopic heterogeneities on the. tertiary gravity drainage process have also been investigated. Experiments for this purpose have been conducted in glass bead packs and in micromodels of pore networks etched on glass. Experimental Procedure for Tertiary Gravity Drainage Experiments After coating with epoxy resin (Devcon High Performance Baking Compound), the cores were flushed with carbon dioxide to replace the air originally present. Carbon dioxide was then miscibly displaced with several pore volumes of brine and the pore volume was determined by weighing the core. Following this, the cores were positioned vertically and flooded with several pore volumes of oil injected at the top. After that, a waterflood was carried out with brine injected at the bottom of the cores. Next, a mixed-wet semipermeable membrane was placed against the bottom face of the core and tertiary gravity drainage was started while injecting nitrogen under low pressure (a few kPa) at the top. The mixed-wet semipermeable membrane permits the passage of oil and brine at the producing end of the core, while preventing gas breakthrough under the excess nitrogen pressure applied. It consists of a mixed-wet paste supported by a stainless steel porous plate. The mixed-wet paste is a mixture of calcium carbonate (water-wet) and active carbon (oil-wet) powders finely ground. The size of the grains should he small enough so that the displacement pressures of the oil- gas and brine-gas interfaces in the pores between the grains are larger than the pressure of gas injection.