affected by the increase in temperature. However, data show that leaching rate and hydraulic conductivity of the grouts decrease with time. The results showed clearly that chemical reactions, presumably accelerated by the elevated temperatures (100 °C), led to the formation of a precipitate in the microcracks and on the surface of the leached specimens. This precipitate is likely the cause of the observed decrease in the hydraulic conductivity and leaching rate.

Key words: Microstructure, temperature effects, Cements

(iii) Study of a new cheap grouting material: clay-hardening grout, Xinghua, W. and Gao, Q., Tunnelling and Underground Space Technology Vol. 12, No. 4, Oct-Dec 1997 Elsevier Sci Ltd, Exeter England, pp. 497-502.
     The paper discusses factors that affect gel time and strength of a new, cheap grouting material: clay-hardening grouts (CHG). A special quality of these grouts is that they may be grouted repeatedly. The concept of a 'groutable period' (GP) is introduced in connection with this quality. Results of a comparison of a CHG with an ordinary cement grout are presented, and an example of a practical engineering application of a CHG is discussed.

Key words: Underground structures, Clay hardening grout, Rheology

(i) Grouting to control coal mine subsidence, Stump, D. E. Jr., Proceedings, Grouts and Grouting, GSP No. 80, ASCE, 1998, pp. 128-138.
     Abandoned underground coal mines are located throughout the coal fields in the United States. The mines are anywhere from a few meters to several hundred meters below the ground surface. In many of these mines coal pillars and/or wooden supports were left in the mine to keep the roof from collapsing during operations. After abandonment these roof supports deteriorate and eventually collapse causing subsidence. To protect the public from subsidence and other adverse effects of coal mining an Abandoned Mine Lands Reclamation Fund was established under Public Law 95-87. This fund is used to finance subsidence control projects. Grout is frequently used to stabilize areas in the mines by filling the mine voids and providing the support needed to minimize future subsidence. However, the placement of the grout may require unique equipment or customized grout materials since abandoned mines are generally inaccessible, the areas requiring stabilization are beneath surface structures, the voids may be flooded, and methane gas may be present. In addition to special considerations in grout placement the areas requiring roof support may be extensive and filling the voids in the entire mine are cost prohibitive. Special grout placement techniques have been evaluated including controlled grout columns or synthetic bags filled with grout and chemical additives to control grout set-up.

Key words: Abandoned mines, Subsidence

(i) Autogenous healing properties of cement-based grouts, Onofrei, M., Roe, L. and Shenton, B., Atomic Energy of Cananda Limited, AECL (Report), No.11753, May 1997, 38p.
     Cement-based grouts have been identified as likely candidate sealing materials by nuclear waste management programs investigating the deep burial of nuclear waste as a disposal strategy. Laboratory, field and modelling studies are under way to gather fundamental data, practical experience and estimates of the longevity of cement-based materials. Cracks in cement-based sealants within a radioactive waste repository could provide fast transport pathways and as such could impair the performance of the cement-based sealants as barriers to radionuclide migration. It has been reported that cracks in concrete can self-seal and this phenomenon is generally accepted to occur in civil engineering practice. Experimental work is required to improve understanding of the self-sealing process, particularly for concrete and grout mixes that contain mineral and chemical admixtures and under the likely chemical conditions within an underground repository. This report presents the results of a study conducted to provide information on the ability of cement-based grouts to self-seal. Autogenous sealing was investigated both on bulk grouts and in thin films of grouts. In both cases, the self-sealing capabilities of the cement-based grouts were investigated with water flowing through the grout. Autogenous sealing was studied through changes in pore structure (decrease in pore radius and volume of pores) and changes in the rate of water flow through the cement-based grouts. Analyses showed that the hydraulic conductivity (k) of the cement-based grouts with imposed porosity decreased with time, but only within limits that depended on the grout's initial porosity and composition. In some cases, k decreased from 10^-7 m/s to 10^-10 m/s. The observed decrease in the hydraulic conductivity was found to depend on the pore-size distribution in the grout, and more importantly, on the chemical reactivity of the grout. Changes in hydraulic conductivity were related mainly to changes in porosity caused by changes in the volume of the solids. The changes in the volume are caused by the formation of new hydration products resulting from the increase in the degree of hydration and associated reactions. The material observed to form in the available pore space and to bond the grout grains in the compacted hardened grout specimens exposed to percolating water was identified as ettringite (3CaO.Al2O3.3CaSO4.32H2O). The results indicate that self-sealing also occurs in thin films of hardened grouts. The infilling material formed in cracks was identified as a mix of calcium silicate hydrate (CSH), calcium hydroxide (Ca(OH)2 and traces of calcite (CaCO3). Self-sealing occurs in both thin-film and bulk-hardened grouts when they are in contact with water. More than one mechanism may be responsible for promoting self-sealing. These include the formation of ettringite and portlandite as well as calcite in the permeable connected porosity. It is concluded that high-performance cement-based grouts have the potential to self-seal and maintain their performance for a very long time. Further experimental work is