To-day's advanced techniques in tunnelling and gallery engineering call for high-quality shotcrete, i.e. a material which develops accelerated set and high early strength to suit the safety requirements in the heading phase, and also final strength requirements for the perliminary concrete lining (New Austrian Tunnelling Method, NATM). High early strength can be obtained with the addition of an accelerator to the shotcrete mixture. Many materials are known for accelerating the setting time of shotcrete including strongly alka-line reacting materials such as alkali metal hydroxides, alkali metal carbonates, alkali metal aluminates and alkaline earth chlorides. The adverse effects of these admixtures are also known. With the development of an efficient alkali-free shotcrete accelerator it has become possible to produce high early strength without undesirable effects on the final strength. Attempts to positively influence the fracture characteristics of cement mortars or concrete with fibres have a long history, while organic fibres were soon discardes, great efforts are still being undertaken with glass and steel fibres. Problems in the processing of steel-fibres reinforced shot-crete motivated us to introduce a new type of fibre. The addition of condensed silica fume increased the strengths and reduced sharply the permability. The resistance to freezing and thawing was also greatly improved. No long-term strength loss was obtained due to the use of a new alkali-free setting accelerator.

The effect on the properties of portland cement binders due to the addition of volcanic tuff, calcined clay, fly ash, coarse coal bottom ash and diatomaceous earth, are compared and evaluated for pozzolanic activity. Durability studies carried out for two years on mortars with addition of fly ash, in different aggressive environments, are also reported. Volcanic tuff is a highly reactive pozzolan, fly ash yields compressive strengths higher than the control after 60 days. The calcined clay reactivity is proportional to the calcination temperature. Coarse coal bottom ash has a pozzolanic activity similar to the calcined clay. Diatomaceous earth failed to produce pozzolanic activity.

Concretes containing fly ash were evaluated to establish the effect of the fly ash on freeze-thaw resistance, resistance to deicer scaling, and chloride ion penetration. The effects of low temperature curing and moisture availability during curing were also evaluated. These tests indicated that the freeze-thaw resistance of air-entrained concrete was reduced by the use of certain fly ashes when cured at low temperature. For other conditions there was no significant influence of fly ash . Deicer scaling resistance tests showed that air-entrained concrete without fly ash generally performed somewhat better than concrete with fly ash, regardless of the type of curing provided. Air-entrained concretes made with some fly ashes were as resistant to chloride ion penetration as air-entrained concrete without fly ash. The class of fly ash did not significantly influence the degree of chloride ion penetration.

Pore structure changes in silica fume-portland cement blend mortars fabricated with 0, 10 and 30% silica fume at a water/binder ratio of 0.60 and a sand/cement ratio of 2.25 have been monitored by mercury porosimetry while being cured for 1 to 180 days. The threshold value for pore intrusion increases with pore size and becomes less abrupt with silica fume addition; it is in the 0.5 to 20 x l03 nm region. Mortars were also made with and without 10% silica fume at a water/cement ratio of 0.60 and sand/cement ratios of 0, 1.0, 1.5, 1.8, 2.0, 2.25 and 3.0; the sand passed ASTM C109. Mercury intrusion measurements were carried out after 14 days of curing. In the presence of silica fume pore volume in the 0.5 to 20 x 103 nm pore diameter range increased with sand/cement ratio. Mortar prisms were subjected to freezing and thawing cycles (two cycles in 24 h) according to ASTM standard test method C 666, Procedure B. Freezing and thawing resistance was monitored by measuring changes in residual length and weight. Results indicate that if the sand/cement ratio is 2.25 or over, expansion is less than 0.02% after 500 cyles. At lower sand/cement ratios 10% silica fume gives little protection.

The use of autoclaving processes in producing cement-based products is well established. The main reason for using autoclaving processes is often to increase production rate and/or to decrease sensitivity towards variations in humidity, such as reducing moisture movements. The latter effect is mainly believed to be caused by an improved crystallinity of the finished material. The use of fly ash as the siliceous component in the base mixture is also well known. However, the crystallinity is ignored by the use of a noncrystalline base material, such as fly ash. From productions of autoclaved cement-based products utilizing silica sand it is known, that additions of small amounts of gypsum to the base mixture may improve strength and reduce moisture move-ments. This paper deals with the production of autoclaved materials utilizing fly ash and gypsum or its derivatives. Materials produced are characterized according to their density and strength-characteristics as well as to their crystallinity. The use of gypsum or its derivatives may cause significant improvements in strength as well as in crystallinity, but the op-timum design is closely related to the actual production process as well as to the chemical properties of the base materials and to the physical properties of the fly ash.