An investigation was carried out to find a suitable additive for acceleration of strength development in blast furnace slag cement at an early stage hydration. Sodium compounds such as NaF, NaCl, NaBr, NaI, NaOH, Na2SO4, and Na2CO3 were used. The strength development, porosity, scanning electron micrographs, and heat liberation characteristics were examined. Addition of a small amount of NaBr, NaCl, NaF, or Na2SO4 increased the strength at early ages; NaBr or NaCl increased the strength even at 91 days. The microstructure of paste with NaBr or NaCl addition showed a structure containing CSH, gel, AFm, ettringite, and other phases having a compact structure.

Cement pastes and mortars prepared with ASTM Type I cement and 50, 65, 70, and 85 percent granulated blast furnace slag have been investigated. The chloride diffusion rate and permeability of the pastes and mortars show pronounced decreases from the control portland cement pastes and mortars. The decreases are related to the refined microstructures. The pore structure of hardened slag-containing paste is dominated by the pores with radiums finer than 5 nm. But for neat cement paste, the majority of pores range from 5 to 20 nm. This difference in pore size distribution makes the pastes behave quite differently in diffusion and transport processes. The reduction of effective porosity makes the pastes blended with slag much more resistant to chloride diffusion.

Traditionally, the utilization of granulated blast-furnace slag (BFS) is based on activation with alkalies released from the hydration of portland cements (PC). In Finland, a special type of alkaline admixture was introduced some years ago for activation of BFS. In the present paper, some experiences based on the activation of a Norwegian BFS with this admixture are reported. To investigate the hydraulicity of the slag, the slag was ground to three different levels of Blaine fineness (420, 540, and 640 mý/kg) and hydrated at five different levels of curing temperature (20 to 60 C). For comparison, a Finnish slag and a Norwegian blended portland cement with 10 percent fly ash were also included in the test program. The test results demonstrate that increasing curing temperature and fineness of the slag significantly accelerate the strength development (more so at early ages than later on). Thus, at 60 C the slag with 640 mý/kg of fineness and a water-cement ratio of 0.33 reached a compressive strength of approximately 40 MPa after a curing period of 4 hr. After a curing period of 72 hr, the heat of hydration of the slag cements was only about 60 percent of that of the modified portland cement.

Hydration of cements containing the supplementary cementing materials fly ash (FA) and silica fume (SF) is discussed and compared with the hydration of ordinary portland cement (OPC). Early stage heats of hydration, changes in the chemistry of the solution (both at early stages, and later pore solution compositions), microstructural development, and pore structure are compared. The hydration rates normally follow the order: SF > OPC > FA. The complex hydration processes may be controlled so that the use of these cements enables development of materials having superior strength and durability.

The effect of different curing conditions on the performance of alkali-activated slag (AAS) pastes, mortars, and concrete has been investigated. The temperature range is from -15 to 105 C. The storing conditions were underwater and at relative humidities of 100, 70, and 50 percent. After storing under these conditions, the compressive strengths were determined, and some microscopic and x-ray diffraction analyses were made on the tested samples. The AAS mortars and concretes perform very well even after an extremely strong heat treatment followed by storing at a low relative humidity. At normal temperature, the drop from 100 to 70 percent relative humidity does not affect the strength properties of the concrete. The AAS concrete can be heat-treated immediately after casting without any detrimental losses in strength. Storage in a dry climate does not have a strong influence on the strength development because the AAS binders form a very dense matrix with a large part of closed pores. Some results from industrial production of concrete with AAS binders are also presented. The results prove the suitability of AAS for the precast industry.