This paper presents results of a study on chemical acceleration of a neutral granulated blastfurnace slag activated using sodium carbonate. As strength development of alkali-activated slag cements containing neutral GBFS and sodium carbonate as activator at room temperature is known to be slow, three accelerators were investigated: sodium hydroxide, ordinary Portland cement and a combination of silica fume and slaked lime. In all cements, the main hydration product is C-(A)-S-H, but its structure varies between tobermorite and riversideite depending on the accelerator used. Calcite and gaylussite are present in all systems and they were formed due to either cation exchange reaction between the slag and the activator, or carbonation. With accelerators, compressive strength up to 15 MPa can be achieved within 24 hours in comparison to 2.5 MPa after 48 hours for a mix without an accelerator.

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This paper reports on the results of an investigation into the possibility of producing dry powder alkali-activated slag cements as a ready-to-use product which can be packed in bags and mixed with water to produce a concrete. The cements were produced using a neutral granulated blast-furnace slag and sodium carbonate. To accelerate strength development at ambient temperatures, a combination of silica fume and slaked lime was used as accelerating admixture in the cement's formulation. Powder sodium lingosulfonate was added into the formulation to reduce the water demand of the cements; it also delayed setting and increased compressive strength. Alkali-activated slag concretes were produced using the developed cements. Compressive strengths in the range from 30 to 85 MPa were achieved for the concretes at 28 d. The accelerating admixture increased 1-d compressive strength for the alkaliactivated slag concretes cured at ambient temperature up to 25 MPa. The optimum concentration of sodium carbonate was equivalent to 3 . 5% and 4 . 5% sodium oxide (Na 2 O) for precast and in situ application, respectively.The accelerating admixture is recommended at 6% for dry powder alkali-activated slag cements. The accelerated ageing test showed that the dry powder alkali-activated slag cement had a sufficient shelf life.

Utilization of the fly ashes is a major problem in many developing countries and in South Africa only about 7% of the fly ash produced annually by coal-fired power stations, has being utilized. Although, fly ashes can be used as an alternative binder in alkali-activated concretes, strength development of these concretes at room temperature is slow limiting application of the material. Direct electric curing is proposed for heat curing of alkali-activated fly ash concrete which will open new opportunities for in-situ applications of these concretes in the construction industry thus increasing the amount of beneficially utilized fly ash. Alkaliactivated fly ash concretes containing unclassified low calcium fly ash, sodium hydroxide and sodium silicate solutions were cured at 60 ºC by means of direct electric curing. The electric resistivity and compressive strength development of the concretes were investigated. The resistivity strongly depends on the type of activator used. Compressive strength up to 33.8 MPa and 48.5 MPa at 2 and 28 days respectively, can be achieved after a short period of 2 direct electric curing. This opens new opportunities for wider application of alkali-activated fly ash concretes and for more extensive utilization of fly ash.

This paper reports on core temperature development and initial shrinkage of fly ash cement pastes activated with sodium hydroxide solution at different concentrations during elevated-temperature curing at 60°C. The results indicate that a high sodium hydroxide concentration might result in a substantial rise in the core temperature of samples, dependent on the mould size and ratio of paste to oven volume. An increase in alkali concentration was also found to increase the initial shrinkage of the pastes during elevated-temperature curing. Excessive initial shrinkage and temperature increase might lead to the appearance of internal stresses in the pastes, which can affect the material performance. mass of mould with sample after elevatedtemperature curing (g) m m mass of empty mould (g) T p peak temperature (°C) t p time of appearance of peak temperature after the start of elevated-temperature curing (min) Notation