Alkali activation of a slag at ambient and elevated temperatures

Strength development of alkali activated slag (AAS) mortars, activated using alkali hydroxide and sodium silicate, was investigated at room and elevated temperatures. Heat evolution at room temperature was measured using isothermal calorimetry. Important differences were observed between critical activation parameters. Heat cured specimens gain strength rapidly, humid oven conditions being favorable, but given sufficient time room temperature curing yields comparable strengths. Both activators are needed for high strength at room temperature, NaOH solution is more critical and its concentration greatly influences strength. At 80 °C however, sodium silicate is essential and even sufficient. KOH is more effective than NaOH at 80 °C, but not at room temperature. Lower water-to-slag ratios give higher strength at early ages. AAS hydration evolves less heat than Portland cement hydration. Time to significant strength gain of mixtures can be predicted using their time and heat evolution at setting. Twenty eight-day strength of AAS mortars is roughly related to total evolved heat and increases nearly linearly with the amount of NaOH activator for fixed water glass content.

► Calorimetry and strength gain of alkali-activated slag at various temperatures.
► NaOH activation is more effective at room temperature; water glass is at higher temperatures.
► Humidity is beneficial to mechanical property development at elevated temperatures.
► Effectiveness of KOH and NaOH appears dependent on curing temperature.
► Heat peaks associated with time of setting and strength gain are related, as is peak height to time.