Properties of alkali-activated slag mortar and prediction of its compressive strength

Abstract

An investigation was carried out on alkali-activated slag AAS mortar mixtures cast with three different water-to-binder ratios W/B; 0.35, 0.40, and 0.45, eight different silica modulus Ms, wide range of Na2O%, activated using a mixture of two different activators; waterglass and sodium hydroxide, and cured with two different curing methodologies; sealing and full immersion in water. The standard 70.6 mm mortar cubes were tested under compression at the age of 3 days and 7 days. Furthermore, their flow, setting time, and longitudinal shrinkage were measured. The results showed that not only the percentage of Na2O is important; but, its source. Additionally, curing methodology affected the compressive strength at both ages where the full immersion in water may cause early development of strength, with no later strength gain. Also, flow test results showed that although increasing the W/B ratio can cause an increase in the flow, however, the strength may be affected negatively at the highest W/B ratio. Drying Shrinkage of AAS mortar mixtures was also found to be very sensitive to the W/B ratio. Furthermore, four mathematical models were used to generate prediction equations of the compressive strength of AAS mortar based on the experimental data, and for both curing methods. A comparison between predictions of different mathematical models was made based on the highest coefficient of determination R2, score, and standard error. The models were verified using four AAS mortar mixtures that were casted and tested at the age of 7 days under compression. The results showed that two models closely predicted the actual strengths, except at compressive strength higher than 60 MPa, where the models were significantly conservative.