Alkaline activation of fly ash and blast furnace slag has gained interest due to a desire to avoid Portland cement in mixtures. Outstanding mechanical performance and durability is reported, but often when the activator dosage is high which can have various negative environmental impacts that can overshadow the carbon reduction benefit. This study investigates the use of a ground slag, and a high-lime fly ash, rich in free lime and sulfates, to activate each other and render mortars which don't incorporate any Portland cement or an added chemical activator, but still have useful strengths. The ash, which does not conform to standards for use in concrete, hence is nearly completely landfilled, is used as-received or after grinding. 28-day compressive strengths surpassing 13 MPa and 20 MPa were recorded, for samples cured at 23 degrees C or at 80 degrees C. Various combinations of the two powders have heats of hydration lower than that of a typical Portland cement. Ettringite and CSH are determined to be responsible for the early and ultimate strength gain. The effect of adding gypsum to the system as a low-impact activator is also investigated. (C) 2016 Elsevier Ltd. All rights reserved.