Interest in alkali-activated slag as a construction material is increasing, primarily due to its environmentally friendly nature. Although strong alkaline activators, such as sodium hydroxide and sodium silicate solution, are preferred for high strength, none of them exists naturally and their manufacturing process is quite energy intensive. Whilst sodium sulfate (Na₂SO₄) can be obtained from natural resources, the early strength of Na₂SO₄ activated slag is usually low. In this paper, the effects of slag fineness and Na₂SO₄ dosage on strength, pH, hydration and microstructure were investigated and compared with those of a pure Portland cement (PC). Test results indicated that increasing the slag fineness is a more effective approach than increasing Na₂SO₄ dosage for increasing both the early and long-term strength of Na₂SO₄ activated slags. In addition, increasing the slag fineness can also increase the strength without increasing the pH of the hardened matrix, which is beneficial for immobilizing certain types of nuclear waste containing reactive metals and resins.