CaO-Activated Ground Granulated Blast Furnace Slag Systems and Its Application to the Recycling of Stone Powder Sludge

Abstract

This study investigated on the CaO-activated ground granulated blast furnace slag (GGBFS) systems, and the production of brick using CaO-GGBFS system and stone powder sludge. Especially, it was proved that the addition of gypsum to CaO-activated GGBFS improves the compressive strength performance of both CaO-activated GGBFS and the brick product which incorporates stone powder sludge in large amount. The compressive strength of the CaO-activated GGBFS system with the addition of gypsum was primarily governed by the amount of ettringite, but more importantly, the morphology of reaction products (e.g., size, slenderness) is proposed to be a more dominant factor. The optimal gypsum content at the best strength produced fine ettringite crystals and removed large pores near 100 µm, which resulted in significant pore-size refinement. However, excessive addition reduced the strength and generated a large volume of detrimental pores with sizes of 0.03 - 0.4 µm, which were probably expansive cracking or large openings between the large sizes of re-precipitated gypsum crystals. The scanning electron microscopy (SEM) images demonstrated that the fine ettringite crystals at the optimal gypsum dosage filled the pores effectively, which leaded to dense matrices, while the coarse crystals of gypsum or ettringite at the excessive addition produced porous matrices. The potential use of stone powder sludge as a replacement for fine aggregate was examined for production of bricks through CaO-activation for GGBFS. The brick with 70 % of stone powder sludge and gypsum showed the best strength among various mixture proportions. The addition of gypsum contributed to the pore size refinement through ettringite formation. The reactivity of the stone powder sludge with CaO-activated GGBFS system was little, and any siliceous minerals that can result in alkali-silica reaction were not observed for the stone powder sludge. The bricks in this study satisfied the requirements of ASTM C55 and demonstrated economical superiority to general Portland cement bricks.