Local Browning Phenomenon of Mortar Block Surface Caused by Crushed Granite Sand

Abstract

A mortar block was manufactured employing crushed granite sand, which was produced from Sanuki-shi, Kagawa, crushed copper slag and blast-furnace slag cement. After lying outdoors for several months, the surface of the mortar block was locally browned. To investigate this browning phenomenon, the following analyses were carried out: 1) chemical composition and crystalline compounds in the employed aggregates, 2) elemental (Si, Al, Fe, S) distributions in the aggregate and in the browned portion of the mortar block by EPMA, 3) identification of the browned component by IR. It was inferred that iron(III) sulfate, which was contained in a part of the crushed granite sand particles, was locally leached by contact with rainwater and reacted with Ca(OH)₂ in the surrounding mortar to afford browned iron oxyhydoxide on the mortar block surface. The browning phenomenon also appeared when Fe₂(SO₄)₃ impregnated sand was buried in a Portland cement mortar. As an effective measure against the browning, Fe leaching with acidic solution from the surface layer of granite sand was proposed.