The size of portlandite crystals in ITZ and its relation with ratios of ingredients and properties of LWAC

Abstract

In this experimental study, nine different lightweight aggregate concrete (LWAC) specimens - in which natural lightweight scoria aggregate was used as coarse aggregate - were prepared for investigating the size of portlandite crystals in their interfacial transition zone (ITZ). Scanning Electron Microscope (SEM) was used to determine the size of portlandite crystals in ITZ of LWAC specimens. The size of portlandite crystals in ITZ of these LWAC specimens was determined quantitatively in order to identify its relation with ratios of ingredients and properties of LWAC that were investigated. It was determined that the size of portlandite crystals in ITZ of nine LWAC specimens is in the range of (0.91-2.047) µm. The size of portlandite crystals in ITZ is found to be increased when the water/cement (W/C) and coarse aggregate/total aggregate (Ac/A) ratios of LWAC get increased. On the other hand, the compressive strength and the oven-dry density of LWAC are found to be decreased when the size of portlandite crystals in ITZ gets increased. The best way to make portlandite beneficial from mechanical, physical and durability points of view is to transform it into so-called secondary hydration products by making it react with materials that have proper chemical properties for this transformation. In this case, the small portlandite crystals dissolve entirely, and the large portlandite crystals become smaller. Lightweight scoria aggregate used in this study is thought to have chemical properties to assist such a transformation in ITZ.