A petrographic study of mortar hydraulicity.

Abstract

The nature of the binder determines the mortar cohesiveness and its intrinsic bond, therefore it greatly impacts its physical properties and durability. In a binder, the hydrated material is mainly responsible for the strength. Mortars containing hydrated material are hydraulic. Hydraulicity is coupled to faster hardening, higher ultimate strengths and a greater durability, and is probably the most important feature to be determined in order to replicate historic mortars for repair. The aim of this paper is to explore mortar hydraulicity through petrographic analysis. Hydraulicity can reside either in the binder or in other mortar components such us the aggregate or pozzolanic additions. The paper evidences these instances through case studies. This paper concludes that very similar hydration products to those formed in mortars made with hydraulic lime and Roman cements are present in lime/brick mortars due to pozzolanic reaction. These include amorphous C-S-H cements and crystalline and layered C-S-H). This work also suggests that the C-S-H formed during hydration of hydraulic lime and lime-brick mortars may exhibit a higher crystallinity than that in portland cement pastes, and this may be due to the higher free lime content of the hydraulic lime and lime-brick pozzolan systems, where more calcium is available for reaction. The results also indicate that the hydrates resulting from pozzolanic reaction in lime/brick mortars are more determined by the pozzolan?s composition and the conditions of the reaction, than by its specific surface. This paper also concludes that clay-bearing aggregate such as shale and greywacke; microsilica-bearing aggregate such as chert and even crystalline igneous rocks such as dolerite are reactive, and can bind substantial amounts of lime being therefore pozzolanic. Finally, the paper states that even though petrographic analysis does not allow us to precisely quantify mortar hydraulicity, it enables to discern whether a mortar is hydraulic and conclude on the hydraulicity source. The presence of both crystalline and amorphous C-S-H; portlandite crystals, relicts of unhydrated clinkers; hydration layers and pozzolanic reaction rims as well as the arrangement of new-formed amorphous silica and alumino-silicates can be determined with petrography, and these comform evidence of hydraulicity.