A petrographic study of the technology of hydraulic mortars at masonry bridges, harbours and mill ponds.

Abstract

Masonry mortars for underwater works such as bridge piers, harbours and mill ponds are of hydraulic nature. This paper investigates the raw materials and production technology of a number of these mortars. It focuses on the assessment of hydraulicity and the phenomena of delayed sulfate formation and alkali silica reaction (ASR) leading to damage. Hydraulicity is probably the most important property of a binder. Not only it governs a mortar?s cohesiveness and intrinsic bond therefore ruling physical properties and durability, but it also determines site application and performance. Samples of mortars from several structures including Annesley Bridge, St John?s Bridge, Dun Laoghaire Harbour and Dundrum Millpond were analyzed with petrography and X-ray diffraction. All the mortars displayed evidence of eminent binder hydraulicity including unhydrated clinker relics of different composition and hydraulic minerals. The identification of specific hydraulic minerals was difficult under the microscope, however, both irresolvable C-S-H, portlandite and other unidentified hydraulic phases were recorded. Blast furnace slags undergoing hydraulic reaction were noted, suggesting the addition of artificial pozzolans; and reactive aggregate showing pozzolanic reaction was also found. In relation to mortar durability, secondary reactions involving microsilica aggregate (ASR) leading to fracturing by expansion were evidenced; and delayed ettringite formation was also common in the mortars studied.