Analysis of Lime Capping Mortars After Two Decades of Carbonating in Ambient Conditions or Exposed to Natural Weathering at Corfe Castle

In 1993, 13 lime-based mortar mixes were produced for trials at Corfe Castle, Dorset, UK [1, 2]. The trials aimed to evaluate mortars used for consolidating a section of the wall head at Corfe Castle. These mortars had to be resilient to weathering at the exposed location of the castle but remain sacrificial to the underlying historic stone. A sample of each binder and each mortar mix was also placed in sealed polythene bags and stored indoors under ambient conditions (unweathered). In 2019 (weathered) samples of each mortar mix were extracted from the mortar surface and 50 mm below the surface. This paper compares the unweathered and weathered samples for five mortar mixes that used Blue Lias lime putties with different hydraulicities, Unilit B Fluid X (Unilit) hydraulic lime, or Unilit and Chards non-hydraulic lime putty blends. Physical and chemical characterisation was conducted using thermogravimetric analysis (TGA), powder X-ray diffraction, optical imaging, and scanning electron microscopy (SEM) imaging with energy dispersive X-ray (EDX) analysis. These tests enabled a comparison between the hydraulicity and chemical composition of these mixes. TGA results demonstrated that binder choice and weathering conditions had no noticeable impact on carbonation after 26 years. Optical microscopy showed lichen growth on the weathered surface samples but not on the unweathered samples. SEM and EDX identified self-healing of cracks in weathered samples only. Moisture transfer, the ability to self-heal and acting as a barrier between lichen and stone are key features that enabled the mortars to protect the historic material.