One-Year Bond Strength Development of Outdoor Cured M0.9 Cement-Lime and Natural Hydraulic Lime Mortar

Natural hydraulic lime (NHL) mortars have long been of interest in both historic masonry and as an alternative to cement or cement-lime mortars. This interest is mainly due to their lower energy demand during manufacturing of the binder, slower setting and hardening, along with different stiffness and strength compared to Portland cement (PC) [1]. Both NHL and PC come in many different compositions and strength classes, which can affect the resulting mortar properties. Masonry mortars are therefore, according to EN 1996-1-1, categorized by their strength class (28-day compressive strength); however, when NHL mortars and cement-lime mortars are compared in academic research, it is rarely based on mortars of the same strength class. Several studies have demonstrated the extended strength development of NHL [1–3], yet comparisons with cement–lime mortars of identical strength class remain limited, and documentation of resulting material property differences is incomplete. This study aims to contribute to the documented strength development of cement-lime and hydraulic lime mortars, which remains relevant as mortar selection for construction purposes is often based primarily on strength class alone. This study focuses on one of these properties: the long-term development of flexural bond strength for two prescribed bedding mortars of strength class M0.9 in interaction with typical Danish clay masonry units. This is an initial investigation in a larger study assessing brick reclamation and reuse.