Limestone-filled, hydraulic-lime mortars for historic and traditional fabrics

Abstract

Hydraulic lime (HL) mortars are considered compatible with historic fabrics and used in repairs. They consist of lime and clinkers (mainly belite). As belite hydrates slowly, some HLs develop strength for up to a year. This has raised concerns with conservationists. This paper partially replaces natural HL (NHL5) with limestone to lower ultimate strengths that can damage for some fabrics. It investigates the hydration of NHL5 in the presence of limestone and the impact on mortar properties. Despite the NHL’s low aluminate content, limestone reaction is evident. Limestone-triggered hydrates appear in the matrix and at interfaces, likely strengthening the transition zone. The limestone fineness and permeability, and the significant specific surface area of its microcrystalline carbonate may have enhanced reactivity. The 10-20% limestone replacements increased the 28-day strength. However, they lowered the NHL5 strength after 90 days (flexural) and 180 days (compressive). In contrast, the NHL5 mortar keeps developing compressive strength, increasing by c.14 between 90 and 180 days and c. 10% between 180 days and 1 year. The strength raise did not significantly affect the hygric properties. When NHL5 is replaced with 10-30% limestone, the finest mortar pores are preserved (water vapour permeability varies insignificantly). Contrary to Portland cement (PC), the limestone lowers the water demand of the NHL5 mortars. To achieve a proper workability using less water has benefits relevant for those involved in mortar design.