In: Civil Engineering Research in Ireland, CERAI. Belfast, 2014. 211 ABSTRACT: Hemp-lime concrete is a low-embodied energy, carbon-negative, sustainable building material made with a lime- based binder and hemp aggregate. The material properties depend on several factors including: binder and hemp type and their ratio, water content, density, manufacturing method and curing conditions. This paper investigates hydration of a commercial binder which includes significant hydraulic additions. During curing, hydraulic binders compete for water with the (high suction) hemp particles. This competition can trigger a shortage of water for binder hydration which can undermine the performance of the concrete. The paper investigates the effect of saturating the concrete in water following an initial curing period. It concludes that hydration restarts following the reintroduction of water: the Scanning Electron Microscope evidenced a significant increase in the amount of hydrates filling pores in the concrete following re-immersion. The results also evidenced that the increase in the amount of hydrates results in a compressive strength enhancement and a reduction in permeability and capillary absorption of the concrete. This paper highlights the importance of considering mixing water content and drying conditions (including time of demoulding, curing humidity and temperature and sample size) to ensure full hydration of hydraulic binder in hemp concrete and obtaining optimum performance of the material. KEY WORDS: Hemp-lime concrete; Hydration; Microstructure; Strength; Permeability; Capillary absorption 1 INTRODUCTION Hemp-lime concrete is an alternative to conventional construction materials. It is a low-embodied energy, carbon- negative, sustainable building material made of a lime-based binder and hemp aggregate. It was developed in France in the 1990s and has since been used in hundreds of houses in Europe. Although an innovative material, organic matter has been added to building materials since antiquity. Hemp-lime concrete is light-weight (average wall density 275-400kg/m 3 ) and non-structural, typically used in combination with load-bearing frames. The material properties depend on several factors including: binder type, binder:hemp ratio, mixing water content, density, hemp shiv properties, manufacturing method, curing conditions and age. It typically exhibits low strength and a ductile compressive failure between 0.2-1.2 MPa [1-7] and an open pore structure (70.6% porosity [8]). Hemp-lime concrete has excellent thermal performance: a high thermal capacity (1000 J/kgK [9], 1240-1350 J/kgK [10], 1560±30 J/kgK [11]) coupled with a medium density and a low thermal conductivity (0.05-0.12 W/mK [12]) provide the material with good insulation capabilities. The hemp-lime concrete comprises of hemp shiv (approximately 5-30mm in length) coated by a binder that forms a matrix that holds the concrete together. Evrard and de Herde (2010) identified three types of pores; 1-10mm air voids that are interconnected and separated from the shiv by a binder coating, micro pores in the hemp shiv c.10um and smaller pores in the lime matrix c.1um [13]. Hydrates in the binder further reduces the pore size. The binder is typically lime based with hydraulic additions to facilitate setting and early strength development. Commercial binders specially formulated for use with hemp are also available in the market. Their formulation are not accurately known but may contain hydraulic additions including Portland cement, hydraulic lime and pozzolans. The hydraulicity of the binder should be low to avoid competition for water with hemp particles. Furthermore, strongly hydraulic binders can lead to trapping residual moisture in hemp particles [1]. Following an investigation of hemp concrete durability (freeze:thaw and salt exposure) which involved saturating the concrete in water after 9 months curing, it was evident that exposing hemp concrete made with a commercial binder to water following curing increased binder hydration [7]. Such an increase in hydration was not evident for less hydraulic binders. This research investigates the effect of this additional binder hydration on compressive strength, permeability and capillary action of hemp concrete made with commercial binder. Additional hydration of hemp-lime concrete, following initial curing has not yet been investigated although De Bruijn et al. (2009) observed an increase in compressive strength of samples with cement binder following freeze/thaw cycling and attributed this to the immersion in water for 24 hours prior to testing [14]. However, Evrard (2003) does not contemplate re-hydration, stating that an initial lack of water for hydration causes the concrete to powder and this is irreversible [1]. In cement mortars, hydration restarts following reintroduction of water however, the material may not achieve its full compressive strength due to cracking and to the formation of Impact of hydration on the properties of hemp-lime concrete Walker_R, Pavia_S Department of Civil, Structural and Environnemental Engineering, Trinity College, Dublin, Ireland email: walkerro@tcd.ie, pavias@tcd.ie