The effect of curing conditions on the hydromechanical properties of a metakaolin-based soilcrete

Abstract

An attractive approach to reduce the carbon footprint of deep soil mixing (DSM) is to replace Portland cement-based binders by geopolymers based on metakaolin. Safe design requires a good understanding of the mechanical and hydraulic properties of the improved ground but very little is known about metakaolin-soil mixtures. For instance, shrinkage during curing is a significant issue for metakaolin-based concretes but has not been previously studied in soilcretes. In this work the permeability and strength of sand and silty sand based metakaolin soilcretes are studied under different curing conditions. The development of microcracks induced by geopolymer shrinkage is confirmed through a microstructural study using mercury intrusion porosimetry, scanning electron microscopy and X-Ray computed tomography. The influence of microporosity and binder filling on permeability and strength is clarified adapting wellestablished soil models. A modified Kozeny-Carman formulation is proposed for permeability. A mixture ratio model is calibrated to represent strength. In general, the metakaolin stabilised materials present excellent mechanical and hydraulic properties, although these are very sensitive to curing conditions.

Exploring the mechanical response of low-carbon soil improvement mixtures

“…It has been reported that dry curing of pure CS results in even higher strength gains than wet curing (Axelsson 2006). On the other hand, dry curing of pure MK results in severe mechanical damage to the material (Spagnoli et al 2021a). This was borne out by our tests, where in dry-D r a f t cured CS treated specimens UCS systematically increased, whereas metakaolin treated specimens (SMK(100)D and SOMK(100)D) reduced strength and became more ductile during the curing period.…”

“…Contrary to what happened with UCS the effect of curing time on the triaxial response of dry-cured MK treated specimens was rather small, showing lesser or no clear decay of peak mobilised strength with time (Figure 8). This suggests that confinement inhibited the mechanical effect of MK micro-cracking that was visible on microscopic images (Spagnoli et al 2021a) Undrained secant Young moduli are presented in Figure 9 (at a small strain level) and Figure 10 (at intermediate strain levels). In these figures the strain level effect (decreasing stiffness as strain level increases) is larger than the stress level effect (stiffness increases as confinement increases).…”

“…Tomographic inspection of permeated specimens (Figure 2) showed that the process resulted in a very uniform and complete filling of pore space by the hardened colloid. For details of the tomographic procedure applied see Spagnoli et al (2021a). The proportions of materials in the resulting CS mixtures are given in Table 4.…”

“…Unconfined compressive strength tests (UCS) followed a standard procedure (ASTM 2166, 2017) which involved an axial-strain rate of around 0.5%/min (see Spagnoli et al 2021a, for more detail).…”

“…This may be related to the strength of the two binders, which is generally much larger for the metakaolin geopolymer. For instance, for wet curing at 28 days Spagnoli et al (2021a) report UCS values of 1.2…”