Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay Research paper Effects of clay's chemical interactions on biocementation Rafaela Cardoso a, ⁎ , Inês Pires b , Sofia O.D. Duarte c , Gabriel A. Monteiro c a CERIS/ICIST, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal b Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal c IBB, Instituto Superior Técnico, Universidade de Lisboa, Lisboa, Portugal ARTICLE INFO Keywords: Ground improvement Clays MICP Tensile strength Porosimetry Osmotic effects on clay ABSTRACT Biocementation, or microbially induced calcite precipitation (MICP), is a technique currently appraised for the improvement of sandy soils. These types of soils are good candidates for MICP-based soil improvement due to their relatively large voids size associated to high permeability and room available for bacteria colonization. Only few studies use soils with controlled percentages of clay fraction (up to 20%), in which MICP is still efficient. However, chemical activity in the clay fraction is not considered, but may be relevant for bioce- mentation because the clay minerals react with the feeding solution (due to pH and the presence of calcium ions). In this paper, the effect of clay chemical interactions on biocementation is investigated considering samples of a uniformly graded sand and samples of the same sand to which kaolin clay was added to reduce the porosity to half. The changes on some physical properties of these artificial soils achieved after treatment were evaluated in 1) saturated samples through changes in compressibility and permeability, and 2) in partially dried samples through changes in tensile strength. The results of the tests are discussed considering the presence of a clay and calcium carbonate coating, both bonding the grains of sand. The presence of clay minerals has two main effects: (i) an increase on the compressibility due to osmotic effects that reduce clay stiffness and (ii) a reduction in the permeability associated to pore size reduction. MICP treatment per se appears not to affect significantly this behavior. Although the tensile strength increases for the sand samples with clay, the increment is larger after MICP, which could be explained by apparent cohesion and physical connections from calcium carbonate mi- nerals. The results allow concluding that chemical interactions between the feeding solution and the clay mi- nerals must be considered when using MICP in soils containing them and, for this reason, this treatment in clayey soils is more complex than in sands without significant percentage of clay. 1. Introduction Biocementation, or microbially induced calcite precipitation (MICP), is a soil treatment technique based on bacterial activity that alters the hydraulic and mechanical properties of porous media (see for example DeJong et al., 2013). MICP provides adequate conditions to specific bacteria, living in the soil, hydrolyses urea ((NH 2 ) 2 CO) (Eq. (1)). The ammonium (NH 4 + ) released from urea hydrolysis results in pH rise that promotes the precipitation of calcium carbonate (CaCO 3 ) (Eq. (2)). + → + + (NH ) CO 2H O 2NH CO 22 2 4 3 2– (1) + → + Ca CO CaCO 2 3 2– 3 (2) Calcium carbonate formed, also known as biocement, accumulates in the soil pores mainly between the grains (DeJong et al., 2006), therefore clogging the pores and reducing permeability. In addition, the biocement introduces stiffness and strength because it is a physical connection (named as bond in this paper). Calcium carbonate can precipitate in three main polymorphs: vaterite, aragonite and calcite (Wei et al., 2015). Environmental conditions significantly influence the type of polymorph formed (Teng et al., 1998; Davis et al., 2000), but the type of microorganisms must also be considered (Cañveras et al., 2001; Dupraz and Visscher, 2005; Wei et al., 2015). Wei et al. (2015) found that vaterite is the main form of calcite produced by the ureolytic gene of the bacterial species used in this work, Sporosarcina pasteurii,a result also found by Cardoso et al. (2017) in XRD tests. Previous studies mainly focused on improving sands and silty sands. Concerning hydraulic properties, permeability can be reduced about one order of magnitude (Whiffin et al., 2007; Cheng et al., 2013; Zamani and Montoya, 2016). Kirkland et al. (2017) found a final por- osity 85% of the initial porosity of a silty sand. Concerning the me- chanical properties, strength increment is evaluated mainly through unconfined compression tests (Al Qabany and Soga, 2013, among https://doi.org/10.1016/j.clay.2018.01.035 Received 25 September 2017; Received in revised form 25 January 2018; Accepted 29 January 2018 ⁎ Corresponding author at: DECivil, Instituto Superior Técnico, Av. Rovisco Pais, 1, 1049-001 Lisbon, Portugal. E-mail addresses: rafaela@civil.ist.utl.pt (R. Cardoso), gabmonteiro@tecnico.ulisboa.pt (G.A. Monteiro). Applied Clay Science 156 (2018) 96–103 0169-1317/ © 2018 Elsevier B.V. All rights reserved. T