Effects of mellowing sulfate-bearing clay soil stabilized with wastepaper sludge ash for road construction Mohamad Nidzam Rahmat ⁎, John M. Kinuthia Civil Engineering Research Unit (CERU), Department of Engineering, Faculty of Advanced Technology, University of Glamorgan, Pontypridd, CF37 1DL, UK abstract article info Article history: Received 25 March 2010 Received in revised form 15 October 2010 Accepted 19 October 2010 Available online 27 October 2010 Keywords: Oxford Clay Mellowing Wastepaper Stabilization Sulfate Engineering properties The main objective of this research was to investigate the effect of mellowing on the swelling properties of stabilized Lower Oxford Clay (LOC), a sulfate-bearing clay soil that has caused concern in the past due to expansion upon stabilization. Two stabilizers were used, quicklime and a more sustainable stabilizer — wastepaper sludge ash (WSA), an industrial by-product used on its own and in combination with either quicklime (CaO) or ground granulated blastfurnace slag (GGBS). Quicklime was used on its own as a control. Compacted cylinders of LOC stabilized with the various stabilizers were made either mellowed (compacted 3 days after mixing with stabilizer) or unmellowed (compacted immediately after mixing with stabilizer). The linear expansion of the stabilized cylinders during moist curing and subsequent soaking was monitored for a total of 100 days. The results obtained showed that the linear expansion of unmellowed stabilized LOC was significantly reduced, and thus in terms of swelling potential, mellowing was not beneficial in the LOC system investigated. The results observed also suggest that there are technological, economic as well as environmental advantages of utilizing WSA and similar industrial by-products, in the stabilization of sulfate- bearing and other clay soils, as an alternative to the traditional stabilizers of lime and/or Portland Cement. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Good quality sub-grade soils are necessary for durable roads. Such soils are not always available, and the Highway Engineer is likely to face weak or unsuitable sub-grade soils as a result of either low load- bearing capacity, and/or high swelling properties. Volume changes of some clayey soils, resulting from changes in their water content, may cause unpredictable movement of structures that are founded on such soils, resulting in heaving, shear failure, excessive settlement, cracking and breaking up. This is a particular problem with road pavements which are founded on such soils (Mowafy et al., 1990). Many problems associated with expansive soils have been reported all over the world by soil engineers since about 1940 (Chen, 1975). Since then, much research has been published on the road construction problems of expansive clays and low load-bearing capacity soils in general. For sulfate-bearing soils in particular, sulfate attack is mostly accompanied by strength loss and large volume changes, resulting in substantial heave in stabilized earthworks (Veith, 2000). Many researchers have reported examples of detrimental effects of sulfates, either naturally present in the ground or artificially added when soils are modified or stabilized with lime and/or cement. Examples include Mitchell (1986) and Hunter (1988) in the USA, and Snedker and Temporal (1990) and Parker (2004) in the UK. Snedker and Temporal (1990) outlined the disastrous results in lime stabilization during the construction of the M40 motorway near Banbury, U.K. where the presence of 0.4% sulfates resulted in 60% heave of the stabilized capping layer. Parker (2004) reported that sulfate attack of the lime- stabilized capping layer of the new carriageways on the 7.5 km A10 Wadesmill bypass U.K. resulted in heave that left up to 25% of the carriageways buckled, cracked and ridged. The expansion in lime-stabilized clay in the presence of sulfates is believed to be partly caused by the growth of ettringite crystals formed on the clay particle surfaces (Mitchell, 1986). Mehta (1983) found that ettringite formed in the presence of lime was needle-like, with a large surface area and unsatisfied negative charge. From the observation of swelling in a Portland Cement (PC) system, he concluded that the colloidal ettringite would imbibe large numbers of water molecules and generate swelling pressure leading to overall expansion of the system. Similarly, Wild et al. (1993), researching on industrial kaolinite clay stabilized with various lime and gypsum contents, agreed with Mehta (1983) that osmotic swelling would take place within the colloidal layer in regions of high sulfate concentra- tion in close proximity to the developing ettringite rods at the clay particle surfaces. When in a saturated condition, ettringite grows and develops from this colloidal product. During this growth, colloidal ettringite possesses the capability of imbibing large amounts of water, dramatically increases the swelling potential of the lime-stabilized soil. The main objectives of the current paper are to investigate the effects of mellowing on the swelling properties of stabilized soils. Engineering Geology 117 (2011) 170–179 ⁎ Corresponding author. Tel.: + 44 1443 654829, + 44 7527101947 (Mobile). E-mail address: mnrahmat@glam.ac.uk (M.N. Rahmat). 0013-7952/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2010.10.015 Contents lists available at ScienceDirect Engineering Geology journal homepage: www.elsevier.com/locate/enggeo