Stabilization/solidification characteristics of organic clay contaminated by lead when using cement

Highlights

• HA and FA weaken PTE stabilizing effects in cement treated soil.

• The result of two opposed HA and FA effects on PTE may be mainly determined by material mixing sequence and timing.

• The stabilized strength reaches a peak at a specific lead content in soil.

• Strength increases and lead leaching decreases with more cement added or longer curing time up to about 28 days.

Abstract

Research about cement treated soil has examined various characteristics of strengthened and stabilized soil, but has mainly focused on either the unconfined compressive strength or potentially toxic element (PTE) stabilizing results respectively in response to cement dosing. This study investigates how factors including cement concentration, lead concentration, humic/fulvic acid content and curing age affect these two geotechnical and environmental characteristics. A laboratory study was conducted to measure unconfined compressive strength, and lead leaching under several test conditions. Knowing that humic acid and fulvic acid can weaken cementation in cement treated soil but can stabilize PTEs such as lead by different chemical reactions, it was found that the acids generally reduce lead stabilization in cement treated soil. In addition, the stabilized strength reaches a peak at a specific lead content in soil. Finally, scanning electron microscopy was used to observe more detailed changes and mechanisms.

Introduction

Cement has been used in chemical stabilization/solidification (S/S) as an effective amendment to increase soil strength and/or remediate contaminated soil [1]. Cement increases soil strength by cement hydration, pozzolanic activity and substitution of calcium for sodium and potassium on soil particles. Cement can remediate groundwater contaminants by encapsulating the contaminants, decreasing the permeability in the formation, and changing the chemical conditions (e.g., increasing pH) to reduce concentrations of contaminants and potential contaminants in groundwater. PTEs such as lead and arsenic are two of the most common groundwater or soil contaminants considered for remediation by stabilization/solidification but some organic contaminants (e.g., creosote, and PCBs) have also been treated by this method [2]. Several different factors associated with the cement amendment process including operating variables such as cement and water contents, water/cement ratio (specifically for cement paste), pH and curing time, etc. have been studied for their influence on soil strength or metal stabilizing respectively, resulting in general findings (e.g., soil strength increases with curing time [3], higher cement concentrations results in higher strength [3], and less metal leaching [4]). There are also several studies that have considered the formulation of cement-based stabilizers with different kinds of cement and additives to analyze strength improvement and/or PTE stabilization, using models [5] or envelope charts [6] to suggest optimal operating conditions (range of operating variables) and stabilizer formulations.

It is widely known that higher concentrations of organic matter in soil inhibit the stabilizing effect of cement-based material geotechnically by adversely affecting cementation reactions between soil and the amendment, which means the typically expected strength increase by the addition of cement is reduced because of the existence and effects of humic acid and fulvic acid (major soil organic matter components [7]). These two acids can combine with calcium and aluminum, as well as decrease pH, which are both fatal to cementation because the formation of C-S-H or hydrated calcium aluminate is the main source of strength increase for cement. Incomplete cementation can also diminish comprehensive encapsulation of PTEs resulting in increased exposure to groundwater and the potential for dissolution, and therefore decreased contaminant stabilization or higher contaminant concentrations in groundwater. In contrast, several studies have shown that humic acid (HA) and fulvic acid (FA) can combine with specific PTEs like Pb, Cd, Cu, and Cr by complexation [[8], [9], [10]] and remove them from the aqueous phase. Overall, the combination of HA and FA can play a major role in both the complexation and cementation mechanism. It is hard to determine a priori if the adverse effect of organic matter consuming calcium in cement or the positive effect of complexing PTEs is more important, as shown in Fig. 1. The two reactions occur at different time scales (complexation reactions that bind metals are completed within approximately four days [11] while cementation reactions occur over the course of more than 7 days [12] (28 days is a common curing time milestone). We hypothesized that there is an optimal set of conditions where metals are successfully bound but soil stabilization is minimally impacted because of the different time scales of the competing reactions. There are very few studies [13] in the literature that have examined this topic, but it could be important to know organic matter’s overall influence on PTEs and soil stabilizing with cement-based material under various conditions.

In addition to the organic acid concentrations’ influence on soil strength, PTE concentration may also affect soil stabilization using cement. Previous studies of lead’s effect on cement strength (e.g. Lee et al. [14] and Gineys et al. [15]) have shown that an increase in lead content will decrease the strength of cement. However, these studies did not consider cemented soil with more common concentrations of lead contamination in soil (generally small). Minocha et al. [16] found that the effect of lead contamination on soil strength is negligible using relatively high lead concentrations in their study (2%–8%, or 20,000 mg/kg–80,000 mg/kg) but did not examine the lower concentrations that are more typical at lead contaminated sites. Yin et al. [17] suggest that the soil UCS can even show a slight increase when lead concentrations range from 500 mg/kg to 25,000 mg/kg. Van Jaarsveld and van Deventer [18] have reported that increased lead concentrations in the range of 1000–2000 mg/kg leads to a stronger structure in geopolymer, which was attributed to the formation of specific phases of lead during synthesis.

In this study, a single-factor experiment was conducted. A systematic series of samples were made with different contents of cement, HA & FA, and lead, and different curing times to investigate their effects on post-stabilization properties of soil. Based on unconfined compressive strength (UCS) measurements, and lead leaching tests, each factor’s effect on strength and PTE stabilizing was evaluated, especially the effects of HA & FA and lead content. In addition, scanning electron microscope (SEM) images were used to observe small-scale changes to help explain the soil strength and leaching observations.