Quantification of the effects of nitrates, phosphates and chlorides on soil stabilization with lime and cement
Research Highlights
► The influence of a compound is a function of the curing conditions. ► The selected compounds are not systematically detrimental for soil stabilization. ► Several curing procedures are needed to check for soil stabilization efficiency.
Introduction
Soil stabilization with lime and cement is a commonly used technique in earthworks, and it has been under continuous development since its introduction in the middle of the past century. Currently, environmental issues are leading to an evolution of the socio-economic context of earthworks. One of the most remarkable changes is the requirement to exclusively use the soils located directly in the land reservation of a project to build the infrastructure. In certain situations, the achievement of that objective imposes the use of soils with very low engineering properties that are unsuitable for making different parts of the structure, such as the capping layer and subbase, with the required performance. In these cases, soil stabilization becomes essential to improve the engineering properties of these materials. However, soil stabilization faces a number of limitations, especially when the soil contains a significant amount of chemical compounds that may alter the effects of the lime and cement. In France, about two million tons of soil per year is recognized as unsuitable for soil stabilization with lime and cement, and the presence of some detrimental chemical compounds has been highlighted to explain these failures.
The most widely known compounds to alter soil stabilization with lime and cement are sulfates (e.g., Mitchell, 1986; Hunter, 1988) and organic matter (e.g., Kuno et al., 1989, Tremblay, 1998). However, several cases of soil stabilization failure have been reported by practitioners with no sulfates or organic matter identified in the soil to be stabilized. In these cases, other compounds are sometimes considered to explain these failures like those contained in agricultural fertilizers (nitrates and phosphates) or chlorides in the vicinity of sea coasts (e.g., LCPC-SETRA, 2000). However, there are very few data available in the literature on the actual effects of these compounds on soil stabilization. In this context, it is not possible to conclude whether or not the presence of agricultural fertilizers (nitrates and phosphates) or chlorides in a soil can explain soil stabilization failures. In addition, if these compounds are likely to alter soil stabilization, it is necessary to determine the concentration threshold beyond which a compound may have a negative impact on the effectiveness of soil stabilization with lime and cements. Moreover, it is known that the curing conditions influence the physico-chemical processes of soil stabilization and, therefore, the mechanical performance and the swelling of the stabilized soil. In the case of sulfate-bearing soils stabilized with lime, considerable variations of the swelling have been demonstrated by Wang et al., 2003, Harris et al., 2004 as a function of the curing conditions (relative humidity and temperature).
This study intended to assess quantitatively the influence of some potential deleterious compounds on the mechanical performance of a soil stabilized with cement and lime. The results permitted to study the possibility to determine a concentration threshold beyond which each compound alters the soil stabilization processes. Besides, several experimental procedures are available to determine the performance of a stabilized soil in the laboratory (LCPC-SETRA, 2000). They are based on several curing conditions (temperature and humidity) and different mechanical parameters to be tested (tensile strength, compressive strength, etc.). These different procedures were followed, to highlight the combined influences of temperature, curing time and humidity on the effects of the tested potential deleterious compounds.
Working with a natural soil could cause difficulties in determining the individual effects of a given chemical compound on soil stabilization (e.g., Cabane, 2005). Thus, an original procedure was used in this study. Suitable soils for stabilization were mixed with one chemical compound at a given concentration. The mechanical behavior of this mixture was then determined and compared to the performance of the stabilized soil without any added chemical compounds. Several experimental procedures, derived from French standards available to assess the performance of stabilized soils, were followed. The combination of these results demonstrated the actual effect of each compound quantitatively.
Two agricultural fertilizers were selected for testing: ammonium nitrate ((NH4)2NO3) and potassium phosphate (K3PO4). Their use is widespread, and their potential effects on soil stabilization are still unknown. The chosen chloride was sodium chloride (NaCl). Although a comprehensive literature review of their effects on the soil stabilization processes is already available, sulfate compounds were also considered in this study because sulfates would give a qualitative comparison point to assess the impact of the other compounds on soil stabilization.
Section snippets
Tested soils
Two soils were selected in the framework of this study, a fine sand and a silt. The main difference between these soils was the presence of clayey particles in the silt (Le Borgne, 2010). The main properties of the two soils selected for this study are given in Table 1. The first soil, “limon Val d'Europe” (LVE), was sampled in the East of Paris. According to the AASHTO soil classification system, it can be classified as a silty soil (A.4). The second soil, Bouër sand (BS), was taken from the
Blending of soil and chemical compounds
The preparation of the mixture of the soil and the desired chemical compound is a critical issue. To obtain homogeneous samples, we adapted a mixing procedure from the international standard ISO 11268 (1993). The procedure used in the present study followed these steps:
- 1.
The chemical compound was dissolved in distilled water.
- 2.
The soil and the solution were thoroughly mixed with a mechanical mixer for a few minutes.
- 3.
A fifteen-day equilibration period was required in an airtight container and at 20 °C.
Influence of sulfates on soil stabilization
In the literature, there is a large volume of evidence that sulfates can alter soil stabilization with lime and cement. As an example, Mitchell, 1986, Hunter, 1988 characterized the adverse effects of sulfates on soil stabilization with lime. Many other studies have been carried out to assess the impact of sulfates on soil stabilization or to mitigate their detrimental effects on soil stabilization (e.g. Mitchell and Dermatas, 1992; Rajasekaran et al., 1997; Wild et al., 1999; Baryla et al.,
Influence of nitrates on soil stabilization
To the knowledge of the authors, there are no references on the influence of nitrates on soil stabilization with lime and cement. Nevertheless, an evaluation of their potential detrimental effects on soil stabilization can be inferred from cement chemistry. Indeed, nitrates are likely to alter concrete structures (e.g., Mohan and Rai, 1978, Ukraincik et al., 1978). This alteration is the result of the attack on the cementitious compounds through an acidic–basic reaction mainly involving
Influence of phosphates on soil stabilization
The possible mechanisms that phosphates use to affect soil stabilization can be inferred from the literature available on cement and concrete chemistry. Phosphates are known to be retarders of cement hydration; the setting time can be strongly delayed, while hardening can be greatly lowered, at least at an early age. A comprehensive study of the hydration of cement pastes in phosphate-rich solutions has shown that the delay increases with the phosphate concentration up to 25 g L− 1 but then
Influence of chlorides on soil stabilization
It is generally accepted that the penetration of chlorides in cement-based materials does not readily lead to the formation of detrimental solid phases that may cause expansion and cracking. On the contrary, the interaction between chlorides in solution and the paste, because it binds penetrating ions, is often considered to have a beneficial influence on the durability of reinforced concrete because it slows the rate of ingress toward reinforcing steel. Moreover, calcium chloride (CaCl2) is an
Conclusion
In this paper, the influence of several potential deleterious compounds on soils stabilized with lime and cement was considered. The influence of the concentration of the compound, the nature of the soil, the type of cement and the curing conditions (temperature and humidity) was considered. It has been demonstrated that the considered compounds (nitrates, phosphates, chloride and sulfate) can have a significant effect on the mechanical performance of a stabilized soil. A synthetic qualitative
Acknowledgement
This research was funded by the “Syndicat Professionnel des Terrassiers de France”.
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