Binders for soil stabilization and reclamation

Abstract

A binder composition for soil stabilization is disclosed, which substantially comprises at least 60 wt% cement, 6 wt% or greater of gypsum, 5 wt% or less of a first additional silicate and 9 wt% or gr

Description

FIELD OF INVENTION

[0001] The present invention relates to soil stabilization and in particular relates to a binders, methods and systems for the stabilization of a soil region and if desired for the reclamation and restoration of the stabilized soil region. BACKGROUND ART

[0002] One approach to permanent and temporary soil stabilization is the use of hydraulic cement-based binders. These may be mixed with the soil either ex-situ or in-situ to provide a soil / binder mixture, which may be treated with water and compacted to provide a converted area with a hardwearing surface capable of sustaining human and / or vehicular traffic. The strength is provided by the cement-based binder undergoing hydrothermal curing to set the mixture. In some instances, this soil / binder layer acts as a sub-base and may be covered with a further layer of stone, which is compacted and the whole structure cured to for a robust and harder wearing surface. In other versions, the stone layer may be replaced or further enhanced with other surface materials such as a tarmac layer.

[0003] In GB2521115, GB2543378, EP0100656, US4545820, US4697961 and GB2566005 there are described various soil stabilization processes and compositions.

[0004] With reference to sites with temporary soil stabilization the stabilized soil should ideally be as easy as possible to reclaim and restore to the pre-stabilized capacity of the land. In these temporary sites the stabilizer compositions are selected and designed to be used at such levels so that the binder / soil layer may be broken up, without removal of the binder, and reconstituted with soil to reclaim the site and to return it as far as possible to its pre-stabilized state.

[0005] Despite developments in this field there is a continuing need to find new binder formulations and methods of stabilization and reclamation that meet increasing demands, especially for the performance of temporarily stabilized sites, whilst still needing to provide temporary sites that are fit for use and can be easily and cheaply reclaimed with the lowest possible environmental impact. One particular problem is effective restoration of the soil at these sites is the high levels of alkalinity that is imparted to the site from the binder technology. One of the primary sources of this alkalinity from the binder systems is the relatively high levels of portlandite in these systems. In current systems it has been challenging to keep portlandite as low as possible without compromising other important properties of the system for site stabilization such as hardness, rate of cure and general integrity of the temporary surface. DISCLOSURE OF THE INVENTION

[0006] The present invention is concerned in part with reversible cement technologies. Reversible cement technologies rely on the ability to convert alkaline hydration products to non-alkalinity producing phases through a neutralization process. A key factor in determining cement reversibility is equilibrium pH conditions and the inherent buffering capacity caused by the alkaline reserve. Portlandite (Ca(OH)z) is a primary hydration product of the initial cementation reaction which strongly buffers the system in the region of pH of 12.4-12.6. Portlandite exerts important crystal-chemical controls on the continual dissolution of silicates within the cementitious matrix leading to reaction pathways that generate crystalline phases which are ultimately responsible for producing strength gain.

[0007] The inventors have found that selecting specific binder formulations the early-stage hydration products are controlled whilst allowing acceptable strength generation in the system and that ensuing that reduced portlandite may be achieved and brought to a desired level for, thus providing a reduced buffering capacity and making reversibility both more technically obtainable and commercially viable and effective.

[0008] The present invention is directed to a binder formulation and method for soil site stabilization using such binders and to an improved method of site restoration.

[0009] The preferred binders comprise specific mixtures of cement, preferably Portland cement, gypsum and additional silicate bearing materials.

[0010] In this specification the percentage for each component in any given composition, where ranges are provided is selected to provide a combined total of 100 wt%. All proportions are given as wt% unless otherwise specified or the context would dictate otherwise. 5

[0011] According to one aspect of the invention there is provided a binder composition which consists of 75 wt% cement, 12 wt% of gypsum, 4 wt% of disodium metasilicate and 9 wt% of a second additional silicate, and the binder composition is free from casting plaster. 10

[0012] The cement may be any selected from hydraulic cements or Portland cements. The hydraulic cements may be mixtures of fine ground limestone, alumina, and silica. These constituents react and start to cure with addition of water and then in addition to this (in some cases) compaction. During these processes, a Calcium Silica 15 Hydroxide gel is formed, calcium hydroxide and additional cementitious reactions. i •4 / Aluminous cements typically contain at least thirty to thirty-five percent alumina. CM QQ

[0013] Preferably the cement is Portland cement such as ordinary Portland cement. The raw materials required to produce Portland cements are composed of 20 calcium carbonate, alumina, silica, and iron oxide (as tetracalcium aluminoferrate), tricalcium aluminate, tricalcium disodium metasilicate, and dicalcium silicate. The cement is preferably ordinary Portland cement (CEM1). Alternatively, any other type of cement, such as CEM2, CEM3, CEM4 and CEM5 may be used. 25

[0014] The first additional silicate bearing material is disodium metasilicate.

[0015] The second additional silicate bearing material may be provided by silica fume, blast furnace slag, metakaolin, fly ash or a combination thereof. The preferred second additional silicate is silica fume. 5

[0016] Silica fume is often referred to alternatively as microsilica and can be provided as an ultra-fine powder with the average particle diameter of 150 nm and is also referred to as precipitated silica; fumed silica; gel silica; colloidal silica; silica flour and silica dust. It is typically 100 times smaller than average cement particle. When 10 OPC reacts with water, silica fume reacts with 25% calcium hydroxide. Silica fume is the by-product of producing silicon metal or ferrosilicon alloys. It generally comprises amorphous silicon dioxide 85%. The silica fume may be replaced by fly ash, preferably pulverised, ground granulated blast furnace slag, metakaolin. 15

[0017] Gypsum is sulphate mineral composed of calcium sulfate dihydrate, LQ Gypsum plaster, or Plaster of Paris or Casting Plaster, and is typically produced by CM heating gypsum. The gypsum may comprise a mixture of fine casting plaster and recycled plaster / plasterboard. The gypsum constituent may be gypsum, fine casting plaster. The gypsum composition preferably has a particle size less than 770 microns 20 diameter, preferably less than 270 microns diameter and most preferably less than 260 microns. The gypsum is present at 12 wt%.

[0018] In the binders of the present invention the level of various phases as determined by XRD analysis are maintained at acceptable levels after 28 days curing, 25 whilst at the same time providing an acceptable minimum strength for the temporary site formation. In particular the 28 day cured binders of the present invention preferably have a portlandite phase level of 30% or less, more preferably 20% or less as determined by XRD. Also, the 28 day cured binders of the present invention preferably have an alite phase level of 30% or greater, more preferably 35% or greater as 30 determined by XRD. Also, the 28 day cured binders of the present invention preferably have an ettringite phase level of 20% or less, more preferably 15% or less as determined by XRD. The binders of the present invention preferably exhibit UCS (Unconfirmed Compressive Strength) values after 28 days curing of at least 4000 kPa 35 and most preferably at least 5000 kPa. These binders when formulated into a bound (Unconfirmed Compressive Strength) values after 28 days curing of at least 4000 kPa and most preferably at least 5000 kPa. These binders when formulated into a bound soil formulation preferably exhibit a CBR (Californian Bearing Ratio) of 16 or more ay 7 days curing and 25 or more at 28 days curing.

[0019] The binder compositions of the present invention have been found to be particularly suitable for the low impact and effective regeneration of soil stabilized sites. The binder compositions whilst highly effective at providing stabilized sites having stabilized soil layers of the requisite strength, they also offer much easier and lower impact regeneration of that site compared to known binder systems.

[0020] Therefore, the present invention further provides a method for the stabilization of an area of soil and for the regeneration of the area of stabilized soil, which method includes a first stabilization step comprising the mixing of a binder according to the present invention with the ground soil to provide a stabilized layer and the method including the subsequent step of regeneration by applying one or more acidogenic compositions to the stabilized soil.

[0021] It is preferred that the acidogenic compositions are solid phase materials that may neutralize the alkalinity imparted to the soil from the binder used in the stabilization step. Suitable acidogenic materials include but are not limited to: sulfur, sulphur coated urea, calcium sulphate, aluminium sulphate, anhydrous aluminium sulphate, aluminium sulphate octahydrate, potassium sulphates, nahcolite and sodium bisulphate, iron (II) sulphate, iron (II) sulphate heptahydrate, iron (II) sulphate monohydrate, iron (III) sulphate, iron (III) sulphate pentahydrate. Also, envisaged by the method of the present invention is the use of two or more acidogenic materials in the acidogenic composition. Also, envisaged by the method of the present invention is that the regeneration step using acidogenic compositions may be undertaken in two or more stages using the same or different acidogenic compositions. In the process of the present invention the one or more acidogenic compositions are formulated and applied with in an amount of acidogenic material and sufficient composition to neutralize the alkalinity from the binder composition and thus returning the stabilized soil to a pH approximating its original pH before mixing with the binder composition. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The present invention is exemplified and will be better understood upon reference to the following detailed examples when read in conjunction with the accompanying drawings. FIG. 1 Shows the XRD time series stack for CEM I system (Ett - Ettringite, Por-Portlandite, Ali - Alite); FIG. 2 Shows the XRD time series stack for Ex 1 Pat system (Ett - Ettringite, Por-Portlandite, Gyps - Gypsum); FIG. 3 Shows the XRD time series stack for Ex 3 Pat system (Ett - Ettringite, Por-Portlandite, Gyps - Gypsum); FIG. 4 Shows the XRD time series stack for Ex 4 Pat system (Ett - Ettringite, Por-Portlandite, Gyps - Gypsum); FIG. 5 Shows the XRD time series stack for Sample 1 system (Ett - Ettringite, Por-Portlandite, Gyps - Gypsum); FIG. 6 Shows the XRD time series stack for Sample 2 system (Ett - Ettringite, Por -Portlandite, Gyps - Gypsum); FIG. 7 Shows the XRD time series stack for Soil 1 system (Ett - Ettringite, Por -Portlandite, Gyps - Gypsum); FIG. 8 Shows the XRD time series stack for Soil 2 system (Ett - Ettringite, Por -Portlandite, Gyps - Gypsum); FIG. 9 Shows the XRD time series stack for Soil 3 system (Ett - Ettringite, Por -Portlandite, Ali - Alite, Gyps - Gypsum); FIG. 10 Shows the CEM I XRD phase proportions as a function of hydration time; FIG. 11 Shows the Ex1 Pat XRD phase proportion as a function of hydration time; FIG. 12 Shows the Ex3 Pat XRD phase proportion as a function of hydration time; FIG. 13 Shows the Ex4 Pat XRD phase proportion as a function of hydration time; FIG. 14 Shows the Sample 1 XRD phase proportion as a function of hydration time; FIG. 15 Shows the Sample 2 XRD phase proportion as a function of hydration time; FIG. 16 Shows the Soil 1 XRD phase proportion as a function of hydration time; FIG. 17 Shows the Soil 2 XRD phase proportion as a function of hydration time; FIG. 18 Shows the Soil 3 XRD phase proportion as a function of hydration time; FIG. 19 Shows the UCS strength gain curves for the cement specimens tested; FIG. 20 Shows the Alite assigned relative phase proportion as a function of UCS for specimens cured for 28 days; FIG. 21 Shows the Portlandite assigned relative phase proportion as a function of UCS for specimens cured for 28 days; FIG. 22 Shows the Ettringite assigned relative phase proportion as a function of UCS for specimens cured for 28 days; FIG. 23 Shows the Calcite assigned relative phase proportion as a function of UCS for specimens cured for 28 days; and FIG. 24 Shows the Portlandite assigned relative phase proportion as a function of DSM + silica fume addition. Examples Experimental

[0023] Table 1 presents an array of binder formulations manufactured and evaluated. Alongside CEM I 52.5 N (Hanson Cement), gypsum, casting plaster, disodium metasilicate (DSM) and silica fume have been used in various proportions. CEM I only specimens have been included in this study as a comparator system manufactured against which all test formations are benchmarked. Table 1: Trials cement formulations (wt %) CEM I Sample 1 78 i Sample 2 Ex 1 Pat / 8 I 78 I Ex 3 Pat 74 8 Ex 4 Pat 66 9 Soil -1 Soil-2 75 Soil - 3 75 Gypsum 6 12 0 12 12 G 12 9 Casting Powder 6 0 12 0 0 6 0 3 DSM 9.3 9.3 9.3 0 9.3 4 4 4 Silica Fume 0 0 0 13.2 13 9 9 9

[0024] Cement performance has been assessed through mechanical strength gain via Unconfined Compressive Strength (UCS) for cement, Californian Bearing Ratio (CBR) of modified soils and automatic Vicat testing for the determination of set times, as described below. The evolving mineralogical composition as a function of curing time has also been assessed through X-Ray Diffraction (XRD) analysis for phase identification (indexed against Bragg reflections) and quantification by the Relative Intensity (RIR) ratio method. The 9 No. cement systems were manufactured at a C / W ratio = 0.4. The hydrated slurry was immediately poured into a 50 mm diameter HDPE moulds. Cement grouts were allowed to cure at 95% relative humidity prior to testing at pre-determined curing times. Californian Bearing Ratio (CBR) determinations were undertaken in general accordance with BS EN 13286-47 following 7 and 28 days curing. Soil test specimens were manufactured using a bulk homogenised slightly silty sandy natural soil incorporating 2% binder addition at optimum moisture content (CMC) + 4 %. The unbound soil produced a CBR value of 6.2, providing a useful benchmark for assessing strength development in hydraulically bound mixtures. The rate of strength gain for each of the binders was assessed by UCS. UCS determinations were undertaken on 50mm specimens manufactured at axial: diametric ratio = 1. Strength gain curves were generated for all mix designs over a 28 day curing period at time (t) = 4 days, 7 days, 14 days, and 28 days. Strength gain assessments were completed by UCS using an electro-mechanical testing instrument under standard operating procedures in accordance with BS EN 13286-41. Setting time testing was performed using an automatic Vicat apparatus in accordance with EN 196-3. An in-water testing kit was included to allow continual specimen immersion throughout the experimental period. X-ray diffraction (XRD) patterns were recorded at (t) = 4 days, 7 days, 14 days, and 28 days using a Panalytical XPERT Pro with a diffraction angle range between 5 -70 °20, scan rate 0.017 °20 / s. Smear slide sample mounts were utilized as a simple quick method to create suitable XRD samples for analysis whilst limiting preferred orientation of particular crystalline phases. Sample preparation techniques included manually grinding samples in an agate mortar and pestle under acetone, with the resultant <10 urn slurry transferred to a glass disk to create a thin film specimen. Mineralogical phases were quantified using Relative Intensity Ratio (RIR) using Match! software developed by Crystal Impact. California Bearing Ratio (CBR) for Soil-Binder Systems

[0025] All CBR values listed below in Table 2 are comparable to the pure CEM I at 28 days with the lower 7 day CBR results possibly indicating delayed strength development due to the presence of sulphate bearing retardant phases. It is notable that the inclusion of silica fume within formulations does not appear to interfere with retarded strength gain effected through the inclusion of sulphate phases. Table 2: CBR values (%) at 7 and 28 days curing Sample 7 day 28 day Sample 1 20 23 Sample 2 19 25 Ex 1 Pat 22 25 Ex 3 Pat 16 24 Ex 4 Pat 15 25 Soil - 1 16 22 Soil - 2 16 25 Soil - 3 18 25 CEM I 23 25 Interpretation of X-ray Diffraction (XRD)

[0026] Alongside hydrated cement specimens, XRD patterns for unreacted CEM I clinker and additive powders are presented to confirm purity and starting compositions. Despite numerous attempts, XRD analysis of Disodium metasilicate (DSM) could not be completed due to problems with obtaining evenly dispersed smear mounted specimens. XRD pattern for the OPC CEM I show alite is present as the major phase, which would be expected, with minor quantities of corundum (AI2O3) and ferrite (Ca2(AI,Fe)20s), assigned as brownmillerite, compositionally equivalent to ferrite phases typically found within portlandite cement. Trace quantities of periclase (MgO) and celite are also identified. The mineralogical composition of starting CEMI used in this study is consistent with that expected for a typical CEM I powder.

[0027] XRD of the casting powder and gypsum show pure phases of bassanite and gypsum respectively, silica fume XRD pattern shows a silicon carbide crystalline phase, with an amorphous reflection at approximately 20 - 25 °20. This has been tentatively assigned as amorphous SiOz due to the position relative to the 111 reflection of cristobalite at 21.47 °20.

[0028] Alite and portlandite are two important phases in cement chemistry, which are monitored by XRD to elucidate changes in mineralogical composition during cement hydration and maturation for these binder systems. Other notable mineral phases formed during cement hydration include ettringite (Ca6Al2(SO4)3(OH)i2-26H2O), formed through the reaction between calcium silicate and aluminate phases (such as alite and celite) with calcium sulphate phases such as gypsum or bassanite. Ettringite formation can disrupt the strength gain reactions of cement due to volumetric expansion. Although omnipresent in cement, preferable binder formulations should ideally reduce the presence of ettringite, however the reactivity of cements with native soil minerals such as pyrite, can induce Delayed Ettringite Formation (DEF).

[0029] Figures 1-9 show XRD stacks tracking cement hydration products for the various binders tested, with stacks ordering from bottom to top: (t) = 1 day, 4 day, 7 day, 14 day and 28 day curing. Figure 10-18 display the cement phase quantity change as a function of curing time.

[0030] Hydration reaction progress has been observed through mineral phase composition of the following mineral phases: alite, portlandite, ettringite, gypsum and calcite. CEM I only sample has been used as a benchmark to observe additive effects on mineralogical composition and provide the mechano-mineralogical link to compressive strength performance and development CEM I shows continual hydration throughout the 28 day testing period via formation of portlandite, and subsequent reaction of alite. Calcite and celite contents remain reasonably constant, expressed in relatively low proportions. CEM I XRD pattern shows early stage ettringite formation, remaining fairly constant through the curing period of investigation. Contrary to this, the specimen analysed at 14 days hydration may be considered an outlier. With the exception of Ex4 Pat and Soil 2, trialed cements show development of portlandite with a respective decline of alite, which is not desirable.

[0031] A lack of bassanite identified within XRD patterns of hydrated cement samples containing casting plaster indicates a conversion to gypsum through hydration and / or reaction with calcium silicate to form ettringite within the initial 24 hours; the formation of ettringite is undesirable. Ex4 Pat appears to show the slowest hydration kinetics, retaining the highest relative proportion of alite out of any sample, alongside the low levels of portlandite formation, but it’s slow hydration kinetics are undesirable. Unconfined compressive strength (UCS) Comparison to Mineralogical Phase Composition

[0032] All trialed cement formulations showed poorer strength gain compared to the CEM I only specimen as may be expected, see Figure 19. All specimens show a general strength increase over the initial 28-day curing period. As illustrated in Figures 20 - 23, the relative strength performance can be correlated to the proportion of mineral phases present: alite, portlandite, ettringite and calcite. Soil 3 has not been included within these figures due to the lack of available 28 day UCS data. This has arisen due to sample mechanical failure during testing.

[0033] Alite, ettringite and calcite all show a general negative correlation with mechanical strength gain. Higher alite proportion may indicate slower hydration kinetics, leading to poorer strength gain. As previously stated, ettringite leads to volumetric expansion, therefore it is expected that higher ettringite concentrations result in lower mechanical performance due to a higher density of defects.

[0034] Inspection of Figure 21 shows the correlation between the relative abundance of portlandite and UCS strength gain.

[0035] Soil 1,2 and 3 formulations show similar mechanical strength gain to other binder formulations with CBR values not dissimilar to CEMI mix designs trialed in soil-binder systems. All samples Soil 1, 2 and 3 formulations show a relative reduction in portlandite when compared to CEM I and other formulations with the exception of Ex 4 Pat, which displayed the lowest residual portlandite concentrations of all trialed formulations considered in this study but had poor properties as a binder.

[0036] Overall Soil 1, 2 and 3 formulations provide an acceptable balance between strength development and lowered residual portlandite, with Soil 2 showing the most promising performance. Ex 4 Pat showed lower portlandite but with exceptionally high levels of added silicate, which has problems and is commercially unattractive as these components are the most expensive components in the compositions also it exhibited inferior retardation behavior.

[0037] This study demonstrates that the expression of portlandite may be controlled by the incorporation of effective blend ratios of silicate admixtures; DSM and silica fume being particularly effective. However, suppression of portlandite as shown in Ex 4 Pat comes with the potential offsetting of early stage strength development whereby Ex 4 Pat showed high residual alite content at 28 days consistent with achieving the lowest UCS. These observations point to very high silica content mixtures potentially retarding cementation reaction progress.

[0038] Figure 24 presents the relationship between portlandite and total Si addition (wt %) as a combination of SF and DSM. Soils 1, 2 and 3 show the lowest levels of portlandite with low levels of silicate. Soil 2 provides the most advantageous properties for deployment as a reversible binder in soil stabilisation systems.

[0039] Throughout the description and claims of this specification, the words "comprise1' and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other components, integers, or steps.

[0040] Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. Where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. Features, integers, characteristics, compounds described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

[0041] All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

Claims

11 03 251. A binder composition which consists of 75 wt% cement, 12 wt% of gypsum, 4 wt% of disodium metasilicate and 9 wt% of a second additional silicate and the composition is free from casting plaster.

2. A binder composition as claimed in claim 1, wherein the second additional silicate material is one or more of: silica fume, blast furnace slag, metakaolin or fly ash.

3. A binder composition as claimed in claim 1, wherein the second additional silicate material is silica fume.

4. A method for the stabilization of an area of soil and for the regeneration of the area of stabilized soil, which method includes a first stabilization step comprising the mixing of a binder according to claim 1, with the ground soil to provide a stabilized layer and the method including the subsequent step of regeneration by applying one or more acidogenic compositions to the stabilized soil.

5. A method as claimed in claim 4, wherein the acidogenic compositions comprise solid phase materials.

6. A method as claimed in claim 5, wherein the acidogenic materials are one or more of one of the following: sulphur, sulphur coated urea, calcium sulphate, aluminium sulphate, anhydrous aluminium sulphate, aluminium sulphate octahydrate, potassium sulphates, nahcolite and sodium bisulphate, iron (II) sulphate, iron (II) sulphate heptahydrate, iron (II) sulphate monohydrate, iron (III) sulphate, iron (III) sulphate pentahydrate.

7. A method as claimed in claim 4, wherein two or more acidogenic materials are used in the acidogenic composition.

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