Research Note pubs.acs.org/IECR
Stabilization of Low-Modulus Sodium Silicate Solutions by Alkali Substitution John L. Provis,*,† Adam Kilcullen,†,‡ Peter Duxson,‡ David G. Brice,‡ and Jannie S. J. van Deventer†,‡ †
Department of Chemical & Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia Zeobond Pty Ltd., P.O. Box 210, Somerton, Victoria 3062, Australia
‡
ABSTRACT: Concentrated sodium silicate solutions of modulus (SiO2/Na2O molar ratio) close to 1.0 are well-known to precipitate hydrous sodium metasilicate crystals; this hinders their industrial-scale utilization in applications including geopolymer concrete synthesis. The substitution of 20−50% of the sodium by potassium in such solutions is seen to greatly reduce or prevent this precipitation, on a time scale of up to 7 years. A potassium substitution rate of 20% does not entirely eliminate precipitation, but does reduce it very significantly; 50% substitution does eliminate precipitation, although the viscosity of the solution increases notably at this level of substitution. This substitution provides a relatively low-cost means of extending the shelf life of concentrated low-modulus alkali silicate solutions for large-scale utilization in the production of geopolymer concretes and in other applications.
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INTRODUCTION Alkali metal silicate solutions, or “waterglasses,” are utilized in a wide range of applications. Aqueous sodium silicate in particular is used as an adhesive or sealant, as a drilling solution, in fire protection, and as a reagent in the industrialscale synthesis of specialty chemical and synthetic mineral products.1−5 The modulus of a silicate solution is defined as the molar ratio SiO2/M2O, where M is an alkali metal; highmodulus solutions (modulus ≥2) are generally stable and have a long shelf life at temperatures above 10 °C, and fill the majority of uses for sodium silicate solutions in industrial application. However, lower-modulus (≤1.5) sodium silicates are of interest in the development of low-CO2 alkali-activated (including “geopolymer”) cements and concretes, which are formed by the reaction of an alkaline solution with a solid aluminosilicate to generate a solid binder which performs similarly to hydrated Portland cement.6 In particular, concentrated alkali silicate solutions are combined with industrial wastes or mineral products such as fly ash, blast furnace slag, and metakaolin to form alkali-activated binders.6 While solid silicate activators are useful in the large-scale production of alkali-activated concretes, the majority of work in the open literature has used silicate solutions rather than solids. Because of the high alkalinity required to initiate the reaction of the aluminosilicate solids, a modulus around 1.0−1.5 is usually desirable. However, such solutions tend to crystallize sodium metasilicate hydrates when stored for extended periods and/or are allowed to cool below ∼20 °C.7 Solutions of modulus 0.5− 1.0 are particularly prone to crystallization; unfortunately, this is also close to the compositional range (modulus approximately 1.5) which gives in the highest strength and durability in geopolymers synthesized from several types of precursor.8,9 The phases which crystallize from moderate-modulus sodium silicate solutions are usually sodium metasilicate hydrates, predominantly those containing 5, 6, or 9 water molecules per © 2012 American Chemical Society
formula unit, although some reports also indicate the formation of an 8-hydrate.1 However, the situation with respect to potassium silicate solutions is known to be rather different. K2SiO3 does not have a reported solubility at 25 °C, as it forms hydrous glasses and gels rather than distinct crystalline products, and thus provides the possibility for more stable solutions. However, the key issue related to the use of potassium silicates in large-scale application as geopolymer activating solutions is related to cost; the activator comprises a very significant fraction of the cost of a geopolymer concrete mix10 (particularly when the solid aluminosilicate source is an industrial waste or byproduct), and potassium silicates are more expensive (in general by a factor of 2−4) than their sodium counterparts. This means that the use of a mixed-alkali solution would be preferable to a pure potassium activator if sufficient shelf life can be achieved. The specific study of the chemistry of mixed Na−K silicate solutions in the literature is not widespread, and the mixed-alkali effects in these systems are not well understood. There are some mixed-alkali effects in the viscosities of high-modulus solutions, with mixed systems showing a higher viscosity than either end-member.3 A weak mixed-alkali effect in conductivity has also been identified at low temperature,11 and it has been noted that mixed-alkali geopolymers often show compressive strengths exceeding those of the pure sodium or potassium aluminosilicate mixes.12−14 The purpose of this paper is therefore to present data related to the long-term (7 years) stabilization of low to moderate modulus sodium silicate solutions at room temperature, by the substitution of up to 50% of the sodium by potassium. This is directly relevant in the commercial preparation of geopolymer concretes by the combination of a silicate solution with a solid Received: Revised: Accepted: Published: 2483
September 18, 2011 January 17, 2012 January 26, 2012 January 26, 2012 dx.doi.org/10.1021/ie202143j | Ind. Eng.Chem. Res. 2012, 51, 2483−2486
Industrial & Engineering Chemistry Research
Research Note
Table 1. Summary of Observations of Solutions after 7 Years of Aging sample number
a
Na/(Na+K) (molar)
SiO2/ M2O
solids
1
1
0.5a
2 3 4 5
1 1 1 0.80
1.0a 1.5a 2.0a 0.5
6
0.80
1.0
7 8 9 10 11 12
0.80 0.80 0.50 0.50 0.50 0.50
1.5 2.0 0.5 1.0 1.5 2.0
transparent crystalline precipitates (∼20% possibly Na2SiO3·9H2O solid polycrystalline block Na2SiO3·6H2O, none none transparent crystalline precipitates (∼10% Na2CO3·H2O, possibly Na2SiO3·9H2O transparent crystalline precipitates (∼10% Na2SiO3·9H2O minor suspended solids None none very minor suspended solids none minor suspended solids
13 14 15 16
0.20 0.20 0.20 0.20
0.5 1.0 1.5 2.0
none none none none
liquid
of sample) Na2SiO3·6H2O, minor Na2CO3·H2O,
viscous clear liquid
minor Na2CO3·H2O, possibly Na2SiO3·9H2O
of sample) Na2SiO3·6H2O, very minor
no free liquid viscous clear liquid viscous clear liquid clear liquid
of sample) Na2SiO3·6H2O, possibly
clear liquid viscous clear liquid viscous clear liquid clear liquid clear liquid viscous clear liquid extremely viscous clear liquid/gel clear liquid clear liquid clear liquid clear liquid
H2O/Na2O molar ratio 6.7 for these samples; 10 for all others.
crystalline product in all samples analyzed by XRD is Na2SiO3·6H2O; there is also a tentative identification of Na2SiO3·9H2O in some samples, but the concentration and/ or crystallinity of this compound seem to be low. A small amount of thermonatrite (Na2CO3·H2O) is identified in the samples containing no K due to carbonation, either through absorption of some CO2 during preparation of the silicate solutions, or inadequate sealing of the samples during aging. Because of this carbonation, detailed characterization of the supernatant solutions accompanying the solid precipitates is not presented. It is likely that thermonatrite is also the phase responsible for the small white suspended particulates in several of the other samples. To aid in explaining the trends in Table 1, and the importance of the stabilization of geopolymer activating solutions, it is necessary to consider in more detail the formation of hydrous sodium metasilicates. Figure 1 shows part of the Na2O-SiO2−H2O system with metasilicate crystallization isotherms at 25 °C, plotted from the data of Wills.7 The solubility of silica (amorphous or crystalline) in NaOH solutions has been studied in a large number of contexts, and solubility-pH diagrams plotted,2,15,16 but the compositional region in which these systems fall is somewhat removed from the solution compositions studied here. It is also possible that there is a metastability relationship involving the hexahydrate and pentahydrate, with the hexahydrate being the phase observed in the systems studied here. Of the solutions shown in Table 1, only the highest-silica compositions (4, 8, 12, and 16) fall in the region of Figure 1 corresponding to a stable solution (far right-hand end of the dark shaded region). The others, and particularly the modulus 0.5 and 1.0 solutions (samples 1, 2, 5, 6, 9, 10, 13, and 14; lefthand end of the dark shaded region) are seen to be unstable. To further illustrate this point, Figure 2 is a diagram adapted from Vail,1 showing the full ternary Na2O−SiO2−H2O system at ambient temperature, divided according to the properties and uses of the materials obtained in each compositional
aluminosilicate powder, as the limited (and sometimes unpredictable) shelf life of low-modulus sodium silicate solutions is a hindrance in this process.
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MATERIALS AND METHODS Alkali hydroxide solutions were prepared by dissolution of NaOH pellets (Merck, 99.5%) and/or KOH pellets (Merck, 85%) in Milli-Q water; solutions with 20, 50, 80, and 100% Na substitution by K were prepared. The 15% water content of the KOH pellets was considered in the formulation of solutions. Alkali silicate solutions with desired SiO2/Na2O ratios, and H2O/Na2O ratios of 10 (except as noted in Table 1), were prepared by dissolving amorphous silica (Cabosil M5, 99.8% SiO2) into alkali hydroxide solutions at room temperature, agitated with a PTFE-coated stirrer bar on a magnetic stirrer. Closed vessels were used to minimize contact with atmospheric CO2. Polypropylene laboratory-ware was used to avoid contamination by the etching of glass vessels. When silica dissolution was complete, the solutions were transferred to 1000 mL polyethylene bottles, which were immediately sealed and stored at ambient temperature (maximum, 30 °C; minimum, 15 °C) for 7 years. Observations regarding the precipitates formed in each system were made after this time. The solids were removed from selected systems by decantation of the supernatant liquid, for analysis by X-ray diffraction (XRD). Samples with only suspended solids were not subjected to XRD, as the quantities of precipitate collected were small. XRD data were collected using a Bruker D8 Advance instrument and LynxEye detector, with Ni-filtered Cu Kα radiation, random powder mounts, a step size of 0.02° 2θ, and 4 s count time per step. MDI Jade 8 was used with the Powder Diffraction File (PDF-4 Minerals) 2010 database for phase identification.
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RESULTS AND DISCUSSION Table 1 presents a summary of the observations of the solutions and precipitates, after 7 years of aging, including the crystal phases identified by XRD where possible. The predominant 2484
dx.doi.org/10.1021/ie202143j | Ind. Eng.Chem. Res. 2012, 51, 2483−2486
Industrial & Engineering Chemistry Research
Research Note
(Table 1) is therefore important in the commercial-scale deployment of geopolymer technology. In particular, the difference between the entirely solidified sodium silicate sample with modulus 1.0 in Table 1, and the predominantly liquid sample at the same modulus with 20% potassium substitution, is highly notable, as a modulus of 1.0 is highly desirable in terms of geopolymer strength development and microstructure.8 Potassium-containing activating solutions give more favorable rheological properties in fresh geopolymer mixes,17 and mixed-alkali geopolymers also tend to show higher strengths,12 thus providing benefits additional to the stabilization of the activating solutions by potassium addition. It is also notable that, although the substitution of 50% of the sodium by potassium entirely prevents precipitation (other than minor suspended particles, most likely carbonates), the viscosity of these mixed-alkali solutions is high (Table 1). Thus, the use of this potassium content is not desirable. The viscosity decreases again at 80% K, and these solutions are also stable, but the cost of using very high potassium contents for largescale geopolymer production is a disadvantage.
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Figure 1. Crystallization isotherms for hydrated sodium metasilicate phases at 25 °C and ambient pressure (data from Wills7). The horizontal axis scale is logarithmic, and the dashed lines are a guide to the eye only. The compositions discussed in this study fall within the darker shaded area (sample IDs are marked for samples 1−8 in Table 1, with total alkali content expressed as molar equivalent Na; samples 9−16 align with samples 5−8 and are not marked). The lighter shaded region shows the full range of solutions which are likely to be useful in geopolymer synthesis.
CONCLUSIONS The ability to extend the shelf life of concentrated low-modulus (
