Synthesis and characterisation of magnesium silicate hydrate gels

Sedimentary talc in Neoproterozoic carbonate successions

A cementitious system for the immobilisation of magnesium rich Magnox sludge was produced by blending an Mg(OH)2 slurry with silica fume and an inorganic phosphate dispersant. The Mg(OH)2 was fully consumed after 28 days of curing, producing a disordered magnesium silicate hydrate (M–S–H) with cementitious properties. The structural characterisation of this M–S–H phase by 29Si and 25Mg MAS NMR showed clearly that it has strong nanostructural similarities to a disordered form of lizardite, and does not take on the talc-like structure as has been proposed in the past for M–S–H gels. The addition of sodium hexametaphosphate (NaPO3)6 as a dispersant enabled the material to be produced at a much lower water/solids ratio, while still maintaining the fluidity which is essential in practical applications, and producing a solid monolith. Significant retardation of M–S–H formation was observed with larger additions of phosphate, however the use of 1 wt% (NaPO3)6 was beneficial in increasing fluidity without a deleterious effect on M–S–H formation. This work has demonstrated the feasibility of using M–S–H as binder to structurally immobilise Magnox sludge, enabling the conversion of a waste into a cementitious binder with potentially very high waste loadings, and providing the first detailed nanostructural description of the material thus formed.

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Structure and properties of binder gels formed in the system Mg(OH)2–SiO2–H2O for immobilisation of Magnox sludge

Mg-silicate minerals (e.g., stevensite, kerolite, talc, sepiolite) play an important role in the construction of facies models in lacustrine and peri-marine environments because they are sensitive to changes in solution chemistry. However, the response of Mg-silicate mineralogy to changing aqueous chemistry is only broadly understood because the mechanisms underpinning the co-precipitation of Mg 2+ and SiO 2 (aq) from surface water, and subsequent Mg-silicate crystallization, are unclear. Here we describe the results of experiments designed to systematically examine the effects of pH, Mg/Si and salinity of the parent solution on the nature of initially precipitated products. Structural interrogation of the products with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis (TGA/DTA) allow comparison of synthetic products with naturally occurring crystalline counterparts. In general, Mg 2+ and SiO 2 (aq) co-precipitation and nucleation of Mg-silicate layer structures first involves the rapid formation of 2:1 layers with trioctahedral occupancy and a mean coherent X-ray scattering domain between 1–2 unit cells with respect to the c axis. Well defined but diffuse hk reflections indicate two-dimensional growth, turbostratic stacking and highly variable interlayer hydration. Diffuse reflectance FTIR shows numerous structural similarities with stevensite, kerolite and sepiolite. However, TGA/DTA analysis indicates the presence of variable kerolite/stevensite interstratification not readily detectable through XRD analyses, as well as a significant degree of surface and interlayer hydration (e.g. 15–20 wt.%). We observe a number of clear trends in the products with respect to solution chemistry. For example, at low salinity, kerolite-like products dominate at high Mg/Si and high pH, whereas sepiolite-like products are formed at lower pH and lower Mg/Si. At high salinity and high Mg/Si, stevensite-like products are favoured at high pH and kerolite-like products dominate at lower pH, whereas a decrease in Mg/Si of the solution leads to sepiolite-like products at low pH and only stevensite-like products at high pH. Higher pH leads to an increase in octahedral vacancies which favour stevensite-like products; this may result from a higher rate of two-dimensional tetrahedral sheet expansion relative to the octahedral sheet, as inferred from studies of silica oligomerization and brucite growth kinetics. Together, our results indicate that the neoformation of Mg-rich silicates from solution may often begin with the rapid nucleation of hydrated 2:1 layers. Subsequent dehydration leads to progressive layer stacking order and could occur in response to wetting/drying cycles, prolonged exposure to high salinity solutions, or burial and heating. The surface and interlayer water associated with these products is undoubtedly an important source of diagenetic water in Mg-silicate-bearing successions, and the chemistry of this water upon later diagenesis should be a focus of future investigation.

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Chemical controls on incipient Mg-silicate crystallization at 25°C: Implications for early and late diagenesis

Mineralogical, petrographic and sedimentological observations document early diagenetic talc in carbonate-dominated successions deposited on two early Neoproterozoic (~ 800–700 million years old) platform margins. In the Akademikerbreen Group, Svalbard, talc occurs as nodules that pre-date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates. In the upper Fifteenmile Group of the Ogilvie Mountains, NW Canada, the talc is present as nodules, coated grains, rip-up clasts and massive beds that are several meters thick. To gain insight into the chemistry required to form early diagenetic talc, we conducted precipitation experiments at 25 °C with low-SO4 synthetic seawater solutions at varying pH, Mg2+ and SiO2(aq). Our experiments reveal a sharp and reproducible pH boundary (at ~ 8.7) only above which does poorly crystalline Mg-silicate precipitate; increasing Mg2+ and/or SiO2(aq) alone is insufficient to produce the material. The strong pH control can be explained by Mg-silica complexing activated by the deprotonation of silicic acid above ~ 8.6–8.7. FT-IR, TEM and XRD of the synthetic precipitates reveal a talc-like 2:1 trioctahedral structure with short-range stacking order. Hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite (hydrated talc) and eventually to talc. This formation pathway imparts extensive layer stacking disorder to the synthetic talc end-product that is identical to Neoproterozoic occurrences. Early diagenetic talc in Neoproterozoic carbonate platform successions appears to reflect a unique combination of low Al concentrations (and, by inference, low siliciclastic input), near modern marine salinity and Mg2+, elevated SiO2(aq), and pH > ~ 8.7. Because the talc occurs in close association with microbially influenced sediments, we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry, specifically pH and alkalinity increases driven by anaerobic Fe respiration.

“Two dimensional” talc was prepared by the hydrothermal reaction of various silicas with magnesium hydroxide. The precipitated and activated silicas were especially available as starting materials. The powder X-ray diffraction data indicated that the resulting talc consisted of no-stratified and random layered structure. According to the IR spectrum, no solid acidity, i.e. Bronsted and Lewis acid sites were found. The specific surface area was over 500 m2 g−1. A large amount of water was adsorbed and partially combined with the surface of lamellar talc. As a result, the artificial talc would receive applications such as emulsifying agents to the mixture of liquid paraffin and water.

Study of Synthetic Clay Minerals. III. Synthesis and Characterization of Two Dimensional Talc

kn-=天然に産出する1:1型粘土鉱物の一種であるフライポンタイト((Zns-xA1x)(Si2-xA1x)O5(OH)4をモデルとし,合成で得られたフライポンタイトの構造について種々検討した。X線回折,熱分析,IRスペクトルから,天然のものと合成のものは類似していることがわかった。NMRから四配位のアルミニウムがほぼ等モルずつ存在することが示され,同形置換によって生じた陽電荷の過不足が四面体シートと八面体シートとの間でつりあう構造であった。このことは,陽イオン交換容量がほとんどないことからも確認された。化学分析値とIRスペクトルから同形置換しているアルミニウムの量(x)は天然のものより多いことが示された,また熱分析,化学分析から,天然のものには認められなかった構造水の存在が認められた。

Study of Synthetic Clay Minerals. II. Structure of Synthetic Fraipontite.

1:1型粘土鉱物の一種であるフライポンタイト((zns-xAlx)(Siz-xAlx)Os(oH)`)を合残すや反応条件について種々検討した。その結果フライポンタイトは,塩化亜鉛と塩化アルミニウムからなる1種の酸性水溶液と,ケイ酸ナトリウムと水酸化ナトリウム,アルミン酸ナトリウムと水酸化ナトリウムからなる2種のアルカリ性水溶液を調合し,この3種の液を同時に添加する塩の複分解法で合成できた。しかし,この時の結晶性は低かっ零。一方,酸化亜鉛,アルミナ,シリカの各金属酸化物を原料とし,オートクレープを用いて比較的穏和な160。C,3時間の水熱条件で反応させた。この反応系にアンモニウム塩を鉱化削として共存させることにより,結晶性の高いフライポンタイトが合成できた。この反応において原料が残存せず,しかもフライポンタイト以外り物質の生成が認められなかったことから,共存するアンモニウム塩が,フライポソタイトの生成を促進させることがわかった。さらに,各金属酸化物に対する塩化アンモニウムの鉱化剤としての作用に着目して反応過程の検討を行った,その結果,塩化アソモニウムは,反応系のpHを制御するとともに,原料の酸化亜鉛を錯イオンもしくは錯体化して反応性を高める作用のあることがわかった,

Study of synthetic clay minerals. I. Synthesis of fraipontite.

A novel porous adsorbent was prepared by adding an aqueous solution having Zn, Al, and Si components into a suspension of silica hydrogel at 40 °C. According to the results of XRD, NMR, TEM etc., the adsorbent was identified as being a new composite consisting of lamellar fraipontite and silica. The layer structure of the resulting fraipontite was not stratified three-dimensionally. NMR data also verified that the structural framework consisted of a single ZnO6 octahedral sheet with Aloct and a single SiO4 tetrahedral sheet with Altet, which bonded with silica. The formation process of this porous adsorbent, incorporated by silica particles, was systematically examined in some detail.

Teiji Satoh

Papers

Study of Synthetic Clay Minerals. III. Synthesis and Characterization of Two Dimensional Talc

Study of Synthetic Clay Minerals. II. Structure of Synthetic Fraipontite.

Study of synthetic clay minerals. I. Synthesis of fraipontite.

Study of Synthetic Clay Minerals. V.^ Lamellar Fraipontite-Silica Composite as a New Adsorbent