Investigation of the Hardening Processes of Lime-Ash Binder by X-ray Analysis and Electron Microscopy
Abstract Generated abstract
This study examines the hardening of lime and fly ash binder through X-ray diffraction and electron microscopy, building on earlier chemical and thermal analyses of the same system. The authors compare initial materials, mechanical mixtures, and hardened lime with silica gel or ash after moist curing, and assess changes after calcination to evaluate crystallization of reaction products. The results indicate that hardening involves chemical interaction between ash and lime in the presence of water, producing hydrated calcium silicate phases with initially disordered structure, alongside free calcium hydroxide and partial carbonation. Electron micrographs support these interpretations by showing plate-like hydrosilicate forms, rounded calcium hydroxide crystals, carbonate-like plates, and less-defined regions associated with incompletely crystallized material.
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INVESTIGATION OF THE HARDENING PROCESSES OF LIME–ASH BINDER BY X-RAY ANALYSIS AND IN AN ELECTRON MICROSCOPE
P. G. USOV, N. S. DUBOVSKAYA
(Presented by Professor, Doctor of Chemical Sciences A. G. Stromberg)
The chemical interaction between the constituent parts of a lime–ash binder during its hardening was detected by us through chemical analysis of the hardening products over the course of a year. The approximate structure of the hardened binder was revealed by us through thermal analysis of the hardening products [1].
Further investigation of the hardened binder, carried out by X-ray analysis, confirmed the presence of the structural elements discovered earlier, while microscopic determinations with the aid of an electron microscope made it possible to establish and record the minerals composing the hardened binder.
Research methodology. In studying the hardening processes of the lime–ash binder by the method of X-ray analysis, the working procedure was borrowed from V. F. Zhuravlev [2], who used it in studying chemical interaction in the system MeO—SiO₂.
Preparation of the specimen for exposure was carried out as follows. The hardened binder is dried to an air-dry state, ground until it passes completely through sieve No. 0085, and the powder is applied in a thin layer to a glass thread previously coated with an adhesive substance (lacquer); then the uniform layer of powder is covered with a thin layer of lacquer, and the specimen is ready for investigation. In its finished form the specimen has the shape of a rod 10 mm long and 0.4–0.5 mm thick.
The X-ray photographs were taken on a URS-70 X-ray unit. Since the ash included in the binder is dominated by silica in a glassy state, and lime was used as the activating additive, and since the chemical processes in the lime–ash binder–water system may be assumed to be analogous to the processes in the CaO—SiO₂—water system, X-ray photographs were taken of the initial materials (ash, lime), chemically pure silica gel and CaO, their mechanical mixtures, and mixtures calcined to 900° after four months of hardening.
The specimen for examination in the electron microscope is prepared by depositing a thin layer of a ground hardened sample from a drop of aqueous suspension placed on a thin collodion film.
When a beam of electrons passes through the specimen, the image is obtained as a result of the unequal absorption of electrons by different parts of the specimen. Thus, shadow images of the observed objects are obtained in the microscope field, making it possible to determine the dimensions of the smallest particles and their external contours.
A similar method has been used in the study of various disperse formations—fine fractions of clay rocks [3], products formed during the hydration of cements and individual minerals composing them [4], [5], and hardened sulfate slags [6].
Results of the X-ray Method of Investigation
The X-ray diffraction pattern of silicalcite (Fig. 1, a) is typical of an amorphous substance and is characterized by the absence of reflection lines.
Fig. 1. X-ray diffraction patterns of the starting materials and hardened mixtures: a—silicalcite; б—ash; в—mixture of silicalcite, lime, and water, hardened for 4 months; г—lime-ash binder, hardened for 4 months; д—calcium hydroxide hydrate.
The X-ray diffraction pattern of the ash contains lines characteristic of crystals of considerable size, and blurred lines of a finely crystalline substance, which indicates imperfection of the crystal lattice (Fig. 1, б).
The X-ray diffraction pattern of a mechanical mixture of silicalcite and lime is distinguished by the presence of lines characteristic of CaO, while in the X-ray diffraction pattern of a mechanical mixture of lime and ash, lines characteristic of lime and ash are clearly distinguished (Table 1, Figs. 2, 3).
The X-ray diffraction pattern of the hardened mixture of silicalcite, lime, and water after storage in a moist environment for 4 months contains a number of weak lines indicating the presence of crystalloid formations with a low degree of order. In addition to these lines, there are very strong and strong lines not characteristic of the X-ray diffraction pattern of lime and presumably belonging to newly formed chemical compounds with a clearly expressed crystalline structure (Fig. 1, в; Table 2, Fig. 2).
The X-ray diffraction pattern of the hardened binder differs sharply from the X-ray diffraction patterns of the starting products (ash and lime).
Table 1
Indices of X-ray diffraction patterns
| Line No. | Ash intensity | Ash $d$ | Lime intensity | Lime $d$ | Silicate + lime (mech. mixture) intensity | Silicate + lime (mech. mixture) $d$ | Ash + lime (mech. mixture) intensity | Ash + lime (mech. mixture) $d$ |
|---|---|---|---|---|---|---|---|---|
| 1 | weak | 4.32 | strong | 4.91 | weak | 3.43 | weak | 3.60 |
| 2 | very strong | 3.35 | medium | 3.10 | medium | 3.15 | very strong | 3.34 |
| 3 | medium | 2.28 | medium | 2.89 | medium | 2.91 | medium | 3.10 |
| 4 | medium | 2.21 | very strong | 2.51 | very strong | 2.62 | medium | 2.90 |
| 5 | medium | 2.12 | very weak | 2.40 | very weak | 2.60 | very strong | 2.62 |
| 6 | strong | 1.81 | very weak | 2.11 | weak | 2.19 | weak | 2.19 |
| 7 | strong | 1.76 | weak | 1.97 | weak | 2.13 | weak | 2.13 |
| 8 | strong | 1.54 | strong | 1.91 | very strong | 1.93 | medium | 2.10 |
| 9 | strong | 1.37 | strong | 1.79 | strong | 1.87 | very strong | 1.90 |
| 10 | medium | 1.69 | medium | 1.85 | strong | 1.86 | ||
| 11 | weak | 1.55 | medium | 1.81 | weak | 1.82 | ||
| 12 | weak | 1.50 | strong | 1.69 | weak | 1.80 | ||
| 13 | weak | 1.44 | weak | 1.60 | strong | 1.68 | ||
| 14 | very weak | 1.60 | ||||||
| 15 | strong | 1.54 | ||||||
| 16 | strong | 1.35 |
Table 2
Indices of X-ray diffraction patterns
| Line No. | Hardened mixture of silicate and lime — intensity | Hardened mixture of silicate and lime — $d$ | Hardened binder — intensity | Hardened binder — $d$ |
|---|---|---|---|---|
| 1 | very weak | 3.14 | medium | 4.58 |
| 2 | weak | 3.02 | medium | 4.30 |
| 3 | very strong | 2.81 | very strong | 4.16 |
| 4 | medium | 2.62 | weak | 3.73 |
| 5 | very strong | 2.41 | weak | 3.54 |
| 6 | medium | 2.28 | strong | 3.22 |
| 7 | strong | 2.19 | weak | 2.89 |
| 8 | weak (medium) | 2.06 | weak | 2.77 |
| 9 | weak (medium) | 1.97 | strong | 2.52 |
| 10 | very weak | 1.84 | weak | 2.45 |
| 11 | very weak | 1.81 | very weak | 2.26 |
| 12 | very weak | 1.78 | weak | 2.13 |
| 13 | very weak | 1.73 | medium | 2.05 |
| 14 | strong | 1.69 | weak | 1.83 |
| 15 | weak | 1.63 | weak | 1.78 |
| 16 | weak | 1.59 | strong | 1.63 |
| 17 | weak | 1.51 | weak | 1.59 |
| 18 | weak | 1.45 | strong | 1.45 |
| 19 | medium | 1.43 | strong | 1.43 |
On the radiograph there are a number of lines of low intensity; there are considerably more such lines than on the radiograph of the hardened mixture of silicagel and lime. This indicates an even greater disordering of the structure of the newly formed chemical compounds, since the fly-ash silica is less chemically active than silicagel, and a hardening period of 4 months is insufficient for the crystallization of the products of chemical reactions.
Fig. 2. Radiographs of materials in graphical representation.
Labels in the figure: lime; lime + silicagel, hardening for 4 months; lime + ash, hardening for 4 months; ash; Ca(OH)\(_2\). Axis: \(d\).
Fig. 3. Radiographs of materials in graphical representation.
Labels in the figure: lime + silicagel (mechanical mixture); lime + silicagel, hardening for 4 months (calcined to 900°); lime + ash (mechanical mixture); lime + ash, hardening for 4 months (calcined to 900°). Axis: \(d\).
On the radiograph, clearly expressed lines of high intensity are observed, common with the radiograph of the hardened mixture of silicagel and lime (Table 2, Figs. 1, 2).
The latter circumstance makes it possible to conclude that the hardening products contain the same hydrated formations, crystallized by the indicated hardening time.
Heating of the hardened mixtures promotes crystallization of the new formations. The radiographs of the hardened mixtures calcined to 900° have the appearance of radiographs of substances with a more ordered crystalline structure (Fig. 3).
Results of the Investigation in the Electron Microscope
The hardened mixture of silicagel and lime after 4 months of storage in a moist environment is characterized by the presence of polymineral intergrowths, among—
a
б
в
Fig. 4. Micrographs of the hardened mixture of siliceous limestone, lime, and water. Magnification 6000×.
a
б
в
Fig. 5. Micrographs of hardened lime-ash binder. Magnification 6000×.
in which dicalcium hydrosilicate can be found, predominantly in the form of hexagonal plates of various thicknesses (Fig. 4, a, b, c).
The edges of most crystals are not sufficiently sharply defined, owing to its incomplete crystallization during 4 months of hardening.
Crystals of calcium hydroxide were not found in the specimen, although the endothermic effect of decomposition of \(Ca(OH)_2\) is present on the thermogram.
In microphotographs of the hardened lime–ash binder, chains of rounded crystals of \(Ca(OH)_2\) are readily observed (Fig. 5, a), as well as multifaceted plates similar in shape to crystals of dicalcium hydrosilicate photographed in the electron microscope by V. F. Zhuravlev [2] in his study of the hydration processes of clinker minerals (Fig. 5, b), and also elongated multifaceted plates, similar in shape to calcium carbonate crystals (Fig. 5, c).
In addition to the crystalline forms named above, in the microphotographs of the hardened lime–ash binder one can readily discern areas of indeterminate form, less dense than the aforementioned crystals and therefore lighter. They are located along the contours of the crystallized compounds and also occur as clusters of grayish color in Fig. 5, a, b, c.
Thus, the hardening of the lime–ash binder is caused by the chemical interaction of ash and lime with the participation of water, as a result of which hydraulically stable calcium hydrosilicate is formed, along with crystallization of free calcium hydroxide and its partial carbonation.
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