Binder from Solid Fuel Ash and Processes During Its Hardening

N. S. Dubovskaya, P. G. Usov

(Presented by Prof. A. G. Stromberg, Doctor of Chemical Sciences)

By its nature, solid-fuel ash is a complex silicate system. A characteristic feature of the ash under investigation is the predominance in it of a glassy component in the form of hollow spherical formations ranging in size from 15–20 microns to 0.4 mm, with a wall thickness of up to 5 microns. The refractive index of the fragments lies within the limits 1.498–1.540, which makes it possible to establish the siliceous character of the glassy component. The chemical composition of the ash under investigation is given in Table 1.

On the basis of its chemical composition, the ash should be regarded as a hydraulic additive [1, 2, 3] with low hydraulic activity.

In order to increase the activity of the ash, the method of dispersion and chemical activation was investigated by simultaneous grinding in a ball mill of the ash and the activating additive.

Grinding the ash promotes exposure of the surfaces of particles having a predominantly siliceous character, while the introduced additive awakens the binding properties of these particles. Taken together, this makes the ash chemically active.

The production of an ash binder is based on this activation of the ash.

As a quantitative characteristic of the degree of ash activation, the mechanical compressive strength was adopted for specimens made with ash binder and sand in a ratio of 1:3 by pressing [4].

The degree of grinding of the ash binder is characterized by the value of its specific surface area.

On the basis of the investigations carried out, the composition of the ash binder contains 70 percent ash and 30 percent unslaked lime.

In the course of the work, the influence of the fineness of grinding of the binder on its activity was studied. The results of the investigation are given in Table 2.

Chemical composition of the ash under investigation

Table 1

Table 2

Effect of storage time on the activity of the ash binder

The binder obtained is sufficiently air- and water-resistant. This is evident from the continuously increasing compressive-strength limit under conditions of both air and water media.

The processes occurring during the hardening of binders of this type have been investigated insufficiently. In the main, the process is reduced to the interaction between silica and calcium hydroxide, as well as alumina and calcium hydroxide [5, 6].

The hardening products of the ash binder were studied by means of chemical and thermal analyses.

To investigate the hardening products of the ash binder, specimens were prepared from paste and stored in an air-humid environment free of carbon dioxide. In the hardening products, at various ages, uncombined calcium hydroxide and chemically bound water were determined. The results of the determinations are given in Table 3.

The data of Table 3 show that during one year of hardening there is a decrease in the amount of calcium hydroxide and an increase in the content of chemically bound water in the hardening products of the ash material. Ta-

Table 3

This phenomenon makes it possible to conclude that calcium hydroxide, during hardening, enters into chemical interaction with the constituent parts of the ash, binding water in addition to that which was bound by the lime during hydration at the moment the binder was mixed with water.

At the same time, newly formed products are produced that impart hydraulic properties to the hardened binder [3, 7].

The hardening products of the ash binder were subjected to thermal analysis. Thermograms of the hardening binder were recorded on an apparatus assembled at the Tomsk Polytechnic Institute in the Department of Silicate Technology.

The standard for the analysis was the investigated material calcined to 1100°. The heating rate during the analysis was 15–20° per minute.

In analyzing the thermal curves of the binder hardening over different periods of time, several endothermic effects can be observed on the curves, depending on the duration of hardening.

For early and late hardening periods, a low-temperature effect in the interval 100–200° is characteristic. A higher-temperature endothermic effect occurs in the interval 500–600°. For the later hardening periods of the binder, in addition to the effects named, endothermic effects in the intervals 700–800° and 900–1000° are characteristic.

The low-temperature endothermic effect (100–200°) on the thermograms of specimens with a long hardening period is more pronounced than on thermograms with a short hardening period.

The differing character of the low-temperature effect and the appearance over time of new effects make it possible to conclude that, during hardening of the ash binder, chemical and physical processes take place that lead to changes in the composition of the hardening material and in the structure of the hardening products over time.

The temperature effects on the curves may be interpreted as follows.

The first endothermic arrest (100–200°) is characteristic of the process of loss of free water, including the water of gel-like products of binder hardening.

The second endothermic arrest at 500–600° arises as a result of the decomposition of calcium hydroxide. The indicated effect on the heating curve of a specimen that had hardened for 9 months has a more clearly expressed character; this, according to P. I. Bazhenov [8], indicates the crystalline structure of the newly formed product by the indicated hardening time.

The third endothermic arrest at 700–800° corresponds to the decomposition of hydrated newly formed products obtained through the interaction of calcium hydroxide with the constituent parts of the ash (either with alumina or with silica).

The presence of these newly formed products imparts hydraulic properties to the hardening products.

The fourth endothermic arrest at 900–1000° is characteristic of the process of decomposition of calcium carbonate, formed through carbonation of calcium hydroxide that had not entered into interaction with the ash during hardening.

Thus, according to the results of our investigation, hardening of the binder is due to the course of chemical and physical processes in the system lime—ash—water.

Chemical processes: 1) interaction of lime with water with the formation of gel-like products; 2) interaction of calcium hydroxide with the constituent parts of the ash with the formation of hydrated newly formed products that impart hydraulic properties to the hardened binder.

... [9], 3) carbonation of calcium hydroxide that has not entered into the reaction. Part of the calcium hydroxide remains in a free crystalline state.

Physical transformations: 1) according to the theory of hardening of hydration binders and new ideas about the mechanism of hardening of quicklime, the first physical transformation will be the dissolution of calcium hydroxide until a supersaturated solution is formed [9]; 2) then the formation of a colloidal system in the form of a gel takes place, i.e., “colloidization” according to A. A. Baikov, or “chemical and adsorption dispersion” according to P. A. Rebinder. Centers of crystallization of calcium hydroxide arise. 3) spontaneous recrystallization of part of the colloidal fractions occurs because of the thermodynamic instability of colloid-dispersed systems.

The products of the interaction of calcium hydroxide with the constituent parts of the ash have a colloidal structure which, striving to reduce its reserve of free energy, in accordance with the second law of thermodynamics, passes partly into crystalline form.

Thus, the structure of the hardened binder proves to be composed of the crystalline phases of calcium hydroxide, the products of interaction of calcium hydroxide with ash, and calcium carbonate.

In order to study the influence of heat treatment on the hardening process of a binder, wall blocks made with an ash binder were manufactured under plant conditions. Steaming of the articles in chambers at 90–100° according to the regime: 3 hours temperature rise, 12 hours holding at 90–100°, 4 hours cooling—produces a noticeable effect in accelerating the hardening processes. And articles made with the binder under study considerably surpass, in mechanical strength, similar articles made with ordinary cement.

The use of ash binder on an industrial scale gives a significant saving in binder expenditures.

LITERATURE

1. Lapin V. V., Elizon I. P. Construction Industry, No. 6. Promstroiizdat, 1953.

2. Surovtsev V. V. Ashes and Slags as Raw Material for the Production of Building Materials. Promstroiizdat, 1953.

3. Budnikov P. P. On reactions between kaolin and calcium oxide hydrate under hydrothermal treatment. Proceedings of the Conference on Cement Chemistry. Promstroiizdat, 1953.

4. Butt Yu. M. Practical Course in the Technology of Binding Substances. Promstroiizdat, 1955.

5. Zhuravlev V. F. Chemistry of Binding Substances. Goskhimizdat, 1951.

6. Baikov A. A. Collected Works. Publishing House of the Academy of Sciences of the USSR, 1943.

7. Krasilnikov K. G. Chemical processes in disperse bodies. Proceedings of the Conference on Cement Chemistry. Promstroiizdat, 1957.

8. Bazhenov P. I., Suvorova G. F. Cement, No. 1. Promstroiizdat, 1957.

9. Osin B. V. Quicklime. Promstroiizdat, 1954.