Лаборатория геологии техногенных процессов
Мaximovich N.G., Blinov S.M. The use of geochemical methods for neutralization of surroundings aggressive to underground structures // Proceeding 7 Int. Congress Ass. of Engineering Geology.-V.5.-Portugal, Lisboa,1994.-P.3159-3164. /0,4/
THE USE OF GEOCHEMICAL METHODS FOR NEUTRALIZATION OF SURROUNDINGS AGGRESSIVE TO UNDERGROUND STRUCTURES
L'UTILISATION DES METHODES GEOCHIMIQUES POUR LA NEUTRALISATION DES ENVIRONNEMENTS AGRESSIFS POUR LES STRUCTURES SOUTERRAINES
N.G. Maximovich & S. M. Blinov
ABSTRACT: Methods are described which are based on the enhancement of chemical resistance of structures and used to combat corrosive media. It is proposed to neutralize such media with various agents. The results of laboratory and experimental works are described which were carried out on the site of an enterprise in the West Urals with the purpose to reduce sulfate-induced corrosion of underground structures when mine waste materials from Kizel Coal Deposit are used for construction. Sulfates were precipitated with barium compounds. The results show that the method proposed by the authors and the new approach based on the geochemical effect on corrosive media are promising.
RESUME: Etude des methodes actuelles de la protection contre les milieux agressifs basees en general sur 1'accroissement de la stabilite chimique des constructions. De differents agents sont proposes pour neutraliser, de tels milieux. Il y a les resultats d'etudes de laboratoire et de travaux experimentaux realises dans l'aire d'operation d'une entreprise d'Oural Occidental afin de diminuer 1'agressivite de sulfate des eaux souterrains ayant lieu dans le cas de 1'utilisation dans les travaux de construction des roches tres sulfureux de terrils du bassin houiller de Kizel. Les sulfates sont deposes/ avec les composes de baryum. Les travaux effectues ont montre 1'avantage de la methode proposee et de la nouvelle approche basee sur 1'action geochimique sur les milieux agressifs.
1 INTRODUCTION
Underground engineering structures are always to some extent subjected to chemical and physico-chemical attacks of the environment. Agressive underground water or soils cause a reduction in the strength properties or even failure of structures. In the practice of constuction the most frequent are the attacks of sulphate and acid corrosive media on concrete structures.
Corrosive media may be both of natural and technogenic origin, the latter being a result of production activities (spillage, leakage, artificial soils) (Maximovich, Gorbunova 1990). Technogenic corrosive media are formed as a result of changes in the hydrodynamic and geochemical parameters of natural media, for example, in the cases of saline soil underflooding, pyrite oxidation, etc. (Rethati 1981; Hawkins, Pinches 1989). Microbiological processes can also be of a certain effect (Boch 1984). The protection of underground structures is mainly restricted to the enhancement of their chemical resistance by some methods: addition of special ingredients to concrete, enhancement of its density, application of water proofing compounds on concrete surfaces. These methods are described in Russian normative documents. Instances are also known when the corrosive soil was replaced or the level of underground water was lowered. All the above methods increase the operating cost of construction considerably. Most of the methods can not be used under the conditions of plants in operation.
The experience of combatting corrosive media shows that it is necessary to find new approaches to this problem with due regard for their economic and technological expediency. One of such methods is the method of geochemical effect on corrosive media.
2 SITE ENGINEERING AND GEOLOGICAL CONDITIONS
Such an approach has been used by the authors on one of the sites of the Industrial Association METANOL in Gubakha (Perm Region, Russia). The site is located on a slope of the river Kosaya (the Kama River basin) in the West zone of folding of the Urals. There the aleurolite and coaly-clay shale of Carbon are overlaied with eluvial clay containing gruss and quartz aleurolite debris 0.5-17.0 m thick. The clay is overlaid with deluvial clay 2.0-16.0 m thick containing inclusions of quartz sand and aleurolite debris and gruss.
During grading and the formatiion of embankment in addition to the soil moved within the site rocks from coal mine dumps of Kizel Coal deposit were used. The soils from the dumps contained high concentrations of sulfur in various kinds of compounds, the concentrations reaching 8.7 wt %. A considerable amount of the sulfur was contained in water-solube compounds. The results of aqueous extract analysis showed that the content of sulfates could be as high as hundreds of grams per kilogram of soil. On the earth surface the rocks were weathered which resulted in a pH decrease in the water contacting with them to 1.0-3.0.
As a result of underflooding in the fill-up soils concrete-corrosive water emerged at elevations above the levels of foundation lower surfaces. The observations conducted starting from 1984 show that there is a tendency of sulfate corrosiveness increase of the water. In some zones the content of sulfates has increased to 4.1 g/1 and this is above the admissivible standard value. The following composition is characteristic of the underground water (in g/1): 0.22 HC1, 2.57 SO4, 0.115 Cl, 0.414 Ca, 0.04 Mg, 0.80 Na+K. The value of pH of the water is 6.6 to 7.9. The composition of the water is formed as a result of its interaction with mine dump waste rocks which is confirmed by the results of laboratory analysis. The relatively high values of pH are caused by acidity neutralization in the process of interaction with clay soil and inclusions of carbonate debris. Stripping of the foundations and execution of corrosion proofing work under the conditions when the continuous cycle manufacturing complex is in operation are practically impossible. The lowering of water level and soil replacement under the existing conditions are unacceptable or too expensive.
3 KNOWN METHODS OF MEDIUM CORROSIVENESS REDUCTION
The experience gained in the reduction of sulfate and acid corrosiveness of the environment is relatively small. Studies are known which have been made on the possibility to use the ash resulting from coal burning as an additive to soils to neutralize their acidity and to precipitate deleterious components. Positive results have been obtained from the use of alkaline additives such as lime, limestone and trona (Sandereggen. Donovan 1984). Some investigators propose various additives to be used if there are sulfides in soils which additives are to suppress their microbiological oxidation in the cases of sulfate corrosiveness (Evangelon et al. 1985). However, these processes are difficult to control.
4 CHEMICAL GROUNDS FOR THE METHOD PROPOSED
For the precipitation of sulfates the authors have offered to use soluble barium compounds.
SO4 + Ba = BaSO4 (barite)
This reaction is practically instantaneous and does not depend on medium pH. It is expedient to use barium hydroxide and barium chloride as the reagents. Barium chloride is highly soluble in water, so concentrated solutions can be used. The solubility of barium hydroxide is one order of magnitude lower than that of barium chloride but its use neutralizes the acid reaction of the medium and no extra components arc to be added to underground water.
Ba(OH)2 + H2SO4 =BaSO4 + 2H2O
The resulting barite is relatively stable under exogenic conditions and practically does not decompose under weathering. It is not toxic, is used in drilling muds and may be used as a concrete filler and also in medicine fox X-raying of the digestive tract.
5 NATURAL GEOCHEMICAL ANALOGUES FOR THE METHOD
In nature the processes of barite formation are rather widespread. The understanding of these processes is important in the development of methods aimed to combat corrosive sulfate water. There are several processes of barite formation: mixing of waters carrying separately barium ions and SO4 ions; reaction of solutions containing barium ions with sulfate rocks: reaction of solution containing sulfate ions with barium-containing rocks; oxidation of solutions containing Ba and S ions.
In underground waters occuring at small depths barium ions are rare for they contain one or other quantity of sulfates.
The major barite deposits were formed under hydrothermal conditions. If the zone of hypergenesis barite occurs in the form of nodules in clay and sand deposits in the coastal areas of seas. Barite modules can also occur in the oozes of recent sediments. Barite is formed as a result of chemical erosion of rocks. Sulfuric acid which results from sulfide oxidation reacts with barium ions and forms barite. With the other minerals evacuated. the so-called barite sypuchkas are formed replacing the ore bodies.
6 RESULTS OF LABORATORY AND FULL-SCALE INVESTIGATIONS
Three kinds of soil with different sulfur contents taken on the site were studied in laboratory with the purpose to determine the possibility of use of barium compounds for the neutralization of corrosive media and to define their optimal concentrations. Through the soils placed in a special devices barium chloride and barium hydroxide solutions of various concentrations and also destilled water were filtered. The filtered solutions were analized for the content of sulfate ions and pH. A total of 29 series of experiments were performed. It was found that when distilled water was flittered through a soil, from 19 % to 62 % of the total sulfur content passed into solution. On the basis of the studies the amount of barium salts required to precipitate sulfate ions was determined. It was found that the processing of soil may precipitate up to 97 % of mobile sulfur. The experiments were carried out on two parts of the site. On the first part where the underground water was of medium
sulfate corrosiveness two holes with a diameter of 60 mm were drilled (Figure la) in which 30 kg of barium chloride (BaCl2 x 8H2O) and 10 1 of demineralized water were pourred. In the well located down the stream of underground water the composition of water was analized. A day after the beginning of the experiment the content of sulfates in the well under observation decreased to zero. A sulfate corrosiveness two holes with a short-time increase in the content of chlorine ions was observed and barium ion appeared. 14 days after there were no Ba ions found in the water and the content of chlorine ions and mineralization were close to the their original values and the content of sulfates was 0.55 g/1. At a later time the content of sulfates and miniralization decreased regularly (Figure lb). By the end of the fourth month of observation the conccentration of sulfate ions was 0.18 g/1, i.e. the water became non-corrosive relative to concrete and its sulfate-calcium composition changed to the chloride-calcium one. During the whole period of observation the content of chlorides was far less than it is required for salt corrosiveness.
On the second experimental part of the site there were two trenches to bury the reagent and four observation holes down the stream of underground water (Figure 2a). The results of analyses of water extracts showed that the content of sulfates in the soils varied within the range of 1.05-7.43 g/kg. The soils in accordance with Russian standards were classified as corrosive. The content of sulfates in the underground water of that site part before the experiments had been 1.09-1.52 g/1 and mineralization had ranged from 2.81 to 3.422 g/1.
The experiments on the decrease of sulfates corrosiveness were performed in several stages. At the first stage 45 kg of barium chloride (BaCl2x8H20) were buried in a trench. The result of chemical analyses showed that in all the observation holes there was a tendency for a decrease in the content of sulfate ions and by the end of the fourth month of observations their concentration was not higher than 0.36 g/1. The content of chlorides varied from 0.01 to 1.08 g/1. The mineralization decreased to 1.50-2.48 g/1 (Figure 2b). The concentration of sulfates varied sharply due to their transport from the zone of aeration by atmospheric precipitation. There was observed a direct relationship between the amount of atmospheric precipitation and the content of SO4, but during the whole
period of observations their amount tended to decrease. At the second stage barium hydroxide (Ba(OH)2 x 8H2O) was used to neutralize the corrosiveness of the medium. A year and a half after the beginning of the experiment 60 kg of the reagent were buried in trench 2. The average content of sulfates decreased to 0.04 g/1. A decrease in water miniralisation was observed. At the final stage of observations it was from 0.39 to 1.40 g/1 (Figure 2b).
The experiments on the sites part showed that the underground water which had possessed medium and strong corrosiveness became non-corrosive relative to concrete The content of chlorides and pH were normal during the whole period of observations. As a result of reagent introduction the geochemical activity of the soils changed considerably. Analyses of water extracts showed that the content of soluble salts in the soil of site part 2 decreased by a factor of 2.5 and by the end of observations it was 2.69 g/kg; the contents of sulfides decreased by a factor of 3 and was 1.30 g/kg. The content of soluble salts at a distance of 1 m from the trench with the reagent was not more than 0.07 g/kg and in the water extracts hydrocarbonate and calcium ions were prevailing. To achieve a positive effect 29 kg of barium chloride or 22 kg of barium chloride or 22 kg of barium hydroxide were required per a cubic meter of soil. Those values were close to the results of calculations on the basis of laboratory analyses.
7 SIDE EFFECTS
The treatment of soils with soluble barium compounds caused changes in the mineral composition and properties. A yellow sediment was found at the bottom of the trench filled with barium hydroxide. Roentgenometric analyses showed that the sediment contains 24 % of barite, 15 % of calcite, 30 % of witherite, 30 % of quartz gypsum. The soil in the walls and in the lower part of the trench was cemented and difficult for cracking with a pinch.
To determine the composition of the precipitate separating from the underground water due to the reaction with barium salts, from the hole located near the site part 1 the water was sampled which had the following composition (in g/1): 0.23 HCO3, 4.08 SO4, 0.07 Cl, 0.37 Ca, 0.04 Mg, 1.51 Na+K, 0.02 Fe*** with the total mineral content being 6.31 at pH 5.65. An excess of barium hydroxide or barium chloride was added to the water. The resulting precipitate was collected and rhoentgenographed. In the case of barium chloride 99 % of the precipitate was barite and in the case of barium hydrochloride the precipitate consisted of barite (72 %) and witherite.
It is known that if a solution contains sulfate ions, there occurs an exchange reaction for the solubility of barite is much lower than that of witherite (Bisehberg, Plummer 1986):
BaCO3 + SO4 = BaSO4 + CO3.
This should be considered as a positive factor for if sulfate ions penetrate into the soil under treatment they will settle out in accordance with the above reaction. The formation of barite and witherite and the reaction of the alkaline component with the soil results in a considerable increase of its strength. The filling of pores decreases the water permeability of the soil. These side effects should be considered as positive for the decrese in soil permeability to water and in the intensity of water exchange diminishes the effect of corrosive water on concrete structures and the increase of soil strength enhances the reliability of structure foundations. The use of barium chloride and barium hydroxide does not cause negative changes in the composition of underground water. In holes adjacent to the source barium ions are found only during the first moments after the start of the treatment. In the case of barium chloridee the concentration of chlorine ions increases only at the initial stage, but its content is always lower than the admissible level. No increase in pH was observed when barium hydroxide was used. Barium ions are not corrosive for concrete.
8 PROSPECTS FOR THE METHOD
Dependind on the actual geological conditions and the features of structures various ways are possible for the realization of the new method. If underground water occurs at the small depths trenches may be used to introduce the reagents into the soil, the trenches being upstream of the structure to be protected. If underground water occurs at the great depths, the reagents can be injected. If there is a possibility that corrosive media can appear, the reagents may be introduced into the soil in the course of construction. Thus, the experiments performed have shown that to combat media corrosive to structures on the basis of non-traditional approaches now under developments, i.e. by acting on corrosive by geochemical methods, is an effective way to protect the structures.
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