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Acidic attack of cement based materials under the common action of high, ambient temperature and pressure
Construction and Building Materials 36 (2012) 623–629
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Acidic attack of cement based materials under the common action of high, ambient temperature and pressure Vladimir Zivica ⇑, Martin T. Palou 1, Martin Krizma 2, Lubomir Bagel 3 Institute of Construction and Architecture SAS, Slovakia
h i g h l i g h t s " Acidic geothermal waters and alkalic cementing material are conditions for acidic deterioration. " Water aggressivity factors are species present, concentration, temperature and pressure in well. " Cementing material resistance factors are cement, w/c ratio, processing and curing regime used. " All factors make should be considered at the design of the composition of the cementing mixtures. " Exacting character is increased by the necessity of use negative high w/c ratios for properties.
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Article history: Received 9 November 2011 Received in revised form 28 March 2012 Accepted 25 April 2012 Available online 15 July 2012 Keywords: Acidic attack Cement Bore hole Temperature Pressure Aggressivity
a b s t r a c t The cement materials in geothermal bore holes used to be exposed to the acidic attack under the common action of ambient high, temperatures and pressure. As it is known the waters in the deep geothermal wells are usually very aggressive to the applied cementing materials. Acidic aggressivity used to be a dominating type. Therefore, in the interest of the ensuring of life service of the cementing material it should be high acidic resistant. But it is a crucial problem due to the fact that practically all-protective measurements like e.g. the choice of the cement, increased density of material and others are weak and insufficient. One presumption of the ensuring of acidic resistance of cementing material is the correct evaluation of cooperating factors conditioning its acidic resistance and the factors of the aggressivity of environment. Both are of complex character depending on several conditioning factors. This complexness, as a rule, is not taken into consideration. However, such an evaluation current in the practice is entirely unsatisfied and incorrect. Therefore, further factors should be taken into consideration. These are the subject of the presented paper showing also the complexness of the point at issue. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction An important engineering property of cement-based materials is their durability. It determines the service life of concrete structures very significantly. Due to the interactions of cementitious material with external environments the durability may be threatened and lost. One important and significant threat is acidic attack. In this respect, geothermal waters can represent an effective source of the aggressivity. Geothermal waters exhibit different chemical composition. This composition is usually close to the chemical composition of surrounding rocks. ⇑ Corresponding author. Tel.: +421 59 30 9257; fax: +421 54 77 35 48. E-mail addresses: [email protected] (V. Zivica), [email protected] (M.T. Palou), [email protected] (M. Krizma), [email protected] (L. Bagel). 1 Tel.: +421 59 30 9260; fax: +421 54 77 35 48. 2 Tel.: +421 59 30 9228; fax: +421 54 77 35 48. 3 Tel.: +421 59 30 9256; fax: +421 54 77 35 48. 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.025
Geothermal waters typically contain 1–5 wt.% of soluble salts and in the particular case up to 30% [1–3]. The presence of these components is the result of water rock interaction. Diluted components are mainly sodium chloride, sulfates and bicarbonates and the solutions are always saturated with respect to silica. The last one tends to precipitate at lower temperatures. However, from the chemical point of view, the presence of silica in water environment may not play disastrous role. But, the chemically fatal effect is linked often to the very aggressive CO2 present [4–7]. The principal species C1 , affecting cement stability are SO24 , CO23 and pH. Indeed, pure water can decompose hardened cement compounds by leaching out Portlandite Ca(OH)2 and hydrated phases. The dissolution of Portlandite increases the permeability and decreases the strength of the cement composite. The source of acid media can even represent air pollution in the geothermal wells by gaseous carbon dioxide, sulfur dioxide and nitrogen oxides [8,9]. Acidic attack may also occur as a consequence of bacterial activity. This is particularly so when ground
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water contains ferrous iron or reduced sulfur compounds. The bacterial activity is able to produce acidic products like hydrogensulfide. Anaerobic bacterial action can break down proteins and hydrocarbons present in clays to release hydrogensulfide and methane. Furthermore, it can reduce sulfate within soils to form iron sulfide, which in turn react to form hydrogensulfide. The hydrogensulfide produced is oxidized by anaerobic bacterial action under the formation of sulfuric acid. Natural soft waters containing aggressive CO2 may be very aggressive against concrete. CO2 is present in greater or smaller quantities in the predominant majority of natural waters. The sources of CO2 in waters are biochemical processes in water or in contacted soils. Often the source of CO2 is the decaying of plant residues. The CO2 may be also a result of the decomposition of carbonate soils by the ground waters. Aggressive CO2, which is dangerous for concrete, is the amount of free carbon dioxide present in water that is in excess of the quantity required for the stabilization of the calcium hydrogen carbonate equilibrium. Aggressive CO2 reacts in a cement matrix with Ca(OH)2. Further more, the low soluble calcium carbonate formed is transformed into highly soluble calcium hydrogen carbonate Ca(HCO3)2 which is gradually leached [10,11]. Based on this process, the equilibrium between Ca(OH)2 and the hydration products is destroyed, resulting in a gradual hydrolytic decomposition of the hydration products destroying the integrity of the material. 2. Principle of acidic attack 2.1. Chemical effects The acidic attack of cement-based materials is based on their alkalinity. Therefore, it is an ever-willing partner of the acidic-alkaline reaction. At first, calcium hydroxide is attacked connected with the gradual hydrolytic decomposition of the hydration products, mainly after the consumption of calcium hydroxide. In this case, limestone aggregate is attacked too. These changes are characteristic: calcium oxide and water bound content decrease, and insoluble residue content increase. Under very severe conditions; the process of the attack may result in the total decomposition of the cement matrix with silica, alumina and ferric hydrogels as final reaction products. 2.2. Degradation processes A practical consequence of the chemical effects of acidic attack is a gradual degradation of engineering properties of cementing materials. At first, there is a deterioration of the surface of concrete showing crushing and dropping of surface material (Fig. 1). With the progress of acidic attack, a gradual degradation of strengths of concrete occurs.
Fig. 1. View of mortar test specimens after 180 days action of CO2 water [63].
Fig. 2. Mechanical properties decrease of mortar due to the action of aggressive carbon dioxide water [64].
It is a direct consequence of the decomposition of the hydration products and the leaching away of the products of this decomposition. The degradation of mechanical properties of mortar due to acidic attack is shown in Fig. 2. 3. Deterioration reactions rate factors Some reactions are naturally faster than others. The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can influence greatly the rate of a reaction. 3.1. Solution aggressiveness 3.1.1. Acid kind First of all, the chemical nature of the anions present is important. The ionic strength and the solubility of the calcium salt formed in the process play significant roles [12]. The dissociation degree of the acid is a further important factor. However, pH values give no correct information about the real quantity of the acid in the solution, because the pH value is dependent on dissociation degree of acid. Therefore, a strong acid having a high degree of dissociation (e.g. hydrochloric acid, nitric acid) may achieve very low values of pH, due to relatively small quantities of acid in the solution. The weak acids owing to their low dissociation degree, achieve higher pH values. It is evident that the concentration of the aggressive species in the acting solution is a more reliable parameter than the pH value [13]. The severinity of the acidic attack is significantly dependent on the solubility of the calcium salt formed. In the case of the formation of highly soluble salts, the severinity of the attack is very high. The possibility of the dissolving and leaching of the formed salt from the attacked material cause this. In the case of the formation of insoluble calcium salts such as calcium oxalate and fluoride, the effect of the acidic solution is entirely different. A dense insoluble layer is formed enhancing the development of the acidic attack showing a protective effect. This is used in practice for the protective purposes. Among the highly aggressive media are the solutions of hydrochloric and nitric acids because the solubility of their calcium salts with values of ca 46 and 56 wt.% is very high. The hydrolytic decomposition of cement hydration products is a conducting factor contributing to the severity of the attack. Presence of carbonate in some geothermal waters presents a serious problem for Portland cement systems. Calcium silicate hydrates become instable in such chemical environment, even at ordinary temperatures. After they have been exposed to carbonates
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solutions, calcium silicate hydrates convert finally into a mixture of calcium carbonate and amorphous silica. The phenomenon has been observed in well cements by numerous researchers. Geothermal waters usually contain magnesium and sodium sulfate. In such environment, calcium hydroxide present in hardened cement reacts with magnesium and sodium sulfate solution to produce calcium sulfate, magnesium and sodium hydroxide. Then, calcium sulfate reacts with C3A to produce ettringite or monosulfate which are larger in volume and cause the disintegration of hardened cement paste by expansion. Sulfate aggression occurs at temperature below 90 °C. 3.1.2. Solution concentrations The reaction rate increases with concentration, as described by the rate law and explained by collision theory. As reactant concentration increases, the frequency of collision increases. The significance of the concentration in the acidic attack is a consequence of the fact that the rate of the acidic attack, like that of other chemical reactions, is greatly dependent on the concentration of reaction participants. Therefore, the estimation of the concentration of the acting solution should be the first step at the evaluation of the solution aggressivity [14–17]. Generally, the increase in rate of acidic deterioration with an increase concentration in acidic solution is accepted. An example of the dependence of the rate of the acidic attack on the concentration of the acidic solution according to our results is shown in Fig. 3. It may be seen the increase in rate with the increase of the acidic component—carbon oxide—in the acting solution, but only to a certain limit. After that the increase of the rate is practically negligible. The chemistry of the reservoir fluids varies from fresh water to saline brines with greater than 200,000 mg/l total dissolved solids [18]. 3.1.3. Contact conditions A key is the quantity of the solution, which comes into contact with the attacked surface of the material over time unit. In principle two action modes are possible: (1) static conditions when no practical movement of the acting solution is present; (2) dynamic conditions, which occur with a change of the level or flow of the acting solution. In this case, the flow rate may have a greater influence than the concentration of the solution [14]. Moreover high velocities of the flowing ground water may disturb the attacked layer and contribute to the development of the attack. Dynamic conditions contribute to the transport of the aggressive species into the pore system of concrete and to the drainage out of the decomposition products [14,15]. Both effects contribute to the
Fig. 3. Dependence of decrease of compressive strength of mortar on carbon dioxide concentration in the acting water [65].
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intensification of the attack: the first affects the transport of aggressive partner of the attack, the second one contributes to the hydrolytic decomposition of cement matrix disturbing the chemical equilibrium in the system. A decrease in the level of the acting solution may be a further factor of the intensification of the attack. In this case, already attacked surfaces of concrete may be exposed to the effects of climatic factors, like sunshine, frost and wind contributing to the deterioration process. In some cases, the effect of drying the concrete surface may start the crystallization of the reaction products of the attack present in the pore liquid of the surface layer. Then crystallization may be a source of expansion tension of cracks propagation in the attacked part of the concrete surface. Alternate wetting and drying caused by repeated changes of the level in the acting solution may contribute to the severity of the processes [14]. Ambient temperature and relative humidity may influence the acidic attack, especially in the presence of the changes of the level of the acting solution [18]. Abrasion effects of deterioration may contribute to the intensification of acidic deterioration.
3.1.4. Ambient temperature rise Usually conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory. However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions. The influence of temperature is described by the Arrhenius equation. As a rule of thumb, reaction rates for many reactions double for every 10 °C increase in temperature, [2] though the effect of temperature may be very much larger or smaller than this. Cementing materials during the application procedure at bore hole and during the geothermal wells exploitation is exposed to different temperature – from atmospheric level to 350–400 °C depending on bore hole deep. This circumstance is of important significance as a factor of the control of rate of chemical reaction and a factor conditioning the phenomena of expansion or shrinkage of the solid. It is known that temperature level controls the chemical reaction rate. Moreover, it influences the kind and amount of the hydration products formed in those dependent properties of the hardened cement paste. A significant practical consequence of the temperature rise is setting and strength development acceleration. A significant consequence of elevated temperatures is the unfavorable change of calcium silicate gel C–S–H into a-dicalcium silicate hydrate, a-C2SH which begins to occur over 110 °C [19–21]. The product shows high porous structure with low strength. The unfavorable change of C–S–H gel into a-C2SH can be eliminated by the addition of silica-reach material in the quantity needed to reach CaO/SiO2 ratio value 1 or lower [22–24]. In addition, temperature rise can cause the thermal expansion of the cement-based materials under the producing the stress and crack propagation in the material. An average value for the coefficient of thermal expansion is about 10 millionths per degree Celsius. Several factors influence the behavior of concrete at high temperatures: moisture content, aggregate type and thermal stability, cement content, duration of exposure, the temperatures rise rate, age, and any restraint. Some authors reported that the effect of a number of tenCelsius rises on the rate of acidic attack of the cement-based materials is not significant [14,19]. But the effect of a number of hundred-Celsius rises is unclear. But when all known effects (reaction rate acceleration, calcium salts solubility increase, presence of critical solutions) are considered then temperature rise can be expected as a factor of the increase of acidic aggressivity. High relative humidity in bore hole is able to contribute to the fur-
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ther aggressivity increase due to the attack by air pollutants like CO2, SO2 and NOx [18]. 3.1.5. Environment pressure rise The rate of gaseous reactions increases with pressure, which is, in fact, equivalent to an increase in concentration of the gas. The pressure dependence of the rate constant for condensed-phase reactions (i.e., when reactants and products are solids or liquid) is usually sufficiently weak in the range of pressures normally encountered in industry that it is neglected in practice [25]. But, it is reported that effect of hydrostatic pressure up to 250 MPa on structurization kinetics and morphology of network polymers based on epoxy oligomers is significant [26]. High temperature in geothermal bore holes makes possible the presence of water vapor–air medium with related pressure, corresponding to the given bore hole deep and to the known dependence of water vapor pressure on ambient temperature (Table 1) [27]. A significant circumstance represents the water state change beginning at temperature 374 °C and pressure 22 MPa when the system vapor–liquid results in one phase – homogenous supercritical liquid. It shows the increased penetration ability into pore structure of the solids. Therefore, it can be expected an increased danger of the attack by the aggressive supercritical liquid [28–32]. A little information about influence of ambient pressure on hydration rate and cement hardening is mentioned in research literatures. It is reported that the pressure improves contact between water and cement particles and decreases the setting and increases the cement strength. Also it is reported that pressure does not have influence on type of created hydration products. They are identical with these formed under atmospheric pressure [33]. The importance of ambient pressure as hydration rate factor seems to be in comparison to temperature less important and ambiguous. But its co-operation is unconditional condition for the possibility of the hydration and hardening of cement at high temperatures. This dependence is based on the preventing of water evaporation by the ambient pressure. 3.1.6. Temperature and pressure common effect The values of high temperature and pressure represent hydrothermal conditions. Their positive accelerating effect on hydration and hardening of cement composites is utilized in praxis using technology process of concrete production so-called autoclaving. In accordance to this possibility, the hardened composite mixtures applied in bore holes are in principle similar to the cement composites made by autoclaving. This circumstance provides the possibility of knowledge utilization in field of concrete technology production using autoclaving for designing of cement composites for bore hole application. 3.1.7. Aggressiveness combination Geothermal brines can contain various chemical constituents able to evocate the combination of acidic and expansion deterioration. Expansion deterioration is a consequence of the reaction of
Table 1 Dependence of water vapor tension on ambient temperature [27]. Temperature (°C)
Pressure (MPa)
100 150 200 250 300 350 373
0.1013 0.4762 1.550 3.976 8.588 16.529 22.064
sulfate ions present in the acting solution and hydration products of C3A in cement under the formation of the bulky ettringite or monosulfate. The deterioration effect is showing by strength decrease, expansion and cracking of the attacked materials. Under the common presence of aggressive CO2 and sulfates in the solution its aggressivity can be increased under the appearance of the sulfate and acidic deterioration. The presence of chloride ions causes an increase in aggressivity of sulfate deterioration, mainly at the presence of Mg2+ ions [34–36]. 3.2. Factors of acidic resistance of cement based materials 3.2.1. Cement type The type of cement used has also an important bearing on the performance of concrete in an aggressive environment. It is known that acidic attack is always strong on any type of cement. It behaves more or less in the same manner, i.e. bad [37–39]. It seems that high-alumina cements may be resistant to acidic attack solutions with pH values between 4 and 5. The better performance of these cements compared to that of Portland cements could be attributed not only to the absence of calcium hydroxide but to the presence of more stable calcium-aluminate hydrates in the cement matrix and also to the presence of aluminum hydroxide. This may encapsulate the hydration products and protect them from acidic attack [24]. Blended Portland cements by pozzolans and slags are considered to be more resistant to the acidic attack than Portland cement alone. Some authors have claimed that the use of pozzolans Portland cements had only a weak positive effect on the increase of the acidic resistance. The data in literature are contradictory regarding whether cement-containing pozzolans are significant in improving the acidic resistance. However, it should be considered that the efficiency of blending for acid resistance may influence many factors e.g. the type of cement used for blending; the amount; the fineness of pozzolans used; and especially curing conditions [40,41]. However, it is well known that no type of cement has a satisfactory and long-term acidic resistance. This unfavorable situation consists of the fact that cement based materials are of alkaline nature. Therefore, they are ever a willing partner in the reaction with acidic species. 3.2.2. Cement content The effect of this factor is based on the fact that during the process of the acidic attack the quantity of the acid is consumed proportionally to the content of cement in the attacked materials, or more precisely to the content of the present hydration products. It has been reported that the rate of acidic attack decreases with an increase of cement content. For the main reason the neutralization capacity of the cement matrix is considered [42]. Nevertheless, the amount of cement used is important for the workability of the concrete mix, and to a certain extent, also for the curing sensitivity of fresh concrete. Both workability and curing sensitivity may result in influencing the quality of concrete, including its acidic resistance. Normally a cement content of 300–400 kg/m3 is sufficient to get a satisfactory low permeability and a sufficient acidic resistance of concrete if the w/c ratio is kept below 0.5, and adequate compaction and curing is provided. Naturally, the mentioned factors of the aggressivity should be taken into consideration. Cement content in concrete should not be so high as to induce cracking due to the drying shrinkage in the sections and due to thermal stresses. A minimum cement content provision is essential to produce a workable and cohesive concrete mix. Increasing the cement content with the attendant changes in the w/c ratio will be beneficial from an acidic resistance and strength viewpoint. For completeness, it should be mentioned that the behavior of the aggregates under the acidic attack is dependent on their solubility or insolubility in acids. Insoluble silica sand and river gravel
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usually do not take part in the deterioration process, but soluble dolomite and calcite decomposed in acidic solution may contribute to the determination process under the acidic attack of concrete. 3.2.3. Water–cement ratio The w/c ratio values of cementing mixtures are in the range of 0.6–0.8 and more. It is conditioned by the used cementing technologies, which need relatively thin composite mixtures. These values exceed the values of w/c 0.25–0.50 commonly used in technology of concrete and its related materials. It is known that the value of w/c ratio has the significant influence on cement hydration and hardening and subsequently on quality of engineering properties of hardened materials. It is also known that the quality of the engineering properties of concrete is increased with the w/c ratio decrease and the other way round [43–47]. It illustrates Fig. 4 showing the dependence of compressive strength and total porosity on the w/c ratio values. The necessary consequence of high w/c ratio used for composite mixtures assigned for bore hole cementing is that hardened material will exhibit lower values of strength, bulk volume and higher porosity and potentially lower resistance to aggressive influence in comparison to cement composites prepared with commonly used values of w/c. Since the mechanism of all the deterioration phenomena are permeability oriented, it is essential that the acidic resistance concrete should be sufficiently dense. This consideration is a direct consequence of the fact that for equals hydration, pore size distribution and permeability of the hardened cement paste is a direct function of w/c ratio. It is reported that the permeability is significantly reduced to a w/c ratio below 0.45. Therefore, in the interests of ensuring of an acid resistant concrete, it is necessary that the w/c ratio should be less than 0.45 and approximately 0.40, and at the same time ensuring a suitable workability of the concrete mixtures. The use of plasticizers and superplasticizers is useful in this respect [48–53]. 3.2.4. Curing conditions Curing conditions represent a further important processing factor. The quality of pore structure and permeability of concrete are significantly influenced. Even the crack propagation is dependent on the curing conditions. Due to insufficient curing, the surface layer of the concrete may be dried causing an increase in permeability and a decrease in acidic resistance. In this respect the wind and elevated ambient temperature may very negatively influence the quality of surface layer. Suitable curing conditions must begin
Fig. 5. Content of pores in mortar [54].
immediately after the concreting and they are not to be interrupted. When the hardening of concrete is interrupted once it may not continue, and latter applications of curing measures are usually useless. Special attention should be paid to curing in cases when higher values of w/c ratio and blended cements are used. Both increase the curing sensitivity of fresh concrete [37,38]. Especially the curing factors, mainly curing time is of special significance in the use of pozzolanic cements known as low hardening binders. These usually need a longer curing time as opposed to Portland cement [19]. According to the literature, a finely ground Portland cement clinker with the addition of an anion-active surface agent and an inorganic salt for the regulation of setting is acidic resistant [21,22]. The binding systems based on the modified silica fume show increased acidic resistance [23]. From viewpoint of curing the conditions in bore holes with lasting high humidity and temperatures seem to be favorable for the applicated cementing material hardening. 3.2.5. Pore structure The significance of pore structure quality is not surprising. It is well known that pore structure is a vital factor of engineering properties of cement-based materials. The significance of pore structure for acidic resistance of cement-based materials is conditioned by the fact that it is an important cooperating factor in transport phenomena [54–56]. Both the transport of aggressive species and the draining away of the products of the attack are realized through a pore system. Pore structure is an inseparable condition of the hydrated product formation of cement-based materials, and a primary key of their engineering properties. The formation of pore structure begins immediately after the addition of water into the mixture, and it progresses during the mixing and consolidation of the fresh mixture, and through further hardening of the material. As it is shown in Fig. 5 the acidic resistance of cement-based material is significantly dependent on pore size distribution and increases with micropore content [54]. 4. Protective measures
Fig. 4. Dependence of compressive strength of hardened cement paste on the values of total porosity and w/c ratio used [66].
The principle of acid attack—dissolution of the cement matrix and its general alkaline nature—often makes the protection of concrete structures difficult task. In principle, there are two possibilities how to protect the concrete against acidic attack: Group 1. Its possibilities are based on the fact that both the concrete and the structure built with it can be designed to afford desired protection. In many instances the protection can take the form of self-protection through judicious selection of concreting materials, and the appropriate design of concrete mix and the
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structure, without a need to resort to special protective measures [57,58]. The protective measures applied to the preparation of the concrete represent the most convenient way of ensuring the satisfied resistance of the concrete in aggressive media. These measurements are based on the choice of the cement used, application of the admixtures, design of the suitable composition of the concrete mixtures, effective conditions of the consolidation of concrete mixture and curing of the hardening of fresh concrete. All these protective measurements are aimed at gaining maximal compactness and minimal porosity of the concrete. As it has been already mentioned as a rule, current cements do not show many significant differences in their acidic resistance Hence attention should be paid to the protective measurements enabling the possibilities of the decrease of water permeability of concrete. In this respect, the application of modern superplasticizers is useful and effective. The superplasticizers used give the possibility of a significant decrease of the ways used, under the observing satisfied workability of the fresh concrete mixture. This last positive effect is very important for the high compactness and low porosity of concrete, and in the end for its low water permeability and acidic resistance. The mentioned measures may contribute to the acidic resistance of concrete. However, it should be stated that the concrete material remains of an alkaline nature willing to react with acids compounds and undergo acidic attack. Therefore, this property of concrete represents a durable threat for the concrete structures when exposed to the acidic attack. Group 2. Its possibilities are based on the protection of concrete structures by the application of various coatings on their surface. Surface coatings have a significant role to play in protecting and preserving new and existing concrete structures. This protective measurement has the unique advantage that it can be applied to protect existing concrete structures [59–62]. It is evident that protective measures of group I are for protection of cementing material of geothermal wells actual.
5. Conclusion Chemical aggressivity of geothermal waters represents a potential danger for the durability of the cementing materials in bore hole wells with the possibility of the treating of the service life of the construction of the wells. Due to the universal alkaline character of the cementing materials the danger of its acidic attack is ever real. Therefore, the protective measurements should be an indispensable task in the choice of the materials and composition of the mixtures for the cementing of geothermal wells. It has been shown that the rate of acidic attack of cement-based materials is dependent on numerous factors conditioning the aggressivity of the acting medium and resistance of the attacked material. The resulting rate of the attack is a consequence of their mutual interaction. Therefore, this complexity should be taken into consideration at the evaluation of the service life of concrete structures in acidic media, and the choice of protective measures. With respect to the acidic resistance, cement based materials were shown ever to be vulnerable solid. On the other hand, the occurrence of the acidic media in the geothermal environment is prevailing and therefore ever real. The fact of general alkalinity of the cement-based materials makes them an ever-willing partner of alkali – base reaction and the object of acidic attack. Therefore, the protection of the cementing materials in deep bore holes and ensuring of their durability represent a pretentious task. It seems that only binders of new generation with increased acidic resistance should be an effective a prospective way of the solution of the problem.
Acknowledgements This article has been produced with the financial assistance of the European Regional Development Fund (ERDF) under the Operational Programme Research and Development/Measure 4.2 Transfer of knowledge and technology from research and development into practice in the Bratislava region /Project Center for applied research of composite materials for deep geothermy/.
References [1] Robins NS, Milidowski AE. Bore hole cements and the downhole environment a review. Quart J Eng Geol Lond 1986;19:175–81. [2] Milestone NB, Sugama T, Kukacka LE, Cerciello NR. Carbonation of geothermal grouts – Part I: CO2 attack at 150 °C. Cem Concr 1986;86:941–50. [3] Sugama T, Carciello NR. Carbonation of hydrothermally treated phosphatebonded calcium aluminate cements. Cem Concr Res 1992;22:783–92. [4] Sugama T, Carciello NR. Carbonation of calcium phosphate cements after longterm exposure to Na2CO3 – laden water. Cem Concr Res 1993;23:1409–17. [5] Kutchko B, Strazivar B, Dzombak D, Lowry GV, Sthaulow N. Degradation of well cement by CO2 under geologic sequestration conditions. Environ Sci Technol 2007;41:4787–92. [6] Kutchko B, Strazivar B, Shuerta N, Lowry GV, Dzombak D, Sthaulow N. CO2 reaction with hydrated class H well cement under geologic sequestration conditions: effects of fly ash admixtures. Environ Sci Technol 2009;43:3947–52. [7] Ecob CR, King ES. Aggressive under ground environment: factors affecting durability of structure and specification of appropriate protective systems. In: Dhir RK, Green JW, editors. Proc of the int conf University of Dundee; 1990. p. 507–17. [8] Harrison WH. Chemical resistance of concrete. Concrete 1997;115:35–7. [9] Lea FM. The chemistry of cement and concrete. Chemical Publishing Company, Inc.; 1971. [10] Mehta PK. Concrete, properties and materials. Prentice Hall, Inc.; 1986. [11] Turkel S, Felekoglu B, Dulluc S. Influence of various acids on the physicomechanical properties of pozzolanic cement mortars. Sadhana 2007;32:683–91. [12] Revertega E, Richet C, Gegout P. Effect of pH on the durability of cement paste. Cem Concr Res 1992;22:259–72. [13] Grube H, Rechenberg W. Durability of cement paste in acidic water. Cem Concr Res 1989;19:783–92. [14] Fattuhi NI, Hughes BP. The performance of cement paste and concrete subjected to sulphuric acid attack. Cem Concr Res 1988;18:545–53. [15] Zˇivica V. Experimental principles in the research of chemical resistance of cement based materials. Constr Build Mater 1998;12:365–71. [16] Jambor J. Influence of pore structure of hardened cement paste on the rate of its deterioration by sulphate and CO2 waters. Research report, Institute of Construction and Architecture of SAS, Bratislava; 1976 [in Slovak]. [17] Gaurina-Medimurec N, Matanovic D, Krlec G. Cement slurries for geothermal wells cementing. Rudarsko-Geolosko-Naftni Zbornik 1994;6:127134. [18] Nelson EB. Well cementing. Amsterdam: Elsevier; 1990. [19] Guman GO, Elis RC. Cementing handbook. Houston: Gulf Publishing Co.; 1977. [20] Meller N, Hall Ch, Kyritsis K, Giriant G. Synthesis of cement based Ca–Al2O3– SiO2–H2O (CASH) hydroceramics at 200 and 250 °C: ex situ and in situ diffraction. Cem Concr Res 2007;37:823–33. [21] Taylor HF. The calcium silicate hydrates. In: Taylor HFW, editor. The chemistry of cements. London: Academic Press; 1964. p. 168–232. [22] Luke K. Phase studies of pozzolanic stabilized calcium silicate hydrates of 180 °C. Cem Concr Res 2004;34:1725–32. [23] Jupe AC, Wilkinson AP, Luke K, Funkhouser GP. Class H cement hydration at 180 °C and high pressure in the presence of added silica. Cem Concr Res 2008;38:660–6. [24] Kalousek GL, Prebs AF. Crystal chemistry of hydrous calcium silicates. III. Morphology and other properties of tobermorite and related phases. J Am Ceram Soc 1958;42:124–32. [25] Isaac NS. Physical organic chemistry. 2nd ed. Section 23,8,3, Harlow UK: Adison Welez Longman; 1995. [26] Beloshenko VA, Beresnev BI, Zaika VI, Zaika TP, Pakter MK. Structural and kinetic aspects of hydrostatic pressure effect on polymer network formation. High Press Res: An Int J 1990;3(1 & 6):278–81. [27] http://www.hyperphysics.phy-astr.gsu.edu/hbase. [28] Onan DD. Effects of supercritical carbon dioxide on well cements. In: Permian basic oil and gas recovery conference, Midland Texas. SPE Paper 12593; 8–9 March, 1989. [29] Nave CR. Hyper physics. Georgia State University, Department of Physics and Astronomy; 2005. [30] Jones R, Tinglez L. In: Poliakoff M, George MW, Howdle SM, editors. Proceedings of the 6th meeting on supercritical fluids: chemistry and materials, Nottingham; June 1999. p. 65. [31] Shaw RW, Thomas BB, Antony AC, Charles EA, Franck EU. Supercritical water – a medium for chemistry. Chem Eng News 1991:23–6.
V. Zivica et al. / Construction and Building Materials 36 (2012) 623–629 [32] Short NR, Purnell P. Preliminary investigations into supercritical carbonation of cement paste. J Mater Sci 2001;36:35–41. [33] Zˇivica V. Study on some combination of physically and physico-mechanically acting measures on the hydration of Portland cements. Final report, Institute of Construction and Architecture of SAS, Bratislava; 1975 [in Slovak]. [34] Borec T. Study of common action of sulphate and aggressive carbon dioxide water on the mechanism, chemism and kinetic of deterioration processes in Portland cement hardened paste. Final report, Institute of Construction Architecture of SAS, Bratislava; 1971 [in Slovak]. [35] Sabadoš Lˇ. Water: cement ratio change influence on the porosity of hardened cement paste, mortar and concrete and kinetic of sulphate and aggressive carbon oxide water deterioration. Final report, Institute of Construction Architecture of SAS Bratislava; 1971 [in Slovak]. [36] Zˇivica V. Estimation of influence of common action of Mg2+ and C1 ions and alkalic carbonates and hydrocarbonates on the chemism and mechanism of the action of sulphate and aggressive carbon dioxide waters. Final report. Institute of Construction Architecture of SAS, Bratislava; 1971 [in Slovak]. [37] Alegre R. Behaviour of cement in aggressive media. Ann IIBTB 1978;374:72–81 [in France]. [38] Calleja J. In: Durability, proceedings of 7th int congress on the chemistry of cement, vol. 1. Principal report, subtheme VII, Paris; 1980. p. VII 2–48. [39] De Ceukelaire L. The effects of hydrochloric acid on mortar. Cem Concr Res 1992;12:903–14. [40] Al-moudi OSB. Studies on the evaluation of permeability and corrosion resisting characteristics of Portland pozzolan concrete. MS thesis, King Fahd University of petroleum and minerals, Dhahran; 1958. [41] Rasheeduzzafar MM, Al-moudi OS, Alo-Many AT. Concrete durability in a very aggressive environment, concrete technology, past, present, and future. In: Proc of V Mohan Malhotra symposium, American Concrete Institute, Detroit, SP-144; 1993. p. 191–212. [42] Oberholster RE, van Aardt JHP. In: Barnes P, editor. Durability of cementitious systems. London: Applied Sciences Publishers; 1983. p. 365–413. [43] Schulze J. Influence of water–cement ratio and cement content on the properties of polymer modified mortars. Cem Concr Res 1999;29:909–15. [44] Jambor J. Influence of phase composition of hardened cement paste on its pore structure and strength. Pore structure and properties of materials. In: Proc int symposium RILEM, Part II, D75–D96, Academia Prague; 1973. [45] Appa Rao G. Role of water-binder ratio on the strength development in mortars incorporated with silica fume. Cem Concr Res 2001;31:443–7. [46] Danielson V. Heat of hydration of cement as affected by water–cement ratio. In: Paper IV.57, proc of the 4th int symp on the chemistry of cement, Washington DC, USA; 1962. p. 519–26. [47] Nilson LO. In: Aguado A, Gettu R, Shaheds SP. The relation between composition, moisture content and durability of conventional and new concrete, concrete technology, new trends, industrial applications. In: Proc of the int RILEM workshop, Barcelona, 7–9 September 1994, E and P N Spon, London; 1995. p. 199.
629
[48] Iizuka M, Mizununa T. The properties of concrete with new high range water reducing agent. In: Rev 41st meeting, Cem Ass Japan; 1987. p. 76–9. [49] Ramachandran VS, Beaudoin JJ, Shihua Z. Control of slump loss in superplasticized concrete. Mater Struct 1989;22:107–11. [50] Ramachandran VS, Mailvaganam NP, Malhotra VM, editor. In: New development in chemical admixtures, advances in concrete technology, energy, mines and resources, Ottawa, Canada, MSL; 1992. [51] Rixom MR, Waddicor J. Role of lignosulfonates as superplasticizers, international conference on developments in the use of superplasticizers. ACI SP 1981;68:359–70. [52] Anderson PJ, Roy DM. The effect of superplasticizer molecular weight on its adsorption and dispersion. Cem Concr Res 1988;18:980–6. [53] Pomeroy CD. In: Dhir RK, Green JW, editors. Benefits of concrete as a construction material. In: Protection of concrete proceedings of international conference Dundee, London: E and FN Spon; 1990. p. 1–12. [54] Jambor J, Zˇivica V. Porosity of mortar and its influence on resistance against corrosion caused by aggressive carbon dioxide. In: RILEM – IUPACf int symp prague, Preliminary Report, Part II, F83-F93; September 18–21, 1987. [55] Rostacy RR, Weiss E, Wiedebmann G. Changes of pore structure of cement mortar due to the temperature. Cem Concr Res 1980;10:157–64. [56] Fu YF, Wong YL, Poon CS, Tang CA, Lin P. Experimental study of micro/macro crack development and stress–strain relations of cement-based composite materials at elevated temperatures. Cem Concr Res 2004;34:789–97. [57] Fattuhi NT, Hughes BP. Effect of acidic attack on concrete with different admixtures or protective coatings. Cem Concr Res 1983;13:655–65. [58] Mehta PK. Studies on chemical resistance of low water cement ratio concretes. Cem Concr Res 1985;15:969–78. [59] Leeming MB. CIRIA review and the future for surface treatments. Conference on permeability of concrete and its control. London: Concrete Society; 1985. [60] ACI Committee 515 guide for the protection of concrete against chemical attack by means of coatings and other corrosion resistant materials. J Am Concr Ind Inst 1966;63(12):1305–91. [61] Swamy RN, Takinawa S. In: Dhir RK, Green JW, editors. Surface coatings to preserve concrete durability. Protection of concrete. Proc int conf Dundee, E & FN Spon; September 1990. p. 93–104. [62] Mitchel WC. Effect of waterproof coating on concrete durability. J Am Concr Inst 1957;29:51–7. [63] Zivica V. Concrete deterioration. Rehabilitation of concrete structures. Zdruzˇenie pre sanaciu betónovy´ch konštrukcií. Bratislava 1977:44–61 [in Slovak]. [64] Zivica V, Bajza A. Acidic attack of cement-based materials – a review. Part 1: Principle of acidic attack. Constr Build Mater 2001;15:331–40. [65] Zivica V, Bajza A. Acidic attack of cement-based materials – a review. Part 2: Factors of rate of acidic attack and protective measures. Constr Build Mater 2002;6:215–22. [66] Jambor J. Influence of w/c ratio on the phase composition, structure and strengths of the hardened cement pastes. Stavebnícky Cˇasopis XXII, cˇ 1974;12:806–30 [in Slovak].
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Acidic attack of cement based materials under the common action of high, ambient temperature and pressure Vladimir Zivica ⇑, Martin T. Palou 1, Martin Krizma 2, Lubomir Bagel 3 Institute of Construction and Architecture SAS, Slovakia
h i g h l i g h t s " Acidic geothermal waters and alkalic cementing material are conditions for acidic deterioration. " Water aggressivity factors are species present, concentration, temperature and pressure in well. " Cementing material resistance factors are cement, w/c ratio, processing and curing regime used. " All factors make should be considered at the design of the composition of the cementing mixtures. " Exacting character is increased by the necessity of use negative high w/c ratios for properties.
a r t i c l e
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Article history: Received 9 November 2011 Received in revised form 28 March 2012 Accepted 25 April 2012 Available online 15 July 2012 Keywords: Acidic attack Cement Bore hole Temperature Pressure Aggressivity
a b s t r a c t The cement materials in geothermal bore holes used to be exposed to the acidic attack under the common action of ambient high, temperatures and pressure. As it is known the waters in the deep geothermal wells are usually very aggressive to the applied cementing materials. Acidic aggressivity used to be a dominating type. Therefore, in the interest of the ensuring of life service of the cementing material it should be high acidic resistant. But it is a crucial problem due to the fact that practically all-protective measurements like e.g. the choice of the cement, increased density of material and others are weak and insufficient. One presumption of the ensuring of acidic resistance of cementing material is the correct evaluation of cooperating factors conditioning its acidic resistance and the factors of the aggressivity of environment. Both are of complex character depending on several conditioning factors. This complexness, as a rule, is not taken into consideration. However, such an evaluation current in the practice is entirely unsatisfied and incorrect. Therefore, further factors should be taken into consideration. These are the subject of the presented paper showing also the complexness of the point at issue. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction An important engineering property of cement-based materials is their durability. It determines the service life of concrete structures very significantly. Due to the interactions of cementitious material with external environments the durability may be threatened and lost. One important and significant threat is acidic attack. In this respect, geothermal waters can represent an effective source of the aggressivity. Geothermal waters exhibit different chemical composition. This composition is usually close to the chemical composition of surrounding rocks. ⇑ Corresponding author. Tel.: +421 59 30 9257; fax: +421 54 77 35 48. E-mail addresses: [email protected] (V. Zivica), [email protected] (M.T. Palou), [email protected] (M. Krizma), [email protected] (L. Bagel). 1 Tel.: +421 59 30 9260; fax: +421 54 77 35 48. 2 Tel.: +421 59 30 9228; fax: +421 54 77 35 48. 3 Tel.: +421 59 30 9256; fax: +421 54 77 35 48. 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.04.025
Geothermal waters typically contain 1–5 wt.% of soluble salts and in the particular case up to 30% [1–3]. The presence of these components is the result of water rock interaction. Diluted components are mainly sodium chloride, sulfates and bicarbonates and the solutions are always saturated with respect to silica. The last one tends to precipitate at lower temperatures. However, from the chemical point of view, the presence of silica in water environment may not play disastrous role. But, the chemically fatal effect is linked often to the very aggressive CO2 present [4–7]. The principal species C1 , affecting cement stability are SO24 , CO23 and pH. Indeed, pure water can decompose hardened cement compounds by leaching out Portlandite Ca(OH)2 and hydrated phases. The dissolution of Portlandite increases the permeability and decreases the strength of the cement composite. The source of acid media can even represent air pollution in the geothermal wells by gaseous carbon dioxide, sulfur dioxide and nitrogen oxides [8,9]. Acidic attack may also occur as a consequence of bacterial activity. This is particularly so when ground
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water contains ferrous iron or reduced sulfur compounds. The bacterial activity is able to produce acidic products like hydrogensulfide. Anaerobic bacterial action can break down proteins and hydrocarbons present in clays to release hydrogensulfide and methane. Furthermore, it can reduce sulfate within soils to form iron sulfide, which in turn react to form hydrogensulfide. The hydrogensulfide produced is oxidized by anaerobic bacterial action under the formation of sulfuric acid. Natural soft waters containing aggressive CO2 may be very aggressive against concrete. CO2 is present in greater or smaller quantities in the predominant majority of natural waters. The sources of CO2 in waters are biochemical processes in water or in contacted soils. Often the source of CO2 is the decaying of plant residues. The CO2 may be also a result of the decomposition of carbonate soils by the ground waters. Aggressive CO2, which is dangerous for concrete, is the amount of free carbon dioxide present in water that is in excess of the quantity required for the stabilization of the calcium hydrogen carbonate equilibrium. Aggressive CO2 reacts in a cement matrix with Ca(OH)2. Further more, the low soluble calcium carbonate formed is transformed into highly soluble calcium hydrogen carbonate Ca(HCO3)2 which is gradually leached [10,11]. Based on this process, the equilibrium between Ca(OH)2 and the hydration products is destroyed, resulting in a gradual hydrolytic decomposition of the hydration products destroying the integrity of the material. 2. Principle of acidic attack 2.1. Chemical effects The acidic attack of cement-based materials is based on their alkalinity. Therefore, it is an ever-willing partner of the acidic-alkaline reaction. At first, calcium hydroxide is attacked connected with the gradual hydrolytic decomposition of the hydration products, mainly after the consumption of calcium hydroxide. In this case, limestone aggregate is attacked too. These changes are characteristic: calcium oxide and water bound content decrease, and insoluble residue content increase. Under very severe conditions; the process of the attack may result in the total decomposition of the cement matrix with silica, alumina and ferric hydrogels as final reaction products. 2.2. Degradation processes A practical consequence of the chemical effects of acidic attack is a gradual degradation of engineering properties of cementing materials. At first, there is a deterioration of the surface of concrete showing crushing and dropping of surface material (Fig. 1). With the progress of acidic attack, a gradual degradation of strengths of concrete occurs.
Fig. 1. View of mortar test specimens after 180 days action of CO2 water [63].
Fig. 2. Mechanical properties decrease of mortar due to the action of aggressive carbon dioxide water [64].
It is a direct consequence of the decomposition of the hydration products and the leaching away of the products of this decomposition. The degradation of mechanical properties of mortar due to acidic attack is shown in Fig. 2. 3. Deterioration reactions rate factors Some reactions are naturally faster than others. The number of reacting species, their physical state (the particles that form solids move much more slowly than those of gases or those in solution), the complexity of the reaction and other factors can influence greatly the rate of a reaction. 3.1. Solution aggressiveness 3.1.1. Acid kind First of all, the chemical nature of the anions present is important. The ionic strength and the solubility of the calcium salt formed in the process play significant roles [12]. The dissociation degree of the acid is a further important factor. However, pH values give no correct information about the real quantity of the acid in the solution, because the pH value is dependent on dissociation degree of acid. Therefore, a strong acid having a high degree of dissociation (e.g. hydrochloric acid, nitric acid) may achieve very low values of pH, due to relatively small quantities of acid in the solution. The weak acids owing to their low dissociation degree, achieve higher pH values. It is evident that the concentration of the aggressive species in the acting solution is a more reliable parameter than the pH value [13]. The severinity of the acidic attack is significantly dependent on the solubility of the calcium salt formed. In the case of the formation of highly soluble salts, the severinity of the attack is very high. The possibility of the dissolving and leaching of the formed salt from the attacked material cause this. In the case of the formation of insoluble calcium salts such as calcium oxalate and fluoride, the effect of the acidic solution is entirely different. A dense insoluble layer is formed enhancing the development of the acidic attack showing a protective effect. This is used in practice for the protective purposes. Among the highly aggressive media are the solutions of hydrochloric and nitric acids because the solubility of their calcium salts with values of ca 46 and 56 wt.% is very high. The hydrolytic decomposition of cement hydration products is a conducting factor contributing to the severity of the attack. Presence of carbonate in some geothermal waters presents a serious problem for Portland cement systems. Calcium silicate hydrates become instable in such chemical environment, even at ordinary temperatures. After they have been exposed to carbonates
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solutions, calcium silicate hydrates convert finally into a mixture of calcium carbonate and amorphous silica. The phenomenon has been observed in well cements by numerous researchers. Geothermal waters usually contain magnesium and sodium sulfate. In such environment, calcium hydroxide present in hardened cement reacts with magnesium and sodium sulfate solution to produce calcium sulfate, magnesium and sodium hydroxide. Then, calcium sulfate reacts with C3A to produce ettringite or monosulfate which are larger in volume and cause the disintegration of hardened cement paste by expansion. Sulfate aggression occurs at temperature below 90 °C. 3.1.2. Solution concentrations The reaction rate increases with concentration, as described by the rate law and explained by collision theory. As reactant concentration increases, the frequency of collision increases. The significance of the concentration in the acidic attack is a consequence of the fact that the rate of the acidic attack, like that of other chemical reactions, is greatly dependent on the concentration of reaction participants. Therefore, the estimation of the concentration of the acting solution should be the first step at the evaluation of the solution aggressivity [14–17]. Generally, the increase in rate of acidic deterioration with an increase concentration in acidic solution is accepted. An example of the dependence of the rate of the acidic attack on the concentration of the acidic solution according to our results is shown in Fig. 3. It may be seen the increase in rate with the increase of the acidic component—carbon oxide—in the acting solution, but only to a certain limit. After that the increase of the rate is practically negligible. The chemistry of the reservoir fluids varies from fresh water to saline brines with greater than 200,000 mg/l total dissolved solids [18]. 3.1.3. Contact conditions A key is the quantity of the solution, which comes into contact with the attacked surface of the material over time unit. In principle two action modes are possible: (1) static conditions when no practical movement of the acting solution is present; (2) dynamic conditions, which occur with a change of the level or flow of the acting solution. In this case, the flow rate may have a greater influence than the concentration of the solution [14]. Moreover high velocities of the flowing ground water may disturb the attacked layer and contribute to the development of the attack. Dynamic conditions contribute to the transport of the aggressive species into the pore system of concrete and to the drainage out of the decomposition products [14,15]. Both effects contribute to the
Fig. 3. Dependence of decrease of compressive strength of mortar on carbon dioxide concentration in the acting water [65].
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intensification of the attack: the first affects the transport of aggressive partner of the attack, the second one contributes to the hydrolytic decomposition of cement matrix disturbing the chemical equilibrium in the system. A decrease in the level of the acting solution may be a further factor of the intensification of the attack. In this case, already attacked surfaces of concrete may be exposed to the effects of climatic factors, like sunshine, frost and wind contributing to the deterioration process. In some cases, the effect of drying the concrete surface may start the crystallization of the reaction products of the attack present in the pore liquid of the surface layer. Then crystallization may be a source of expansion tension of cracks propagation in the attacked part of the concrete surface. Alternate wetting and drying caused by repeated changes of the level in the acting solution may contribute to the severity of the processes [14]. Ambient temperature and relative humidity may influence the acidic attack, especially in the presence of the changes of the level of the acting solution [18]. Abrasion effects of deterioration may contribute to the intensification of acidic deterioration.
3.1.4. Ambient temperature rise Usually conducting a reaction at a higher temperature delivers more energy into the system and increases the reaction rate by causing more collisions between particles, as explained by collision theory. However, the main reason that temperature increases the rate of reaction is that more of the colliding particles will have the necessary activation energy resulting in more successful collisions. The influence of temperature is described by the Arrhenius equation. As a rule of thumb, reaction rates for many reactions double for every 10 °C increase in temperature, [2] though the effect of temperature may be very much larger or smaller than this. Cementing materials during the application procedure at bore hole and during the geothermal wells exploitation is exposed to different temperature – from atmospheric level to 350–400 °C depending on bore hole deep. This circumstance is of important significance as a factor of the control of rate of chemical reaction and a factor conditioning the phenomena of expansion or shrinkage of the solid. It is known that temperature level controls the chemical reaction rate. Moreover, it influences the kind and amount of the hydration products formed in those dependent properties of the hardened cement paste. A significant practical consequence of the temperature rise is setting and strength development acceleration. A significant consequence of elevated temperatures is the unfavorable change of calcium silicate gel C–S–H into a-dicalcium silicate hydrate, a-C2SH which begins to occur over 110 °C [19–21]. The product shows high porous structure with low strength. The unfavorable change of C–S–H gel into a-C2SH can be eliminated by the addition of silica-reach material in the quantity needed to reach CaO/SiO2 ratio value 1 or lower [22–24]. In addition, temperature rise can cause the thermal expansion of the cement-based materials under the producing the stress and crack propagation in the material. An average value for the coefficient of thermal expansion is about 10 millionths per degree Celsius. Several factors influence the behavior of concrete at high temperatures: moisture content, aggregate type and thermal stability, cement content, duration of exposure, the temperatures rise rate, age, and any restraint. Some authors reported that the effect of a number of tenCelsius rises on the rate of acidic attack of the cement-based materials is not significant [14,19]. But the effect of a number of hundred-Celsius rises is unclear. But when all known effects (reaction rate acceleration, calcium salts solubility increase, presence of critical solutions) are considered then temperature rise can be expected as a factor of the increase of acidic aggressivity. High relative humidity in bore hole is able to contribute to the fur-
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ther aggressivity increase due to the attack by air pollutants like CO2, SO2 and NOx [18]. 3.1.5. Environment pressure rise The rate of gaseous reactions increases with pressure, which is, in fact, equivalent to an increase in concentration of the gas. The pressure dependence of the rate constant for condensed-phase reactions (i.e., when reactants and products are solids or liquid) is usually sufficiently weak in the range of pressures normally encountered in industry that it is neglected in practice [25]. But, it is reported that effect of hydrostatic pressure up to 250 MPa on structurization kinetics and morphology of network polymers based on epoxy oligomers is significant [26]. High temperature in geothermal bore holes makes possible the presence of water vapor–air medium with related pressure, corresponding to the given bore hole deep and to the known dependence of water vapor pressure on ambient temperature (Table 1) [27]. A significant circumstance represents the water state change beginning at temperature 374 °C and pressure 22 MPa when the system vapor–liquid results in one phase – homogenous supercritical liquid. It shows the increased penetration ability into pore structure of the solids. Therefore, it can be expected an increased danger of the attack by the aggressive supercritical liquid [28–32]. A little information about influence of ambient pressure on hydration rate and cement hardening is mentioned in research literatures. It is reported that the pressure improves contact between water and cement particles and decreases the setting and increases the cement strength. Also it is reported that pressure does not have influence on type of created hydration products. They are identical with these formed under atmospheric pressure [33]. The importance of ambient pressure as hydration rate factor seems to be in comparison to temperature less important and ambiguous. But its co-operation is unconditional condition for the possibility of the hydration and hardening of cement at high temperatures. This dependence is based on the preventing of water evaporation by the ambient pressure. 3.1.6. Temperature and pressure common effect The values of high temperature and pressure represent hydrothermal conditions. Their positive accelerating effect on hydration and hardening of cement composites is utilized in praxis using technology process of concrete production so-called autoclaving. In accordance to this possibility, the hardened composite mixtures applied in bore holes are in principle similar to the cement composites made by autoclaving. This circumstance provides the possibility of knowledge utilization in field of concrete technology production using autoclaving for designing of cement composites for bore hole application. 3.1.7. Aggressiveness combination Geothermal brines can contain various chemical constituents able to evocate the combination of acidic and expansion deterioration. Expansion deterioration is a consequence of the reaction of
Table 1 Dependence of water vapor tension on ambient temperature [27]. Temperature (°C)
Pressure (MPa)
100 150 200 250 300 350 373
0.1013 0.4762 1.550 3.976 8.588 16.529 22.064
sulfate ions present in the acting solution and hydration products of C3A in cement under the formation of the bulky ettringite or monosulfate. The deterioration effect is showing by strength decrease, expansion and cracking of the attacked materials. Under the common presence of aggressive CO2 and sulfates in the solution its aggressivity can be increased under the appearance of the sulfate and acidic deterioration. The presence of chloride ions causes an increase in aggressivity of sulfate deterioration, mainly at the presence of Mg2+ ions [34–36]. 3.2. Factors of acidic resistance of cement based materials 3.2.1. Cement type The type of cement used has also an important bearing on the performance of concrete in an aggressive environment. It is known that acidic attack is always strong on any type of cement. It behaves more or less in the same manner, i.e. bad [37–39]. It seems that high-alumina cements may be resistant to acidic attack solutions with pH values between 4 and 5. The better performance of these cements compared to that of Portland cements could be attributed not only to the absence of calcium hydroxide but to the presence of more stable calcium-aluminate hydrates in the cement matrix and also to the presence of aluminum hydroxide. This may encapsulate the hydration products and protect them from acidic attack [24]. Blended Portland cements by pozzolans and slags are considered to be more resistant to the acidic attack than Portland cement alone. Some authors have claimed that the use of pozzolans Portland cements had only a weak positive effect on the increase of the acidic resistance. The data in literature are contradictory regarding whether cement-containing pozzolans are significant in improving the acidic resistance. However, it should be considered that the efficiency of blending for acid resistance may influence many factors e.g. the type of cement used for blending; the amount; the fineness of pozzolans used; and especially curing conditions [40,41]. However, it is well known that no type of cement has a satisfactory and long-term acidic resistance. This unfavorable situation consists of the fact that cement based materials are of alkaline nature. Therefore, they are ever a willing partner in the reaction with acidic species. 3.2.2. Cement content The effect of this factor is based on the fact that during the process of the acidic attack the quantity of the acid is consumed proportionally to the content of cement in the attacked materials, or more precisely to the content of the present hydration products. It has been reported that the rate of acidic attack decreases with an increase of cement content. For the main reason the neutralization capacity of the cement matrix is considered [42]. Nevertheless, the amount of cement used is important for the workability of the concrete mix, and to a certain extent, also for the curing sensitivity of fresh concrete. Both workability and curing sensitivity may result in influencing the quality of concrete, including its acidic resistance. Normally a cement content of 300–400 kg/m3 is sufficient to get a satisfactory low permeability and a sufficient acidic resistance of concrete if the w/c ratio is kept below 0.5, and adequate compaction and curing is provided. Naturally, the mentioned factors of the aggressivity should be taken into consideration. Cement content in concrete should not be so high as to induce cracking due to the drying shrinkage in the sections and due to thermal stresses. A minimum cement content provision is essential to produce a workable and cohesive concrete mix. Increasing the cement content with the attendant changes in the w/c ratio will be beneficial from an acidic resistance and strength viewpoint. For completeness, it should be mentioned that the behavior of the aggregates under the acidic attack is dependent on their solubility or insolubility in acids. Insoluble silica sand and river gravel
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usually do not take part in the deterioration process, but soluble dolomite and calcite decomposed in acidic solution may contribute to the determination process under the acidic attack of concrete. 3.2.3. Water–cement ratio The w/c ratio values of cementing mixtures are in the range of 0.6–0.8 and more. It is conditioned by the used cementing technologies, which need relatively thin composite mixtures. These values exceed the values of w/c 0.25–0.50 commonly used in technology of concrete and its related materials. It is known that the value of w/c ratio has the significant influence on cement hydration and hardening and subsequently on quality of engineering properties of hardened materials. It is also known that the quality of the engineering properties of concrete is increased with the w/c ratio decrease and the other way round [43–47]. It illustrates Fig. 4 showing the dependence of compressive strength and total porosity on the w/c ratio values. The necessary consequence of high w/c ratio used for composite mixtures assigned for bore hole cementing is that hardened material will exhibit lower values of strength, bulk volume and higher porosity and potentially lower resistance to aggressive influence in comparison to cement composites prepared with commonly used values of w/c. Since the mechanism of all the deterioration phenomena are permeability oriented, it is essential that the acidic resistance concrete should be sufficiently dense. This consideration is a direct consequence of the fact that for equals hydration, pore size distribution and permeability of the hardened cement paste is a direct function of w/c ratio. It is reported that the permeability is significantly reduced to a w/c ratio below 0.45. Therefore, in the interests of ensuring of an acid resistant concrete, it is necessary that the w/c ratio should be less than 0.45 and approximately 0.40, and at the same time ensuring a suitable workability of the concrete mixtures. The use of plasticizers and superplasticizers is useful in this respect [48–53]. 3.2.4. Curing conditions Curing conditions represent a further important processing factor. The quality of pore structure and permeability of concrete are significantly influenced. Even the crack propagation is dependent on the curing conditions. Due to insufficient curing, the surface layer of the concrete may be dried causing an increase in permeability and a decrease in acidic resistance. In this respect the wind and elevated ambient temperature may very negatively influence the quality of surface layer. Suitable curing conditions must begin
Fig. 5. Content of pores in mortar [54].
immediately after the concreting and they are not to be interrupted. When the hardening of concrete is interrupted once it may not continue, and latter applications of curing measures are usually useless. Special attention should be paid to curing in cases when higher values of w/c ratio and blended cements are used. Both increase the curing sensitivity of fresh concrete [37,38]. Especially the curing factors, mainly curing time is of special significance in the use of pozzolanic cements known as low hardening binders. These usually need a longer curing time as opposed to Portland cement [19]. According to the literature, a finely ground Portland cement clinker with the addition of an anion-active surface agent and an inorganic salt for the regulation of setting is acidic resistant [21,22]. The binding systems based on the modified silica fume show increased acidic resistance [23]. From viewpoint of curing the conditions in bore holes with lasting high humidity and temperatures seem to be favorable for the applicated cementing material hardening. 3.2.5. Pore structure The significance of pore structure quality is not surprising. It is well known that pore structure is a vital factor of engineering properties of cement-based materials. The significance of pore structure for acidic resistance of cement-based materials is conditioned by the fact that it is an important cooperating factor in transport phenomena [54–56]. Both the transport of aggressive species and the draining away of the products of the attack are realized through a pore system. Pore structure is an inseparable condition of the hydrated product formation of cement-based materials, and a primary key of their engineering properties. The formation of pore structure begins immediately after the addition of water into the mixture, and it progresses during the mixing and consolidation of the fresh mixture, and through further hardening of the material. As it is shown in Fig. 5 the acidic resistance of cement-based material is significantly dependent on pore size distribution and increases with micropore content [54]. 4. Protective measures
Fig. 4. Dependence of compressive strength of hardened cement paste on the values of total porosity and w/c ratio used [66].
The principle of acid attack—dissolution of the cement matrix and its general alkaline nature—often makes the protection of concrete structures difficult task. In principle, there are two possibilities how to protect the concrete against acidic attack: Group 1. Its possibilities are based on the fact that both the concrete and the structure built with it can be designed to afford desired protection. In many instances the protection can take the form of self-protection through judicious selection of concreting materials, and the appropriate design of concrete mix and the
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structure, without a need to resort to special protective measures [57,58]. The protective measures applied to the preparation of the concrete represent the most convenient way of ensuring the satisfied resistance of the concrete in aggressive media. These measurements are based on the choice of the cement used, application of the admixtures, design of the suitable composition of the concrete mixtures, effective conditions of the consolidation of concrete mixture and curing of the hardening of fresh concrete. All these protective measurements are aimed at gaining maximal compactness and minimal porosity of the concrete. As it has been already mentioned as a rule, current cements do not show many significant differences in their acidic resistance Hence attention should be paid to the protective measurements enabling the possibilities of the decrease of water permeability of concrete. In this respect, the application of modern superplasticizers is useful and effective. The superplasticizers used give the possibility of a significant decrease of the ways used, under the observing satisfied workability of the fresh concrete mixture. This last positive effect is very important for the high compactness and low porosity of concrete, and in the end for its low water permeability and acidic resistance. The mentioned measures may contribute to the acidic resistance of concrete. However, it should be stated that the concrete material remains of an alkaline nature willing to react with acids compounds and undergo acidic attack. Therefore, this property of concrete represents a durable threat for the concrete structures when exposed to the acidic attack. Group 2. Its possibilities are based on the protection of concrete structures by the application of various coatings on their surface. Surface coatings have a significant role to play in protecting and preserving new and existing concrete structures. This protective measurement has the unique advantage that it can be applied to protect existing concrete structures [59–62]. It is evident that protective measures of group I are for protection of cementing material of geothermal wells actual.
5. Conclusion Chemical aggressivity of geothermal waters represents a potential danger for the durability of the cementing materials in bore hole wells with the possibility of the treating of the service life of the construction of the wells. Due to the universal alkaline character of the cementing materials the danger of its acidic attack is ever real. Therefore, the protective measurements should be an indispensable task in the choice of the materials and composition of the mixtures for the cementing of geothermal wells. It has been shown that the rate of acidic attack of cement-based materials is dependent on numerous factors conditioning the aggressivity of the acting medium and resistance of the attacked material. The resulting rate of the attack is a consequence of their mutual interaction. Therefore, this complexity should be taken into consideration at the evaluation of the service life of concrete structures in acidic media, and the choice of protective measures. With respect to the acidic resistance, cement based materials were shown ever to be vulnerable solid. On the other hand, the occurrence of the acidic media in the geothermal environment is prevailing and therefore ever real. The fact of general alkalinity of the cement-based materials makes them an ever-willing partner of alkali – base reaction and the object of acidic attack. Therefore, the protection of the cementing materials in deep bore holes and ensuring of their durability represent a pretentious task. It seems that only binders of new generation with increased acidic resistance should be an effective a prospective way of the solution of the problem.
Acknowledgements This article has been produced with the financial assistance of the European Regional Development Fund (ERDF) under the Operational Programme Research and Development/Measure 4.2 Transfer of knowledge and technology from research and development into practice in the Bratislava region /Project Center for applied research of composite materials for deep geothermy/.
References [1] Robins NS, Milidowski AE. Bore hole cements and the downhole environment a review. Quart J Eng Geol Lond 1986;19:175–81. [2] Milestone NB, Sugama T, Kukacka LE, Cerciello NR. Carbonation of geothermal grouts – Part I: CO2 attack at 150 °C. Cem Concr 1986;86:941–50. [3] Sugama T, Carciello NR. Carbonation of hydrothermally treated phosphatebonded calcium aluminate cements. Cem Concr Res 1992;22:783–92. [4] Sugama T, Carciello NR. Carbonation of calcium phosphate cements after longterm exposure to Na2CO3 – laden water. Cem Concr Res 1993;23:1409–17. [5] Kutchko B, Strazivar B, Dzombak D, Lowry GV, Sthaulow N. Degradation of well cement by CO2 under geologic sequestration conditions. Environ Sci Technol 2007;41:4787–92. [6] Kutchko B, Strazivar B, Shuerta N, Lowry GV, Dzombak D, Sthaulow N. CO2 reaction with hydrated class H well cement under geologic sequestration conditions: effects of fly ash admixtures. Environ Sci Technol 2009;43:3947–52. [7] Ecob CR, King ES. Aggressive under ground environment: factors affecting durability of structure and specification of appropriate protective systems. In: Dhir RK, Green JW, editors. Proc of the int conf University of Dundee; 1990. p. 507–17. [8] Harrison WH. Chemical resistance of concrete. Concrete 1997;115:35–7. [9] Lea FM. The chemistry of cement and concrete. Chemical Publishing Company, Inc.; 1971. [10] Mehta PK. Concrete, properties and materials. Prentice Hall, Inc.; 1986. [11] Turkel S, Felekoglu B, Dulluc S. Influence of various acids on the physicomechanical properties of pozzolanic cement mortars. Sadhana 2007;32:683–91. [12] Revertega E, Richet C, Gegout P. Effect of pH on the durability of cement paste. Cem Concr Res 1992;22:259–72. [13] Grube H, Rechenberg W. Durability of cement paste in acidic water. Cem Concr Res 1989;19:783–92. [14] Fattuhi NI, Hughes BP. The performance of cement paste and concrete subjected to sulphuric acid attack. Cem Concr Res 1988;18:545–53. [15] Zˇivica V. Experimental principles in the research of chemical resistance of cement based materials. Constr Build Mater 1998;12:365–71. [16] Jambor J. Influence of pore structure of hardened cement paste on the rate of its deterioration by sulphate and CO2 waters. Research report, Institute of Construction and Architecture of SAS, Bratislava; 1976 [in Slovak]. [17] Gaurina-Medimurec N, Matanovic D, Krlec G. Cement slurries for geothermal wells cementing. Rudarsko-Geolosko-Naftni Zbornik 1994;6:127134. [18] Nelson EB. Well cementing. Amsterdam: Elsevier; 1990. [19] Guman GO, Elis RC. Cementing handbook. Houston: Gulf Publishing Co.; 1977. [20] Meller N, Hall Ch, Kyritsis K, Giriant G. Synthesis of cement based Ca–Al2O3– SiO2–H2O (CASH) hydroceramics at 200 and 250 °C: ex situ and in situ diffraction. Cem Concr Res 2007;37:823–33. [21] Taylor HF. The calcium silicate hydrates. In: Taylor HFW, editor. The chemistry of cements. London: Academic Press; 1964. p. 168–232. [22] Luke K. Phase studies of pozzolanic stabilized calcium silicate hydrates of 180 °C. Cem Concr Res 2004;34:1725–32. [23] Jupe AC, Wilkinson AP, Luke K, Funkhouser GP. Class H cement hydration at 180 °C and high pressure in the presence of added silica. Cem Concr Res 2008;38:660–6. [24] Kalousek GL, Prebs AF. Crystal chemistry of hydrous calcium silicates. III. Morphology and other properties of tobermorite and related phases. J Am Ceram Soc 1958;42:124–32. [25] Isaac NS. Physical organic chemistry. 2nd ed. Section 23,8,3, Harlow UK: Adison Welez Longman; 1995. [26] Beloshenko VA, Beresnev BI, Zaika VI, Zaika TP, Pakter MK. Structural and kinetic aspects of hydrostatic pressure effect on polymer network formation. High Press Res: An Int J 1990;3(1 & 6):278–81. [27] http://www.hyperphysics.phy-astr.gsu.edu/hbase. [28] Onan DD. Effects of supercritical carbon dioxide on well cements. In: Permian basic oil and gas recovery conference, Midland Texas. SPE Paper 12593; 8–9 March, 1989. [29] Nave CR. Hyper physics. Georgia State University, Department of Physics and Astronomy; 2005. [30] Jones R, Tinglez L. In: Poliakoff M, George MW, Howdle SM, editors. Proceedings of the 6th meeting on supercritical fluids: chemistry and materials, Nottingham; June 1999. p. 65. [31] Shaw RW, Thomas BB, Antony AC, Charles EA, Franck EU. Supercritical water – a medium for chemistry. Chem Eng News 1991:23–6.
V. Zivica et al. / Construction and Building Materials 36 (2012) 623–629 [32] Short NR, Purnell P. Preliminary investigations into supercritical carbonation of cement paste. J Mater Sci 2001;36:35–41. [33] Zˇivica V. Study on some combination of physically and physico-mechanically acting measures on the hydration of Portland cements. Final report, Institute of Construction and Architecture of SAS, Bratislava; 1975 [in Slovak]. [34] Borec T. Study of common action of sulphate and aggressive carbon dioxide water on the mechanism, chemism and kinetic of deterioration processes in Portland cement hardened paste. Final report, Institute of Construction Architecture of SAS, Bratislava; 1971 [in Slovak]. [35] Sabadoš Lˇ. Water: cement ratio change influence on the porosity of hardened cement paste, mortar and concrete and kinetic of sulphate and aggressive carbon oxide water deterioration. Final report, Institute of Construction Architecture of SAS Bratislava; 1971 [in Slovak]. [36] Zˇivica V. Estimation of influence of common action of Mg2+ and C1 ions and alkalic carbonates and hydrocarbonates on the chemism and mechanism of the action of sulphate and aggressive carbon dioxide waters. Final report. Institute of Construction Architecture of SAS, Bratislava; 1971 [in Slovak]. [37] Alegre R. Behaviour of cement in aggressive media. Ann IIBTB 1978;374:72–81 [in France]. [38] Calleja J. In: Durability, proceedings of 7th int congress on the chemistry of cement, vol. 1. Principal report, subtheme VII, Paris; 1980. p. VII 2–48. [39] De Ceukelaire L. The effects of hydrochloric acid on mortar. Cem Concr Res 1992;12:903–14. [40] Al-moudi OSB. Studies on the evaluation of permeability and corrosion resisting characteristics of Portland pozzolan concrete. MS thesis, King Fahd University of petroleum and minerals, Dhahran; 1958. [41] Rasheeduzzafar MM, Al-moudi OS, Alo-Many AT. Concrete durability in a very aggressive environment, concrete technology, past, present, and future. In: Proc of V Mohan Malhotra symposium, American Concrete Institute, Detroit, SP-144; 1993. p. 191–212. [42] Oberholster RE, van Aardt JHP. In: Barnes P, editor. Durability of cementitious systems. London: Applied Sciences Publishers; 1983. p. 365–413. [43] Schulze J. Influence of water–cement ratio and cement content on the properties of polymer modified mortars. Cem Concr Res 1999;29:909–15. [44] Jambor J. Influence of phase composition of hardened cement paste on its pore structure and strength. Pore structure and properties of materials. In: Proc int symposium RILEM, Part II, D75–D96, Academia Prague; 1973. [45] Appa Rao G. Role of water-binder ratio on the strength development in mortars incorporated with silica fume. Cem Concr Res 2001;31:443–7. [46] Danielson V. Heat of hydration of cement as affected by water–cement ratio. In: Paper IV.57, proc of the 4th int symp on the chemistry of cement, Washington DC, USA; 1962. p. 519–26. [47] Nilson LO. In: Aguado A, Gettu R, Shaheds SP. The relation between composition, moisture content and durability of conventional and new concrete, concrete technology, new trends, industrial applications. In: Proc of the int RILEM workshop, Barcelona, 7–9 September 1994, E and P N Spon, London; 1995. p. 199.
629
[48] Iizuka M, Mizununa T. The properties of concrete with new high range water reducing agent. In: Rev 41st meeting, Cem Ass Japan; 1987. p. 76–9. [49] Ramachandran VS, Beaudoin JJ, Shihua Z. Control of slump loss in superplasticized concrete. Mater Struct 1989;22:107–11. [50] Ramachandran VS, Mailvaganam NP, Malhotra VM, editor. In: New development in chemical admixtures, advances in concrete technology, energy, mines and resources, Ottawa, Canada, MSL; 1992. [51] Rixom MR, Waddicor J. Role of lignosulfonates as superplasticizers, international conference on developments in the use of superplasticizers. ACI SP 1981;68:359–70. [52] Anderson PJ, Roy DM. The effect of superplasticizer molecular weight on its adsorption and dispersion. Cem Concr Res 1988;18:980–6. [53] Pomeroy CD. In: Dhir RK, Green JW, editors. Benefits of concrete as a construction material. In: Protection of concrete proceedings of international conference Dundee, London: E and FN Spon; 1990. p. 1–12. [54] Jambor J, Zˇivica V. Porosity of mortar and its influence on resistance against corrosion caused by aggressive carbon dioxide. In: RILEM – IUPACf int symp prague, Preliminary Report, Part II, F83-F93; September 18–21, 1987. [55] Rostacy RR, Weiss E, Wiedebmann G. Changes of pore structure of cement mortar due to the temperature. Cem Concr Res 1980;10:157–64. [56] Fu YF, Wong YL, Poon CS, Tang CA, Lin P. Experimental study of micro/macro crack development and stress–strain relations of cement-based composite materials at elevated temperatures. Cem Concr Res 2004;34:789–97. [57] Fattuhi NT, Hughes BP. Effect of acidic attack on concrete with different admixtures or protective coatings. Cem Concr Res 1983;13:655–65. [58] Mehta PK. Studies on chemical resistance of low water cement ratio concretes. Cem Concr Res 1985;15:969–78. [59] Leeming MB. CIRIA review and the future for surface treatments. Conference on permeability of concrete and its control. London: Concrete Society; 1985. [60] ACI Committee 515 guide for the protection of concrete against chemical attack by means of coatings and other corrosion resistant materials. J Am Concr Ind Inst 1966;63(12):1305–91. [61] Swamy RN, Takinawa S. In: Dhir RK, Green JW, editors. Surface coatings to preserve concrete durability. Protection of concrete. Proc int conf Dundee, E & FN Spon; September 1990. p. 93–104. [62] Mitchel WC. Effect of waterproof coating on concrete durability. J Am Concr Inst 1957;29:51–7. [63] Zivica V. Concrete deterioration. Rehabilitation of concrete structures. Zdruzˇenie pre sanaciu betónovy´ch konštrukcií. Bratislava 1977:44–61 [in Slovak]. [64] Zivica V, Bajza A. Acidic attack of cement-based materials – a review. Part 1: Principle of acidic attack. Constr Build Mater 2001;15:331–40. [65] Zivica V, Bajza A. Acidic attack of cement-based materials – a review. Part 2: Factors of rate of acidic attack and protective measures. Constr Build Mater 2002;6:215–22. [66] Jambor J. Influence of w/c ratio on the phase composition, structure and strengths of the hardened cement pastes. Stavebnícky Cˇasopis XXII, cˇ 1974;12:806–30 [in Slovak].