J. MARKU ALKALI-SILICA EXPANSION POTENTIALITY OF ERZENI RIVERBED ... JOZEFITA MARKU Review paper UDC:620.193.4:669.97 Alkali-silica expansion potentiality of Erzeni riverbed aggregates (Albania) and the risk of corrosion in concretes produced with them The environmental and fuel efficiency reasons led the industry of building raw materials to develop new technologies for production of cements. The cements being produced have high alkali content which in combination with some sorts of aggregates may influence the chemistry of the concrete mixture that may cause long term harmful effects to concrete structures. In this paper it is succinctly reviewed a part of scientific research work done on the possibility of the manifestation of the harmful effects caused by Alkali-Silica Reaction (ASR) on concretes made of high alkali cements and riverbed aggregates of Albania, in particular of those of Erzeni River, which is the most exploited one in Albania. The treatment of this topic has been a necessity and is considered very important for building industry in Albania, where traditionally low alkali cements have been produced and used. Especially in the period of transition, when high alkali cements started to be used both from import and domestic production, it is considered very important the study of the suitability of the riverbed aggregates in concrete produced with high alkali content cement since there are no ways of preventing ASR after the concrete has been placed. The results and recommendations of this study serve to increase the builder’s awareness of the need of proper investigation, from this point of view, of the raw materials prior to construction. They also contribute to environment protection both by recommending alternative aggregates to riverbed ones and by helping to avoid the degradation, in timeframe, of concrete structures. Keywords: Alkali-Silica Reaction (ASR), riverbed aggregates, chert, siliceous aggregate, limestone aggregate. INTRODUCTION There are three essential factors that cause the Alkali-Silica Reaction: alkalis, certain alkaline re- Deterioration of concrete structures because of active amorphous silica minerals, and moisture ha- alkaline attack of aggregates and ASR is causing a ving the role of the reagent and a transport media. great concern in many countries all over the world The alkalis are mainly supplied by the cement pas- and in almost all European countries, both because te of concrete or can under certain circumstances of the potential threat to the service life of concrete be derived from the aggregate material, mix water, structures and the extra costs of repairing the ground water, chemical admixtures, pigments, etc. structures affected by this reaction. Moisture is mostly provided by “exposure condi- ASR is a so-called heterogeneous chemical re- tions”, e.g. atmospheric water or service conditions. action between sodium and potassium hydroxides The mechanism of silica dissolution is not very in cement paste and certain alkaline reactive amor- much controlled by alkalis, as by dissolved OH-ions phous minerals in aggregate producing a hygro- breaking silica bonds which then recombine with scopic gel which, when moisture is present, ab- present alkali-ions. sorbs water and expands causing degradation of The main contributor to the alkalinity of pore appearance (pattern cracking and excessive expa- solution is the soluble alkali in cement. nsion), deterioration in concrete strength and effec- Wet cement is a very active alkaline chemical ting structural behavior of concrete structures. Du- environment and chemical reaction occurs between rability of concrete structures may not be directly the aggregate particles and the cement filler. Ce- affected by ASR, but expansive cracking of cover ments with an alkali content (Na2O+K2O) below concrete may increase the risk of reaching steel 0.6% (low-alkali cements) has been found to cause reinforcements by providing a route for water, air a little expansion with reactive aggregates. On the and chlorides. It will also increase possibility of contrary, the high alkali cement is a very active freeze-thaw attack. alkaline chemical environment for some alkali reac- tive varieties of silica in aggregate, i.e. non-crysta- lline or imperfectly crystalline silica (such as opal, Author's address: Department of Industrial chalcedony, chert, tridimite, christobalite), strained Chemistry, Faculty of Natural Sciences, Albania quartz or microcrystalline quartz (such as quar- Received for Publication: 17. 07. 2014. tzite), glassy to cryptocrystalline matrix of igneous Accepted for Publication: 21. 09. 2014. rocks (such as pumice), etc. Silica is an amphoteric ZAŠTITA MATERIJALA 56 (2015) broj 1 5 J. MARKU ALKALI-SILICA EXPANSION POTENTIALITY OF ERZENI RIVERBED ... material which means it dissolves at extreme pH to determine if any potential risk exists for mani- values, in strongly acidic or strongly alkaline con- festation of ASR in concretes prepared with the ditions and less around neutral pH. The dissolution riverbed aggregates of Albania, to evaluate their of silica is affected by pH as well as by foreign potential reactivity in concrete mixtures made with species in pore solution, foreign species and hyd- high alkali cement and, based on the classification rous species in the solid silica, polymorphism of of their reactivity, to recommend builders the river- quartz, the quality of crystal lattice, silica particle bed aggregates that they must avoid to use in size, coexisting minerals, etc. concrete production; especially in hydraulic struc- tures. The reasons for the recent apparently pre- valence of damage of concrete from ASR are con- It is significant to point out that, this is the first sidered to be the changes in concrete technology work where the topic of the alkali-silica expansion (concrete mixes richer in cements are being used, potentially of riverbed aggregates in Albania and producing higher total alkali contents in concrete) the risk of corrosion in concretes produced with as well as in cement technology. them is considered and evaluated. Changes in cement production methods (espe- In this regard, initially we studied the behaviour cially in developed countries, but even in deve- of aggregates taken from the main riverbeds of loping ones) in order to comply with environmental Albania. regulations have placed controls on the emission of The experiments are carried out according to stack gases into the atmosphere. Therefore, the the method of continuity of tests (UNI 8520-22) that tendency is that the alkali content of cement to be includes the petrographic examinations of aggre- increased, because the dusts from kiln (rich of gates, the accelerated expansion tests and long- alkali) are recycled and returned back to process term expansion tests. The data taken from the adding the alkali load of cement. On the other experimental behavior of samples in these tests hand, in order to use fuel more efficiently, the wet are graphically shown and statistically analyzed process of cement production (where alkalis were and commented. partly leached) is being abandoned and is being The high values of expansions and the visible substituted by dry process. So, the low-alkali ce- cracks of the samples of Erzen (Tirana capital) and ments are unlikely to be produced and found in the Osum (Berat city) riverbed aggregates classified market. them as potentially alkali-reactive aggregates [1]. In Albania, until the start of political transitory Out of these two potentially alkali-reactive period (year 1990), the construction sector needs aggregates we choose the Erzeni riverbed aggre- were satisfied with in country produced cements. gates for further more complex study because, After 1990, in the conditions of the upset of the being in the vicinity of two big cities Tirana and domestic cement production caused by closure of Durres, they are target of high exploitation due to several of cement factories, the cement needs ha- big demand for aggregates from the ever-growing ve been compensated with cement imported from building industry in this region. other countries, like Italy, Croatia, Greece, etc. For this study the aggregate samples are taken Later on, even in Albania, the previous wet at random along Erzen riverbed from Elbasan to process (Fushe Kruja Cement Factory) has been Durres. changed towards dry clinker production process and the other new cement factories are based on Based on the expansion values, a classification dry cement production process, too. of these aggregates from innocuous to moderate reactive aggregates was made [2-11]. So, nowadays in Albania, high alkali content cements are being used in concrete and this is But this classification was not definitive taking likely to happen even in the future. into account that the aggregates containing more than 2% of reactive constituents in the form of po- In order to evaluate the potential risk of the rous chert, flint or chalcedony (that is the case of already built structures of the time period and for Erzeni riverbed aggregates with above 2% mineral comparison reasons, we have examined the che- chert) may exhibit the so-called “pessimum effect” mical content of cements produced in Albania du- in accelerated mortar bar tests and can yield ring the period of time 1980-1995, in the factories misleading results and inappropriate approval of of F. Kruja, Vlora and Elbasan, as well as their raw certain active aggregates to be used in concrete materials. production. Also, we have analyzed several of imported ce- According to Hobbs and Shayan, the reason for ments, used in Albania in the last decades. “the pessimum content” of aggregate in the stan- Therefore, in the conditions of using of these dard accelerated tests, is that for a certain content high-alkali cements and guided by the world expe- of alkalis, the expansion of mortar bars is increased rience and recommendations of many scientific as the content of active aggregate is increased up authorities of this field, we proceeded this research to a point (“negative content”), beyond which there 6 ZAŠTITA MATERIJALA 56 (2015) broj 1 J.