Sülfat etkisine maruz kalmış mineral ve nano katkılı harçların durabilitesi

Abstract

Beton, günümüzde inşaat mühendisliğinin birçok alanında kullanılan en yaygın yapı malzemesidir. Agreaga, çimento, mineral veya kimyasal çeşitli katkı maddeleri ve sudan oluşan karmaşık bir yapıya sahiptir. Sertleşmiş betonun boşluk yapısı, mekanik özellikleri ve yıllar içerisinde maruz kaldığı çevresel faktörler betonun servis ömrünü etkileyen ana öğelerdir.Çevresel etki faktörleri içerisinde sülfat etkisi betonun dürabilitesini ve özelliklerini olumsuz yönde etkileyen en önemli etkenlerden biri olarak karşımıza çıkmaktadır. Betonda sülfat etkisiyle ilgili pek çok araştırma yapıldığı bilinmektedir. Literatürdeki araştırmalar, betonun maruz kaldığı dış sülfat etkisi üzerine yoğunlaşmaktadır.Yapılan bu çalışmada, sülfat etkisine maruz kalmış harçlarda bu etki nedeniyle oluşabilecek hasarlar ve iç yapı oluşumlarının incelenmesi araştırılmıştır. Mikro silika (MS), nano silika (NS), yüksek fırın cürufu (YFC) ve uçucu kül (UK) gibi mineral katkıların sülfat etkisine maruz kalan harçların dürabilite özelliklerini ne yönde etkilediği incelenmiştir.Yapılan çalışma kapsamında, doğal sülfat içeriği ihmal edilebilir düzeyde olduğu bilinen Sakarya kumu ile üretilmiş çeşitli oranlarda mineral katkı kullanılan numuneler ve bunları kıyaslayabilmek adına katkısız üretilen referans numuneler yüzde 5 oranında sodyum sülfat içeren çözeltide bekletilmiş ve dış sülfat etkisine maruz bırakılmıştır. Çalışmada ayrıca, Irak'tan temin edilen ve doğal sülfat içeriği yüksek olduğu bilinen iki çeşit kum kullanılarak üretilen ve yine çeşitli oranlarda mineral katkı içeren numuneler ise su küründe bekletilmiş ve maruz kaldıkları iç sülfat etkileri nedeniyle oluşabilecek dürabilite problemleri incelenmiştir.Yapılan çalışmada 36 ay boyunca numunelerde periyodik olarak boy ölçümleri alınmıştır ve mineral katkı kullanımının sülfat etkisinden kaynaklanan deformasyonları ne derece kısıtladığı tespit edilmiştir. Literatürde ve birçok teknik şartnamede geçtiği üzere sülfat etkisinde boy uzaması için kritik sınır değer %0,1 kabul edilmektedir. Bu değerden daha fazla boy uzaması olması halinde betonun basınç dayanımında da önemli düşüşlerin olduğu bu çalışmada da tespit edilmiştir. 36 aylık etki süresi sonunda tüm numunelerde ultrases geçiş hızı ölçümleri, eğilme dayanımı ve basınç dayanımı deneyleri yapılmıştır. Deney sonuçları mineral katkıların, kullanım oranlarına bağlı olarak, sülfat etkisi karşısında mekanik ve fiziksel özellikler bakımından önemli iyileştirici etkilerinin olduğunu göstermiştir. Dış sülfat etkisine maruz kalmış katkısız numune ve her bir katkı türünden en az ve en çok oranında olmak üzere seçilen numunelerde ise iç yapı oluşumlarını araştırmak amacıyla XRD (X-Ray Diffraction) deneyleri gerçekleştirilmiştir. Sonuçlar, gözlenen hasarın nedenin etrenjit değil, alçı kristalleri oluşumundan olduğunu göstermiştir.

Concrete, which is most commonly used construction material today. It has a complex structure consisting of cement, aggregate, mineral and chemical admixtures, and mixing water. Cement production and concrete are considered to ve important indicators of economic growth.The properties of plain concrete depend on the chemical reactions mainly between cement particles and water, as well as the reactions between the other components of concrete. The chemistry of hydration products, pore structure and mechanical properties of hardened concrete is significantly affected by the process of production and the environmental conditions in which the concrete is exposed during its service life. Concrete in service is often exposed to aggressive environments but it can be relatively simple and economical to produce durable concrete. Sulfate contaminated environment is one of the most important factors that significantly affects the durability of plain and reinforced concrete structures negatively.Sulfate attack is one of the most important factors affecting the durability and service life of concrete structures negatively. As a simplest definition, sulfate attack can be described as a series of complex chemical reaction chains which takes place between the sulfate ions and hydration products of cement. Especially when improper concrete mixture design is used, structural elements exposed to sulfate attack shows significant deterioration diminishing the structural integrity. It is well known that, one way of enhancing concrete durability against external attacks is to improve its permeability properties. Besides the water to cementitious material ratio of the mixture, mineral admixtures having different chemical and physical characteristic improve micro structure of concrete by refining its internal structure. There are a lot of reseaches about this aspect but we can see that they are mainly about external sulfate attack. The sulfate ions can diffuse in concrete from the external environment, however, it is also possible that the source of these ions can also be the concrete mixing materials such as cement, aggregate, and chemical and mineral admixtures.In this study, enhanced durability were investigated against internal and external sulfate attack. In order to investigate external sulfate attack, we used sand provided from Sakarya region that know as there aren't any reserved sulfate naturally inside and proced mortar bars by using this kind of sand were subjected to external sulfate attact by using 5% sodium sulfate solution. On the other hand, in order to investigate internal sulfate attact, we used sands provided from Middle East region that known as these kind of sands reserve sulfate naturally inside (water soluble sulfate amounts of 1.5% and 2.0%) and mortar bars which are produced by sands from Middle East subjected to water cure. The quality of any aggregate, in addition to its chemical and mineralogical nature, depends on its prior exposure to the environment and during processing. Aggregate properties are also can be effective on sulfate attract. Chemical and mineral admixtures are accepted components of modern concrete. They are used to enable easier processing of fresh concrete, to better the proporties of hardened concrete in a structure, and to improve concrete durability and extend its service life. Mineral additives such as fly ash, ground granulated blast furnace slag and micro silica have widely been used in concrete technology for improving strength and durability performance of concrete. In order to enhance sulfate resistance of the mixtures, nano silica (NS), micro silica (MS), fly ash (FA) and ground granulated blast-furnace slag (GGBS) were used with various replacement ratios. The FA used in this work can be classified as Class F fly ash. As it is a new nano technological material, the role of nano silica for limiting the expansions caused by sulfate attack was shown as well.As well as the reference mixtures which do not contain mineral additives, three different replacement ratios by weight of cement were selected for each mineral additive. Replacement ratios were selected as 2%, 4% and 6% for NS, 6%, 9% and 12% for MS, 15%, 30% and 45% for FA and, 20%, 40% and 60% for GGBS. These replacement ratios were selected to investigate a certain range which these mineral admixtures would be used in practical applications. For the mixtures of the samples, a commercially available Type I Portland cement with C3A content of 7.6% was used. All samples' expansions were monitored for 36 months and the effects of mineral admixtures were investigated against length changes. At the end of 36 months, flexural strength, compression strength and ultrasonic pulse velocity of mortars were determined and their results were compared with the referance samples which do not contain mineral additives. The most important engineering properties of hardened concrete is its strength. In this study, it is measured that the strength losses between the reference samples exposed to sulfate environment and cured in water are approximately 38% for compressive strength and 66% for flexural strength.Results have shown that, from the mechanical strength point view of, significant improvements against both internal and external sulfate attack were achieved by using mineral admixtures. Practical solutions to protect concrete against sulfate attack are, in most cases, simple and economical. It should be also mentioned that, the dosage of mineral additive is also another important factor which determines the sulfate resistance of the mixtures. For the micro-structural evaluations, on the other hand XRD tests were also performed on the reference samples exposed to 5% sodium sulfate solution and water cured samples and the samples having the maximum and minimum mineral admixture contents. In accordance with the XRD test results, it was concluded that the reason for the damage of referance mortar samples exposed to sulfare environment is gypsum formation. No ettringite formation was observed. As indicated in the literature, ettringite is not stable in high sulfate concentration environments.