Nanosilika katkılı geleneksel betonların mekanik ve elastik özelliklerinin incelenmesi
Investigation of mechanical and elastic properties of traditional concrete nanosilica
Yazar: ESEN ÖNDER
Danışman: DOÇ. DR. HAKAN NURİ ATAHAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: İnşaat Mühendisliği = Civil Engineering

Özet:
Günümüzde pek çok farklı alanda hem taşıyıcı hem de dekoratif amaçlı olarak kullanılan ve oldukça önemli bir yapı malzemesi olan beton, düşük dayanım ve durabilite, geçirimlilik, segregasyon, çatlaklar, rötre gibi problemlerle karşı karşıya gelmektedir. Bu problemlerin üstesinden gelmek için betona çeşitli katkı maddeleri ilave edilmekte ve beton özellikleri iyileştirilmektedir. Beton özelliklerini iyileştirme amacıyla, ülkemizde de henüz yeni yeni tanınmaya başlayan ve silis dumanının
 (SiO2 ) amorf bir polimorfu olan yaklaşık çimento tanelerinin 1/100'i boyutundaki nano silikanın betona katkısı hakkında çeşitli araştırmalar yapılmıştır.

Betona ilave edilen nano silikanın meydana getirdiği etki, yüksek özgül yüzey alanı ve çok küçük taneli yapısıyla çimento hidratasyon mekanizmasını hızlandırarak, betonun dayanım, geçirimsizlik, işlenebilirlik ve durabilite gibi özelliklerini iyileştirmesinden ileri gelmektedir. Konuyla ilgili yapılan çalışmalarda çimentonun nanometre mertebelerindeki tanecik yapısıyla, betona çimento ağırlığının belirli bir yüzdesi olarak ilave edildiğinde, hidratasyon sonrasında meydana gelen Ca(OH)2  ile reaksiyona girerek ekstra C-S-H jeli oluşumunu sağladığı ve bunun sonucunda daha yoğun matrisli bir çimento hamuru ve beton oluştuğu görülmüştür. Deney ve testlerde öncelikle bir referans karışımı belirlenmiş daha sonra karışımda belirli parametreler sabit tutulmuş, nano silika tanecik boyutu, karışımdaki nano silika içeriği, kürleme süresi, cüruf veya uçucu kül vb. puzolan malzemelerin karışımdaki miktarı gibi parametreler değiştirilerek ilgili numunelerin basınç dayanımı, elastisite modülü, slump, birim ağırlık gibi test sonuçları kendi aralarında karşılaştırılmıştır.  

Bu çalışmada nano silika ve cürufun geleneksel betonlara ilave edildiğinde, betonların mekanik ve elastik özelliklerinde meydana getirdiği değişimleri görmek amacıyla öncelikle C280, C320 ve C360 olmak üzere 3 grup belirlenmiştir. Her grupta su/çimento oranı sabit tutularak karışımların yalnızca cüruf, yalnızca nano silika, cüruf ve nano silikalı bileşimleri üretilerek 28 ve 120 gün sonunda  basınç dayanım ve elastisite modülü değerleri  ölçülmüştür. Karışımlarda cüruf, toplam çimento miktarının ağırlıkça %20 si olarak çimentoyla ikame edilmiş, nano silika miktarı toplam çimento miktarının ağırlıkça %1 i oranında ilave edilmiş, su miktarı 190 kg/m3 olarak sabit tutulmuştur. Ayrıca kıvam kayıplarını önlemek amacıyla deneyler sırasında karışımlara süper akışkanlaştırıcı eklenmiştir.
Yapılan deneyler sonrası elde edilen sonuçlara göre betona belirli oranlarda ilave edilen cüruf ve nano silikanın çimentonun hidratasyon mekanizmasını hızlandırarak,  mikroyapı karakteristiklerini geliştirdiği ve bunun sonucunda yeni meydana gelen betonlarda mekanik ve elastik özelliklerin geliştiği görülmüştür.

Abstract:
Concrete is the most widely used building material in the world. Annual production is around 13 billion tons worldwide. Humans only consume water at a higher rate than concrete. In addition, concrete is easy to construct, easy to shape and economical. Today, concrete is used in many different fields for both carrier and decorative purposes and it is a very important building material. However, although it is a very useful building material, concretes also face problems such as low strength and durability, high permeability, high segregation amount, cracks and risk of cracking and shrinkage. Durability problems that may occur in traditional concrete; corrosion, freeze-thaw effect, wetting-drying, temperature changes, sulfate and acid attack, alkali-silica reactions. When these problems occur, there is a safety risk in concrete which also has to be safe. In addition, the concrete used as a building material is expected to have high strength for many years. Therefore, minimizing the problems that may occur in concrete is an important need. Various studies are being carried out to eliminate the problems that may occur in concrete and to increase the strength. At the same time, in order to prevent these problems, high volumes of various additives are added to the concrete and the concrete properties are improved.
The admixtures added to the concrete may be added to the concrete or mortar alone or in combination with several admixtures in order to achieve the desired concrete properties. Dosages for use are chosen so as to meet the desired performance criteria and at the same time do not impair the fresh and hardened properties of the concrete. There are 3 basic properties that a qualified concrete must provide. The first is machinability, the second is able to meet the desired strength characteristics, and the third is the ability to maintain its durability without compromising its integrity throughout its service life, as well as its steel reinforcement (retention). These three basic properties are intended to be achieved when selecting the types of additives and dosages for use. Another important point to consider here is the compatibility between additives and other materials used in concrete. The chemical properties of cement and mineral additives, physical, chemical and geometric properties of the aggregates used are important parameters that determine the effectiveness of the additives. Therefore, laboratory experiments to determine the compatibility and dosage of additives are of great importance.
It can exhibit the desired stress and deformation behavior of concrete with the additives added to the concrete. At the same time, production can be achieved in a sustainable manner with intelligent functions. Thanks to the additives added to the concrete, both expected properties can be obtained and long-lasting concrete can be produced with low cost and energy.


One of the most widely used chemical concrete admixtures today is the "plasticizer" admixtures. The effect of plasticizer additive on the workability of fresh concrete can affect hardened concrete performance at the highest level. In general, thanks to these additives, it has become possible to produce very low water / cement ratio concretes with high processability values, and high-fineness materials such as cement and mineral additives added to the fresh concrete are dispersed in the concrete without flocculating and thus higher hydration degrees are achieved. Thanks to these two important improvements of plasticizer admixtures in fresh concrete and mortar mixtures, it has been possible to significantly improve the strength and durability of hardened concrete. This makes it possible to produce concrete with almost zero permeability and strengths above 100MPa. On the other hand, in light of the developments in technology and the needs in practice, scientists have been able to produce additives called "new generation ve that improve the workability properties of concrete in particular as compared to conventional plasticizers. It has been possible to produce and apply concretes with high plastic viscosity and low threshold stresses. It is known that such additives may also alter the thixotropic properties of fresh concrete. Thixotropy is the ability to regenerate inter-particle gravitational forces in the static state when shear stresses are ineffective on fresh concrete, thereby increasing its viscosity. Practically, a thixotropic fluid will reduce its viscosity over time under the influence of constant shear stress, or vice versa, increasing its viscosity due to the regeneration of inter-particle tensile forces in the stationary state.
Understanding how chemical additives used for many different purposes affect the performance of concrete in both fresh and hardened conditions has become possible thanks to the macro and micro examination techniques offered by recent technological advances. On the other hand, these developments have allowed scientists to examine, examine and produce materials in the smallest dimensions. Therefore, advances in nano-science have had significant impacts in the field of building materials. One of them is the effect of nanomaterials on concrete technology.
The mechanical properties, compressive strength, strength under bending loads, ductility and microstructural properties of concrete reinforced with nano material are improving.
In order to improve concrete properties, various studies are carried out in our country. One of these is nano silica, which is about 1 / 100th of the cement particles, which is an amorphous polymorph of silica fume (SiO2), which has just started to be recognized. The effects of nano silica on concrete properties are promising.
The nano silica added to the concrete has a high specific surface area due to its nano size and accelerates the hydration mechanism of the cement due to its very small granular structure. With the accelerated hydration mechanism, cement and water reactions take place faster. Thus, the properties of concrete such as strength, impermeability, workability and durability are improved. However, there are some problems with the use of nano materials in cement-bonded composites. The most important of these methods are the effective distribution of nanomaterials into the matrix. The main reason for this is that nano materials have very high surface area. For this reason, it is very important that the concrete doped with nano material is mixed very well and reacts quickly. The first condition for obtaining the desired performance from nano additives is the homogeneous distribution of the nanomaterials effectively in the cement based matrix. Otherwise, the agglomerated nanomaterials worsen the results at extra cost as opposed to providing benefits.
In the studies carried out on the subject, it has been observed that when the cement is added to the concrete as a percentage of the weight of the cement with the granular structure in the order of nanometers, it reacts with Ca (OH) 2 formed after hydration to form extra CSH gel and as a result a denser matrix cement and concrete is formed. . In the experiments and tests, firstly, a reference mixture was determined and then certain parameters were kept constant in the mixture, nano silica particle size, nano silica content in the mixture, curing time, slag or fly ash etc. The parameters such as the amount of pozzolan materials in the mixture were changed and test results such as compressive strength, elasticity modulus, slump, unit weight of the related samples were compared.
In this study, when nano silica and slag were added to traditional concretes, three groups were identified as C280, C320 and C360 to see the changes in the mechanical and elastic properties of the concretes. In each group, the water / cement ratio was kept constant and only slag, nano silica, slag and nano silica compositions were produced and the compressive strength and elasticity modulus values were measured after 28 and 120 days. In the mixtures, the slag was replaced with cement as 20% by weight of the total cement amount, the amount of nano silica was added at a rate of 1% by weight of the total cement amount and the water amount was kept constant as 190 kg / m3. In addition, super plasticizer was added to the mixtures during the experiments in order to prevent loss of consistency.
According to the results obtained after the experiments, it was seen that slag and nano silica added to the concrete in certain ratios accelerated the hydration mechanism of the cement, improving the microstructure characteristics and as a result of this, mechanical and elastic properties developed in the newly formed concrete.