Nano silika ve mikro silika katkılı harçlarda durabilite ve mekanik özellikler
Durability and mechanical properties of mortar mixtures refined with micro and nano-silica
Yazar: MERVE BAŞARAN
Danışman: PROF. DR. HAKAN NURİ ATAHAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / İnşaat Mühendisliği Ana Bilim Dalı / Yapı Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2022
Konu: İnşaat Mühendisliği = Civil Engineering
Dizin: Beton = Concrete ; Nanomateryaller = Nanomaterials ; Silika jel = Silica gel

Özet:
Beton; çimento, agrega, katkı maddeleri ve karışım suyunu içeren karmaşık yapıya sahip olan bir yapı malzemesidir. Aynı zamanda en yaygın olarak kullanılan insan yapımı yapı malzemelerinden biridir ve her yıl yaklaşık 20 milyar metrik ton üretilmektedir. Beton yapımı için üretilen çimento, küresel karbondioksit emisyonunun büyük bir kısmına neden olmakta ve çevre kirliliği açısından büyük endişe yaratmaktadır. Bu etkiyi azaltmak için son zamanlarda birçok araştırmacı çimento kullanımını azaltmak ve betonun servis ömrünü artırmak için çalışmalar yapmaktadır.
Betonun servis ömrünün en önemli göstergelerinin durabilite ve mekanik performans olduğu söylenebilir. Beton, çeşitli boyutlarda ve farklı miktarlarda boşluklar içeren bir yapı malzemesi olduğundan, çeşitli çevresel faktörler betonun özelliklerini etkiler ve değiştirir. Sıvı ve/veya gazın betona sızması bu boşluklar sebebiyle gerçekleşir. Bu olaya betonun geçirgenliği denir. Boşluklara penetrasyon, difüzyon, basınç altında emme veya kılcal emme ile gerçekleşir ve beton içindeki boşlukların miktarına, dağılımına ve boyutuna bağlıdır. En yaygın durum kılcal emmedir, bu nedenle kılcal boşluklar betonun durabilitesi ve mekanik performansında önemli bir role sahiptir.
Betondaki boşluklardan içeri giren sıvılar veya gazlar zamanla betonda fiziksel veya kimyasal reaksiyonlara neden olur, sonrasında ise betonda çatlaklar oluşturur ve onun yapısını bozar. Bu kimyasal saldırılardan biri de sülfat saldırısıdır. Bu reaksiyonlar sonucunda oluşan ürünler birleşerek sertleşmiş betonun genleşmesine neden olur. Bu genişleme nedeniyle derin çatlaklar oluşur. Bu, betonun dayanım ve durabilitesinin azalmasına neden olur. Sonuç olarak da betonun servis ömrünü azaltır.
Betonun durabilitesini ve mekanik özelliklerini arttırmanın en temel yolu betondaki boşlukları azaltmaktır. Betondaki boşluklar azaltıldığında daha yoğun bir yapı elde edilir, böylece gaz ve sıvı girişleri daha zor hale gelir. Bu sayede daha dayanıklı bir beton elde edilir. Bu konuda bugüne kadar birçok çalışma yapılmış ve teknolojinin gelişmesiyle birlikte mikro ve nano boyutta birçok farklı mineral katkı maddesi denenmiştir.
Betonda kullanılan mikro ve nano malzemelerin, özellikle arayüz bölgesinde, filler etkisi nedeniyle genel olarak betonun özelliklerini iyileştirdiği gözlemlenmiştir. Ancak en iyi etki, silika içeriğinden dolayı nano-silika (NS) ve mikro-silika (MS) minerallerinde gözlenmiştir. Filler etkisinin yanı sıra silika içeren mineral malzemeler betonda puzolanik etkiye de neden olmaktadır. Nemli bir ortamda betondaki CH kristalleri ile silis arasındaki reaksiyonla C-S-H yapısının oluşması puzolanik reaksiyon olarak adlandırılır. Bu sayede daha yoğun bir arayüz ve daha güçlü bir yapı elde edilir. 
Nano malzemeler üzerine yapılan çalışmalar incelendiğinde genel olarak beton yerine harç numuneleri üzerinde yapıldığı görülmektedir. Bazı araştırmalarda harçlarda NS ve MS birlikte kullanılır ve bu sinerjik bir etki yaratır. Yapılan bu çalışma kapsamında sinerjik etkinin değerlendirilmesi amacıyla harç numuneleri üretilmiş ve mikro ve nano silika tek tek veya birlikte kullanılmıştır. Üretilen numuneler üzerinde mekanik (basınç ve eğilme) ve durabilite (sülfat direnci, su emme ve ağırlıkça su emme) deneyleri yapılarak bu malzemelerin ve bunların kombinasyonlarının karışımların mekanik ve durabilite özelliklerine etkisinin görülmesi amaçlanmıştır. Üretilen numuneler bir yılı aşkın bir süredir sülfat içeren çözelti içersinde tutulmaktadır ve belirli periyotlarda boy ölçümleri alınmaktadır ve MS ve NS kullanımının dış sülfat etkisinden dolayı kaynaklanabilecek deformasyonları ne ölçüde sınırlayabileceği araştırılmıştır. Sonuçlara göre mikro silika ilavesi basınç ve eğilme dayanımlarını %25'e kadar arttırmıştır. Öte yandan nano silikanın bu parametreler üzerinde önemli bir etkisi gözlenmemiştir. Ayrıca kapiler su absorpsiyon ve sülfat dayanım testleri sonuçları değerlendirildiğinde, mikro ve nano silikanın ayrı ayrı ilave edildiği karışımlarda sorptivitenin azaldığı gözlemlenmiştir. Ayrıca, mikro ve nano-silika kombinasyonu çok daha önemli (sorptivite için %50'ye kadar) bir azalma ile sonuçlanmıştır.

Abstract:
Concrete has a complex structure that includes cement, aggregate, additives and mixing water. It is also one of the most commonly used manmade building material and approximately 20 billion metric ton is being produced every year. The production of cement for concrete is contributing nearly 8% of global carbon dioxide emission and causes great concern in terms of environmental pollution. In order to reduce this effect, many researchers have recently been working to reduce the use of cement and increase the service life of concrete.
Concrete is the most consumed material after water by humans. Humans use concrete because it is easy to construct with and is easy to shape. Despite these advantages, concrete also has some disadvantages. Some of these are about durability problems and they called as sulfate attacks, alkali silica reaction, corrosion, chlorine effect, freeze-thaw effect etc. It should be taken some precautions before these durability problems happen.
Sulphate effect in concrete; These are the effects that occur as a result of chemical reactions, which are formed by the combination of internal or external sulphate ions, and calcium aluminate hydrate and calcium hydroxide, which are the products resulting from the hydration of the cement in the concrete, and affect the durability and strength of the concrete. As a result of these reactions, ettringite and gypsum products are formed and these products cause expansion in the concrete, resulting in cracks and exfoliation. As a result, the service life of concrete is reduced.
One of the important points in preventing the sulphate effect is to keep the amount of C3A and C4AF in the cement within certain limits. These cement components are directly effective in the formation of monosulfoaluminate and ettringite after sulfate attack. In addition, another important point in preventing suphate attack is to increase the use of mineral additives in concrete.
It can be said that the most important indicators of the service life of concrete are durability and mechanical performance. Various environmental factors affect and change the properties of the concrete, since it is a material which contains different amounts of pores in various sizes. Penetration of liquid and/or gas into the concrete occurs because of these pores. This phenomenon is called as permeability of concrete. Penetration into the pores is occurred by diffusion, absorption under pressure or capillary absorption and it depends on amount, distribution, size and connectivity of pores within the concrete. The most common way of situation is capillary absorption, therefore capillary pores have an important role in the durability and mechanic performance of concrete. 
The use of micro and nano mineral additives contribute the sustainability feature of concrete and improve its service life. Nano-silica, which is available in various average particle sizes in the market, has proven itself as an effective nano additive in concrete technology. Providing extremely high surface area, its effect on the fresh and hardened properties of cement-based materials is still being studied by the researchers. Liquids or gases penetrating through the voids in the concrete causes physical or chemical reactions in the concrete over time, they form cracks in the concrete and deteriorate its structure. One of these chemical attacks is the sulfate attack. Calcium hydroxide crystals are formed in concrete as a result of the hydration of cement with water. These crystals combine with sulfate to form calcium sulfate. In addition, tricalcium aluminate (C3A) in the structure of concrete forms monosulphate with water and needle-shaped ettringite crystals. Calcium sulfate and monosulphate formed as a result of these reactions combine and cause the hardened concrete to expand. Because of this expansion, deep cracks occur. This leads to a decrease in the strength and durability of concrete.
When concrete is examined microscopically, three different phases are taken into account; aggregate, cement paste and interfacial transition zone (ITZ). Among these, ITZ could be considered as the most critical one. The ITZ region has more porous structure than the other regions in concrete, and CH and ettringite crystals are more abundant in there. The most basic way to increase the durability and mechanical properties of concrete is to reduce the pores in the concrete. When the voids in the concrete are reduced, a denser structure is obtained, thus gas and liquid entrances become more difficult. Thanks to this, a more durable concrete is obtained. Many studies have been carried out on this subject so far, and with the development of technology, many different mineral additives in micro and nano size have been tried.
It has been observed that the micro and nano materials used in concrete generally improve the properties of concrete due to their filling effect, especially in interface zone. However, the best effect was observed in nano-silica (NS) and micro-silica (MS) minerals due to their silica content. Beside the filler effect, the mineral materials containing silica also cause pozzolanic effect in concrete. In a humid environment, the formation of C-S-H structure by the reaction between CH crystals in the concrete and silica is called as pozzolanic reaction. In this way, a denser interface and a stronger structure are obtained. The chemical reactions of the pozzolanic effect take place as follows;
SiO2 + H2O → H2SiO42-; 
Ca(OH)2 + H2O → Ca+2 + OH-; 
H2SiO4-2 + Ca2+ → C-S-H
In this study, durability and mechanical properties of nano-silica and micro-silica modified cement mortar samples were investigated. Average particle size of nano-silica, in colloidal form, as declared by the manufacturer, is 4 nm and 35 nm. In addition, a reference mixture was also prepared for every mixture type without any additives. Sulfate durability of the mixtures were also measured by monitoring the length change of the samples exposed to 5% Na2SO4 solution. 
Within the scope of this experimental study; In determining the mixing ratios of the mortars, conventional concrete designs with dosages of 350, 400 and 450 kg/m3 and water/cement ratios of 0,55, 0,45 and 0,35, respectively, were planned and the coarse aggregate parts of these designs (crushed stone 1 and 2) The amounts of "water, cement, sand (natural sand + crushed sand)" belonging to the mortar phases were determined by subtracting. Mortar samples of four different groups were produced according to the mineral additive ratios they contain. These were classified as reference samples containing no mineral additives, samples containing only micro silica, samples containing only nano silica, and samples containing both micro silica and nano silica. The MS ratio in the samples was decreased in 2,5 % steps starting from 10%. Then, the calculated amount of NS corresponding to the surface area of the reduced amount of MS was added. Thus, the area of total specific surface MS and/or NS mixtures (10% MS, 7,5% MS+0,075 NS, 5,0% MS+0,15 NS, 2,5% MS+0,025 NS and 0,30% NS) is theoretical. It is set to a fixed value of 0,95 km2 per m3 of mortar. In doing so, the specific surface area of MS and NS was accepted as 15,000 and 500,000 m2/kg, respectively. In addition, additions were made to the samples with a water/cement ratio of 0,45, different from the 9 sets of mixtures made at the beginning. 8 sets of mixtures (0,6% NS, 0,9% NS, 2,5% MS+0,3% NS, 2,5% MS+0,6% NS, 2,5% MS+0,9% NS, % 5 MS+0,3% NS, 5% MS+0,6% NS and 5% MS+0,9% NS) were added.
In the production of the mortar samples, a laboratory mixer that mixes on the vertical axis and whose mixing speed can be controlled was used. After weighing all the mixtures, first of all, dry materials (cement, aggregates, micro silica) were brought together in the mixer's chamber and mixed. Afterwards, most of the water was added and mixing was continued.
When the studies on nano materials are examined, it is seen that they are carried out on mortar samples instead of concrete in general. In some research, NS and MS are used together in mortars and this create a synergistic effect. The phenomenon used to "maximize the performance of concrete" is the synergistic effect. In the presented study, in order to assess the synergistic effect, mortar samples were produced and micro and nano-silica were either used individually or together. It was aimed to see the effect of these materials and their combinations on the mechanical and durability properties of the mixtures by performing mechanical (compressive and flexural) and durability (sulfate resistance, water absorption and sorptivity) tests on the produced samples. Produced samples are kept in sulfate-containing solution for more than one year and length measurements are taken at certain periods, and it has been investigated to what extent the use of MS and NS can limit the deformations that may occur due to the external sulfate effect.  According to the results, the addition of micro-silica increased the compressive and flexural strengths up to 25%. 
In terms of compressive strength results, it was observed that the compressive strength increased with the increase in the use of MS. Although NS does not have a positive effect on its own, it has been observed that NS increased the compressive strength when used with MS. According to the flexural strength results, the flexural strength generally decreased as the use of NS increased. On the other hand, no significant effect of nano-silica on these parameters has been observed. In addition, when the capillary water absorption and sulfate resistance tests results were evaluated, it was observed that the sorptivity decreased in the mixtures which micro and nano-silica were added individually. Moreover, the combination of the micro and nano-silica has resulted in a much more significant (up to 50% for sorptivity) reduction.