Investigating the effects of nano silica usage on rheological, mechanical and microstructural properties of self-consolidating concretes

Concrete constitutes of cement, aggregates, water and if necessary various additive materials, at certain amounts. With the progress in concrete technology, many different types of concretes have been come out in recent years. The raw materials of concrete are abundant in nature. Besides cheap production and easy forming, concrete is the most common building material in the market by its high durability against environmental factors. Besides the parameters of design, vibration is one of the unavoidable needs at concrete casting step for long service life and durable reinforced concrete structures. Thus, voids in the fresh concrete could be removed and in this way it becomes possible to obtain more dense and low void concretes. Nowadays, by the development of construction technology special projects and retrofitting projects with dense reinforcements, narrow sections and rebar spaces are widespread. These developments bring many difficulties about the vibration applications with it. Accordingly, "self-consolidating concrete" have been developed, which can be consolidated under its own weight, passes through the reinforcements easily and fills the formworks excellent, without any internal or external vibration effort. On the other hand, today it can be seen that there are some studies on using "nano technology" for enhancing the properties of concrete. The most common research field of using nano technology on cement based mixtures is the usage of nano particles and nano sized materials. As a result of the studies using nano particles in cement based systems, improvement of mechanical and durability properties of conventional concrete can be seen. In this experimental study, the reduction of total fine material content used in self-consolidating concrete designs have been targeted by means of the addition of colloidal nano silica (CNS) having various average particle size. By this way the total aggregate content of the mixtures was increased. Accordingly, the effect of the modifications made in concrete designs, on the mechanical and elastic properties of concrete has been investigated. Before starting the production of main mixtures, in order to choose the proper aggregate gradation, chemical admixture and binder amount, many preliminary mixtures have been tested. Cement and admixture compatibility tests were performed by using mini slump method in order to choose the proper chemical admixture type for the study. After determining the chemical admixture type, aggregate gradation, cement and fine material dosages, a reference mixture was produced which has the desired self-consolidating properties. The reference mixture contains 400 kg/m3 of cement and 160 kg/m3 of fly ash. The fly ash content of reference mixture was reduced at four steps to 120 kg/m3, 80 kg/m3, 40 kg/m3 and finally to zero. The volumetric reduction occurred by this fly ash decrease was filled by aggregates, without changing its grading. In this way total aggregate content of mixtures was increased. While this process was carried out, colloidal nano silica addition was practiced to see its compensating effects of fly ash in concrete. Three different types of nano silica were used throughout the study with average sizes of 35 nm, 17 nm and 5 nm. For each content of fly ash five different amounts of nano silica were used. These amounts were 0% to 2.5% with 0.5% increments for 35 nm and 17 nm average particle sized nano silica. However, it was not possible to use this range for 5 nm average particle sized nano silica due to the large increase in surface area; thus the rage of 0% to 0.5% with 0.1% increments were used. Mixtures prepared by this way were tested in three major topics. The first topic contains testing the rheological properties of mixtures. Second topic is about investigating the mechanical properties and the third one contains the microstructural investigations. Within the scope of the first topic, which is rheological investigations, standardized single point tests which are T500 test, slump – flow test, V funnel test, static penetration resistance test and sieve segregation test were performed and mixture properties and behaviors were investigated. Visuals of every mixture recorded and categorization was made according to the classification of visual stability index. Besides, rheological parameters such as static yield stress, dynamic yield stress and plastic viscosity were determined by two-point rheological tests by using an ICAR concrete rheometer. Mixtures which exhibit the self-consolidating properties were accepted as "successful mixtures" according to the results of the tests under this topic. On the other hand, mixtures which could not exhibit the desired self-consolidating properties were accepted as "unsuccessful mixtures" and they were excluded from the study. In the second part of the study, which investigates the mechanical properties, only the successful mixtures have been considered. Successful mixtures were produced one more time and were casted to 100 mm diameter and 200 mm height molds. Specimens were cured for 28 days and 120 days. After curing period, compressive strength tests and elastic modulus tests under compression were performed. As a part of the third topic, mortar samples in various sizes and shapes were taken during the concrete production from the previously determined 14 series of mixtures. These samples were cured under the same conditions and duration time with the specimens prepared for mechanical test. After curing period of the specimens, in order to determine the pore size distribution, mercury intrusion porosimetry (MIP) tests, and in order to analyze the mineral composition of samples, X-ray diffraction (XRD) analysis were performed. To understand the amounts of some of the hydration products after the hydration process of samples, thermogravimetric analysis (TGA) were performed and finally by using scanning electron microscope (SEM), micrographs were taken from the samples and visual investigations were performed on the hydrated mortar. According to the results of the rheological tests, by decreasing the average particle size of nano silica, number of unsuccessful mixtures were increased and range of nano silica usage dosage was limited. As a result of evaluating the single point and two-point rheological tests together, the threshold dosages for 35 nm, 17 nm and 5 nm average particle size nano silicas' were 1.5%, 1.0% and 0.3% respectively for the successful mixtures. Rheological parameters of successful mixtures were changed between 150 Pa and 200 Pa for static yield stress, 30 Pa and 40 Pa for dynamic yield stress and 20 Pa.s and 30 Pa.s for plastic viscosity. Mixtures having values under these intervals have shown an increasing segregation tendency, and over these values have presented extremely thickening and flow loss behavior. By decreasing the fly ash content in the mixtures, keeping the cement and water content same, strength decrements were observed. These strength decrements could not be compensated by using nano silica. However, the usage of fly ash and the nano silica together was extremely effective on the increment of elastic modulus, but, its effects on strength development is not very significant. Both on 28th and 120th day, a major part of the samples have higher elastic modulus results compared to the reference sample. Especially on 120th day, elastic modulus results of the high amount of fly ash content samples were noteworthy. The microstructure investigations have shown that nano silica usage created a denser and compact matrix phase. Besides, the existence of interfacial transition zone (ITZ) between aggregate particles and hardened cement matrix phase cannot be observed in the micrographs. Disappearing of the ITZ phase which is called as the weakest chain of the concrete structure can be the explanation of the extraordinary elastic behavior of the samples. As a result of the experimental studies, it can be concluded that the nano silica usage in self-consolidating concrete is a very effective method in order to decrease the total fine material amount. Besides, one of the main concerns of self-consolidating concretes, regarding their lower stiffness tendency compared to the conventional concrete mixtures, can be overcome with the use of nano silica by optimizing its size and the amount. In this regard, it has been shown that the use of nano silica in modern concrete technology as a multi functioning material which improves the fresh state, mechanical and elastic properties of self-compacting concretes and also yields the design of special concrete mixtures is possible.