Effect of synthetic C-S-H compounds on fresh and hardened properties of self compacting concrete

With the development of the building construction in the world, the sector now designs higher and more specialized buildings, uses prefabricated buildings within the sector more often, highlights architectural details in projects and enjoys a higher pace of productivity, which made it essential to manufacture special kinds of conrete. Self-compacting concrete (SCC), which was first developed in the Tokyo University in the beginning of 1987, is a special type of concrete which spreads homogenusly and settles into the desired area without any external force (vibration, jolting etc.) but its own weight. SCC also minimizes the risk of workmanship defects born out of the machinability on the worksite with the help of its fluid consistency, its high weathering resistance and its ability to easily settle in the densely packed and narrow section of structural elements. As SCC is a fluid concrete, it is essential to know their rheological and fresh state properties. A negativity that could occur in fresh state properties would affect the SCC's mechanical and durability properties negatively as well. That is why is it important to know SCC's self-compacting mechanism well while creating a good design. For self-compactablity, the mortar phase of the concrete should be highly fluid and it should flow through reinforcements without decomposing with the coarse aggregate. Three criteria have been identified for the occurance of this mechanism. These are: the limiting of the maximum aggregate diameter, a low water/dirt ratio and the use of fluidizing additives. Superfluidizing chemical additives are one of the main components for SCC. An important part of the high dosage fine material that is supposed to be in SCC matter has been replaced by blast furnace slag (BFS), which is an industrial waste material, since its pozzolanic property has been discovered, in order to decrease costs and minimize environmental damage. In today's world, time is the most important parameter. In order to get to the next step after construction production and concreting, it is necessarry to wait for the concrete to reach the proper strength. It is possible to reduce this waiting time by hydrating the binding material. The temperature and hydration time of the concrete and the pozzolanic binding materials are highly related. The hydration time of concrete poured in warm temperatures (20-30) is a lot faster compared to concrete poured in 5-10 degrees. That is why it is necessary to take some extra precautions for concrete poured in cold weather, such as a hot steam cure, heating the mold or using accelerating additives. Of course, some of these methods can be laborious, whereas some could harm some properties of the concrete. Studies done in recent years use the C-S-H nanoparticules synthetically acquired from C-S-H compounds, which is a hydration product, in order to accelerate concrete hydration. With this method, nucleation reaction, which are required under normal circumstances for crystal growth, become unnecessary and the dead stade of concrete hydretian is shortened. With this method where hydration kinetics are improved with nanoparticul additions, we see that the nuclei of the hydration products are not only formed on and between concrete parts but also on the nanoparticules. Later on, hydration units form with a crystal growth mechanism with the help of these nuclei. Thanks to the nanoparticule addition, hydration units will start to form between concrete pieces as well, therefore the formation of closely-graded microstructures will occur shortly, compared to a reference mixture without nanoparticules. This way it is possible to acquire more closely-graded concrete-based material with high compressive strength at early ages. In this dissertation we have analyzed the affects of the usage of C-S-H compounds as an alternative to use blast furnace slag (BFS) commonly used to correct the negative affects of slow hydration in cold weather and their effects on SCC's fresh and solid properties. This lab study has been conducted in 8 degrees of ambient temperature. Firstly, the amount of water fully meeting the requirements have been identified as 185 lt and 500 dosages of cement binding reference cement has been produced. Then, 40% of the cement was subtracted, and 4 different SCC's have been prepared, substituting them with BFS at the rates of 1:1, 1:1.25, 1:1.5 respectively. To these mixtures, 2% of synthetic C-S-H was added on top of the fine material. Four additional mixtures were prepared, and after conducting SCC fresh state experiments on all these 8 mixtures, they were put in 10x10x10 cm steel forms. We observed that all mixtures achieved a 65 cm spread within the standard time time frame, and that the BFS addition increased the spread diameter. After the forms have been disassembled after 9 hours, half of the samples were put into a 8 degree water pool, while the others were put into a 20 degree water pool. The sample cured in two different temperatures underwent pressure tests that lasted for 9-hours, 1-day, 3-days, 7-days, 28-days and 90-days, respectively. The 8-degree-cold water pool's water temperature was regularly measured and ice was added when needed in order to keep a fixed temperature. Also, the viscosity coefficient was identified with the measured torque value corresponding to the rotational speed of a reometer device with 5 min intervals when the 8 mixtures were in a fresh state and after a total 1 hour, the differences in the viscosity coefficient were observed in order to identify viscosity loss. The results of all the experiments showed higher pressure resistance in the 8-degree -cured samples, especially when analyzing the pressure strength of the 1, 3 and 7 day long experiments. All of the synthetic C-S-H added samples showed better pressure resistance when compared to the ones without additives. However the 20-degree-cured samples showed that after the 3rd day, all samples, with or without synthetic C-S-H additives, caught up to the same rates of their own groups. Additionally we can say that there has been no negative effect on the cement at advanced age when added synthetic C-S-H compounds. Another important fact is that especially BFS added SCCs showed within their own groups (including the 28-day-group) that 8-degree-cured samples had lower resistance compared to the 20-degree-cured samples. This shows that BFS's puzzolanic effect is better on higher temperatures. In samples with synthetic C-S-H additives we see that, especially in the 8 degree cured samples, the negative effects of cold weather conditions were substantially fixed and that form diassembly could be done earlier. Experiments done with the reometer device, which have been done for an hour with 15 min intervals, showed no significance difference in the viscosity coefficient of the samples within the same group. However corresponding to the fixed amount of water, the increase of BFS has caused an increase in the shear stress treshold and the viscosity coefficient. It has been observed that synthetic C-S-H additives slightly decreased the viscosity coefficients. The results of capillarity tests showd that by increasing BFS substitution rates the capillarity decreased and that by using both BFS and synthetic C-S-H additives decreased capillarity even more. To sum up, SCCs where BFS are used as mineral additions benefit from synthetic C-S-H compounds added into the concrete, when SCC has to be manufactured in cold weather conditions, as these help reduce the negative effects of low temperature and positively affect concrete performance.