Effect of curing conditions on the efficiency of ground granulated blast-furnace slag in high performance concrete

In order to obtain an impermeable and durable concrete, having a sufficient strength, curing applications are of great importance. The poor curing after casting on site may cause insufficient characteristics of concrete, in an irreversible way. In the last decade, self compacting concrete which is also widely used in repair and strengthening of structures, in the production of prefabricated elements and for the structures which include heavy reinforcement and complicated molding systems. SCCs have significant amount of fine material such as ground granulated blastfurnace slag (GGBS), fly ash and silica fume are commonly used in concrete because they improve durability and reduce porosity; improve the interface with the aggregate. Economics (lower cement requirement), energy, and environmental considerations have had a role in the mineral admixture usage as well as better engineering and performance properties. The lower cement requirement also leads to a reduction for CO2 generated by the production of cement. The engineering benefits from the use of mineral admixtures in concrete result partly from their particle size distribution characteristics, and partly from the pozzolanic and cementitious reactivity. However, in order to provide sufficient level of pozzolanic reaction, water curing should be extended in longer periods than ordinary portland cement concretes. For this reason, the importance of the application of curing becomes more important. Recently, construction processes are shortened due to the improved technology. Concrete is expected to accomplish sufficient strength as soon as possible in order to pass to the next step of the construction. Particularly in precast industry, strength gaining of concrete is accelerated by applying steam curing to structural elements. So as to achieve high early strength without any negative effect on concrete properties, numerous studies have been carried out to develop alternative methods. Recently in studies, it is observed that early strength values were substantially increased due to addition of synthetic C-S-H, hydration product of cement, to the fresh concrete. Sensitivity of pozzolan blended concrete to curing conditions is well-established. Utilization of pozzolan in self compacting concrete has been excessively studied. However, publications upon the impact of curing compounds on the properties of pozzolan added concretes are very limited. Besides, no researches on the properties of synthetic C-S-H added concretes cured in different conditions were found. In the studies on synthetic C-S-H, it can be seen that more emphasis is put on the effect of C-S-H formulation and on the cement hydration process. And in the studies on curing materials, mechanical and physical properties of concretes together with shrinkage due to water loss are examined but are not focused on the micro structural properties of concrete. In the scope of this experimental study, fresh and hardened properties of self compacting concrete prepared with or without blast furnace slag which not only reduces cost but also improve some mechanical and physical properties of concrete, were investigated. For this purpose, 10 different mixes were produced. The first mixture contained only Portland cement as cementitious material, and in the remaining four concrete mixtures, 40% of the cement was replaced with blast furnace slag by 1:1 or 1.1,25 or 1:1,5 or 1:2. A commercial synthetic C-S-H admixture was also added (2% of binder content) in the same concrete mixture as a second series. Water/cement ratios and water reducing admixture dosages adjusted so as to obtain 70±3 cm spread. Additionally, spread time, V-funnel, air content, unit weight, rheometry and segregation tests were performed on fresh concrete. Concretes were casted in 10 x 10 x 10 cm cubes, 7 x 7 x 28 cm prismatic molds. Furthermore, mortar phase of concrete obtained from fresh concrete passing 4 mm sieve, and were casted in 4 x 4 x 16 cm prismatic molds. The specimens were divided into 6 groups of curing conditions after demoulding. The first group of concretes were continuously kept in 21±2oC water, continuously stored in air and stored under wet burlap for 7 days then continuously stored in air. Acrylic, paraffin and resin based curing compounds applied on the surface of concrete specimens of the remaining three groups and kept in laboratory conditions. Compression, splitting, conductivity and capillary tests performed on the concrete samples and shrinkage, weight loss, bending tests and SEM measurements were carried out on mortar specimens. According to the experimental results of fresh concretes, required properties was maintained except 1:2 substitution. The target spread diameter was achieved with less super plasticizer when incorporated blast furnace slag. Similar results were also seen in the V-funnel test. Positive impact of slag on the workability of concrete was seen. Due to addition of synthetic C-S-H, more super plasticizer admixture is required to obtain certain workability. This shows the negative impact of synthetic C-S-H on the workability of concrete. Viscosity was measured by the relation between the measured torque and shear rate of the rheometer test. The viscosity value decreased with blast furnace slag addition but increased with decreasing the water / binder ratio. The hardened properties of concrete were examined at 9 hours, 1, 3, 7, 28, 90 days. The synthetic C-S-H contributed to hardened concrete mechanical properties developed in a positive direction at early age but no developments were seen in further ages. The highest performance was provided for the water curing method. The resin based curing material showed the best performance at an early age, acrylic based curing compound yielded the best concrete properties in further ages among the chemical cures. Mechanical strength of blast furnace slag added concretes increased as the amount of blast furnace slag increase. After an optimum point, at around %50 of the total binder content, the additional blast furnace slag did not improve the compressive strength. This can be attributed to the presence of unreacted blast furnace slag, acting as a filler material in the paste. Efficiency coefficient was reduced with increase of blast furnace slag amount. The reason of reduction is the remnants of unhydrate blast furnace slag particles in cement paste. Efficiency of blast furnace slag was also calculated for each curing methods. It is clearly shown that all the curing methods led to higher values than air curing. Furthermore, according to the efficiency values, after 28 days strength gain of concretes, which contain blast furnace slag, had more than concretes which produced only Portland cement was understood. Positive effects of curing applications were observed on the durability test such as capillarity and conductivity because of reduction the loss of water. Performance ranking was water, wet burlap, acrylic, paraffin, resin and air curing methods. In addition, blast furnace slag added concrete's resistivity had higher values, 5 to 7 times, against only Portland cement specimens. Capillarity coefficients were decreased by increasing of blast furnace slag amount. Concrete specimens cured by applying curing compounds exhibited higher efficiency in decreasing drying shrinkage deformation than specimens kept in air. Among the three curing compounds investigated, resin based curing compound had the best performance at shrinkage, acrylic-based curing compound had the best performance at weight loss test. In the ESEM images, microstructures of the samples were studied at 1, 3 and 28 days of ages. C-S-H, CH and ettringite formations were monitored. According to performance-cost values, acrylic based cure material is better option than other chemical curing compounds. Under wet burlap have close performance to water curing methods. Therefore, in the future studies, using under wet burlap then curing compound can be investigated.