Effects of Seashell and Lumashell limestone powder on concrete thermal resistance

The structure must be able to maintain its stability and resistance in the event of a fire to protect human life. From time immemorial, concrete has been known to have fire-retardant properties. Thatchr('39')s why the biggest concern with concrete structures at the time of the fire was the reinforcement and their non-flow. But with the development of concrete technology, the focus has also shifted to improving the mechanical properties of concrete to increase its fire resistance. The use of pozzolans and additives in concrete to achieve high-strength and durable concrete has been in the concrete industry for several years. In this study, the role of seashell and lumashell powder and their effects on the mechanical properties of concrete and achieving the optimal percentage of using shellfish powder to achieve high fire resistance and durability have been studied. For this purpose, laboratory tests involving slump evaluation, water absorption percent, and compressive strength under high temperature were conducted on samples in which the replacement ratios of Portland cement with the same weight of shell powder were 2.5, 5, 10, 15 and 20% weight percent. Experimental results showed that seashell and lumashell powder increase the hydration rate and consequently caused an increase in the heat of hydration which resulted in a faster loss of water in the concrete. Furthermore, Seashell and Lumashell powder absorbed more water than cement due to their finer particles. All these ultimately resulted in a reduction in concrete slump such that regardless to the shell powder type, adding 2.5, 5 and 15% of shell powder, in average led to 13.5, 27.5 and 52% reduction in concrete slump respectively and it became approximately constant when the used shell powder was in excess of 15%. In addition, results showed that the presence of seashell and lumashell powder decrease water absorption in samples and made them more impenetrable. It happened because by filling the void in the cement paste with fine powder particles, the permeable cavities have been reduced and the connection paths of the cavities have been somewhat blocked. Replacement of cement with 2.5%, 5% and 10% of Seashell and Lumashell powder led to (27%, 44%, 73%) and (7%, 59%, 73%) reduction in concrete water absorption values respectively and it became approximately constant when the used shell powder was in excess of 10%. The results of this study also showed that the replacement of cement with Seashell and Lumashell powder slightly increases the thermal resistance of concrete and the amount of replacement of 5% by weight of cement with shell powder is reported as the optimal percentage. Adding more than 5% shell powder as a substitute for cement, regardless of its type, is harmful and significantly reduces the thermal resistance of concrete. Also, the results of laboratory tests showed that when concrete is exposed to high temperatures, properties such as load-bearing capacity and durability are reduced, leading to cracking, loss of compressive strength and concrete divot. Finally, it can be concluded that the optimal percentage of using seashell and lumashell powder instead of Portland cement can lead to a suitable concrete in terms of respect for the environment.