“Caking” Process in Green Cement Composites under the Impact of Environment

Introduction. The problems of resource and energy saving during production of cements and concretes on their basis are of great importance in the world. An important trend in solving these problems is the development of green cements, which contain various mineral additives of natural and anthropogenic origin. The advantages of the green cements compared to the clinker-based cements are revealed from both the environmental point of view — reduction of carbon dioxide emissions into the atmosphere, use of the industrial by-products, and the technical point of view — improvement of the technological properties of concrete mixes, improvement of the physical, mechanical and operational properties of con[1]cretes. At the same time, replacement of the clinker component in Portland cement by the mineral components leads to the change of the physical and mechanical properties of cement, including that taking place during the “caking” process in conditions of interaction with the environment. The research presented in the paper aims at studying the effect of long-term storage of plain cements and green cement composites in conditions of free interaction with the environment on the physical and mechanical properties of the cement stone.

Materials and Methods. Various types of cements were used in the experiments. Green cement composites were obtained in the laboratory by intergrinding with the slag and fly-ash respectively. To improve the grinding process and inhibit the sorption process, a grinding intensifying agent was added into the composition of green cement composites. The impact of the environment on the cement sorption process was assessed by measuring indirect parameters: specific surface area, angle of natural repose, bulk density and cement stone compressive strength.

Results. The obtained results indicate that the specific surface area of cement samples stored in the open air for 30 days reduces significantly due to the “caking” process, on average by 25% in type I cements, whereas in green cement composites this process is less intense and amounts to 15%. A similar consistent pattern is observed when measuring the angle of repose and bulk density. The decrease in the angle of repose and high values of bulk density for type I cement are related to the active sorption of moisture from the environment, which leads to the formation of hydrate “bridges” upon interaction of particles. Compared to the control samples, the compressive strength in the cement stone samples tested at the age of 28 days of curing decreases on average by 25–30% in type I cements and by a less extent of 15–20% in green cement composites.

Discussion and Conclusion. When storing different types of cement in conditions of free interaction with the environment, moisture is sorbed and the hydrate “bridges” are formed on the surface of the particles. This process goes more intensely in type I cement due to the highly active surfaces of its particles, which hydrate faster, resulting in a decrease of cement activity by 30%. At the same time, the green cement composites lose their activity less significantly – by 20%. This consistent pattern can be explained by the less active surfaces of the particles of mineral additives included into the composition, as well as by addition of a grinding intensifying agent into a composition.

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