Analysis of Pore Structure in Cement Pastes with Micronized Natural Zeolite

Abstract

The continuous development of urban areas around the world led to an increase in construction material use and demand, with concrete seeing significant market uptake. Although significant progress has been made to reduce the environmental impact of concrete, there is still a stringent need for improvement. One of the most widely used methods to reduce the environmental impact of the cement industry and the construction industry alike is the replacement of ordinary Portland cement (OPC) by supplementary cementitious materials (SCM). Aside from by-products of industry, SCMs could also come from natural sources. Taking into account the porous structure of zeolites and their contribution to the improvement of the mechanical and durability properties of cement-based materials, the analysis of pore structure in cement pastes incorporating micronized natural zeolite is deemed necessary. In this research, the OPC was replaced by zeolite in three different percentages: 10%, 20%, and 30% by mass. The evolution of pore structure was investigated by means of nuclear magnetic resonance relaxometry at the curing ages of 1, 7, and 28 days. The microstructure of cement pastes was assessed by scanning electron microscopy investigations at 1, 7, 14, 21, and 28 days. The obtained results show that smaller pore sizes are present in cement pastes containing zeolites during the first 7 days. However, at the age of 28 days, the reference mix exhibits a similar pore structure to the mix containing 10% micronized zeolite due to the presence of larger amounts of hydration products. Increasing the replacement percentage to 30% results in larger pores, as indicated by larger values of the relaxation time.

The Use of Natural Zeolites in Cement-Based Construction Materials—A State of the Art Review

“…While the pozzolanic properties of zeolites make them suitable as supplementary cementitious materials, leading to improved mechanical properties of cement-based construction materials [8], their high porosity and the configuration of their internal structure, as shown in Figure 1, leads to two different effects. In the fresh state, zeolites containing cementitious materials exhibit a lower workability, as reported by several recent studies [3][4][5]9], but without significant influence on the setting time [4,10]. In the hardened state, the water absorbed by the zeolites is gradually desorbed and contributes to the long-term hydration of the cement particles, thus acting as an internal curing agent [5,8].…”

“…This was attributed to the so-called dilution effect [32]. This effect occurs in cases of high replacement percentages and more significantly impacts the com- It is generally agreed that due to the low amount of CH produced at the early ages of cement hydration, the pozzolanic reaction of natural zeolite and, consequently, the secondary formation of CSH [4] is limited. This leads to lower values of mechanical properties of cement-based materials with natural zeolites at early ages [32].…”

“…Moreover, the initial setting time values did not change with the increase in the replacement percentage, while higher percentages than 20% had no influence on the final setting time. The flow, however, decreased with the increase in the zeolite content in an almost linear manner [4].…”

“…In the construction industry, especially when referring to cement-based materials, natural zeolites are mostly known for replacing Portland cement due to their pozzolanic nature [2]. The large percentages of SiO 2 and Al 2 O 3 found in zeolites can react with Ca(OH) 2 produced during the hydration process of cement and form additional phases of calcium-silicatehydrate (C-S-H) [2][3][4]. This results in a denser structure of the cementitious matrix, leading…”

“…Coatings 2024, 14, x FOR PEER REVIEW 2 of 28 silicate-hydrate (C-S-H) [2][3][4]. This results in a denser structure of the cementitious matrix, leading to improved strength and durability characteristics of the material [5].…”