# Experiment and Analytical Model for Pore Structure of Early-Age Composite Cement Pastes by LF-NMR
## 1 Introduction  
Concrete is a material that plays a fundamental role in engineering projects due to its versatility, durability, and cost-effectiveness. However, cracks often develop at early ages [1], which can significantly compromise the safety and durability of concrete structures [2]. Fly ash, slag, and silica fume are industrial by-products that can negatively impact the environment and human health when directly disposed of in landfills or oceans [3, 4, 5]. However, these materials are also widely considered volcanic ash additives, which can improve their workability, mechanical properties, and resistance to volume deformation when added to concrete [6]. Moreover, the replacement of cement with these pozzolanic materials can save energy and reduce emissions [7]. The characteristics of concrete blended with fly ash, slag, and silica fume have been studied widely [8, 9]. Current research on concrete incorporating supplementary cementitious materials such as fly ash, slag, and silica fume primarily focuses on their macroscopic properties, including permeability, shrinkage, strength, and durability [10, 11, 12]. However, pore structure is a critical characteristic of cement-based materials (CBMs), as it directly influences macroscopic properties and plays a key role in the crack resistance and long-term durability of concrete [13] Moreover, mineral admixtures like fly ash, slag, and silica fume significantly impact the pore structure [14, 15, 16]. In the case of fly ash, its reaction reduces the pore sizein concrete, resulting in a denser pore structure. [PERSON] et al. [17] demonstrated that the fineness of fly ash has a substantial effect on the pore structure of blended cement paste, and the uniform distribution of its particles can further optimize pore size and distribution. With latent hydraulicity, slag forms a group of cement with better reactivity and performance combined with Portland cement. Many studies focusing on the properties of slag-blended cement have been carried out. [PERSON] [18] found that slag accelerated the hydration of cement and refined the pore structure of cement pastes. [PERSON] et al. [19] studied the shrinkage of cement pastes blended with slag. The results showed that the replacement of slag can effectively restrain the shrinkage of cement pastes. Silica fume has been found to reduce the pore size of cement pastes due to its extremely fine particles and high activity. [PERSON] et al. [20] examined the effect of silica fume on the microstructure of cement pastes and found that its addition resulted in a finer pore structure compared to ordinary Portland cement (OPC) paste. In spite of extensive research on pore structures of composite cement pastes, a proper model to evaluate the effect of mineral addition on pore structure have not been established. As for the technology of microstructure measurement, there are many methods to characterize pore structures, such as mercury intrusion porosimetry (MIP), BET, and scanning electron microscopy (SEM). MIP is the most widely adopted method for research on pore structure. However, MIP has limitations. Firstly, it measures the largest entrance to a pore rather than the actual internal size, and secondly, it cannot be used to analyze closed pores, as mercury cannot enter these pores. Nitrogen adsorption and BET analysis are also widely used for testing microporous materials [21; 22]. However, there are arguments about the application of nitrogen adsorption in studying the surface area of cement paste [23]. The controversy surrounding nitrogen includes the \"bottleneck\" theory that nitrogen molecules would be excluded from pores with small openings due to their being larger than water particles [24]. The pre-drying of specimens would cause damage to pores. SEM is extremely useful in examining the microstructure of mortars and concretes [25]. It can directly capture images of pores for analysis. However, this method analyses information on 2D cross sections and may have a one-sided expression on pore structure.