From the initiation of hydration when water is added to cement until the end of service life of cement-based materials, the influence of water permeates almost the entire lifecycle, comprehensively affecting both the development and degradation of their properties.Water transport is a critical fundamental factor determining the durability of cement-based materials (CBMs) and service life of structures. Since most concrete structures under service are unsaturated, capillary water absorption is the primary and efficient mechanism for water transport in concrete. An in-depth analysis of capillary water absorption process and sorptivity is of great significance for quantitatively studying and improving the durability of CBMs.

This study systematically investigates the capillary water absorption process and its anomalies in CBMs. While existing theories attribute sorptivity anomalies to factors such as gravitational effects, material heterogeneity, and secondary hydration of unhydrated cement particles, certain experimental phenomena remain inadequately explained. Through macro-and micro-scale testing combined with theoretical modeling, it is hypothesized that the unique physicochemical interactions between water and C-S-H gel may underpin these critical anomalies.

Although sorptivity testing is operationally simple, its results are highly sensitive to experimental conditions (e.g., temperature, humidity, and initial saturation). The moisture state of specimens plays a decisive role in sorptivity measurements, while inconsistencies in testing protocols can lead to disparate outcomes. Consequently, adopting a scientifically rigorous and practical testing methodology is imperative. This paper reviews international standards for sorptivity testing, including the widely recognized and scientifically reliable ASTM C1585—20 and ISO 15148:2002. However, these standards exhibit limitations, such as short testing durations that overlook the significance of secondary sorptivity. Given the anomalous long-term absorption behavior of CBMs, a systematic consideration of the two-stage sorptivity process is essential. Furthermore, China urgently requires the development of a national testing standard tailored to its specific material and environmental conditions.

To elucidate the influencing factors of sorptivity, this study compiles and analyzes experimental data from diverse studies. The results reveal that cement content, water-to-cement ratio, and the dosage of supplementary cementitious materials profoundly alter hydration kinetics, hydration products, and pore structure, thereby significantly affecting sorptivity. Beyond material composition, variations in pretreatment methods (e.g., drying protocols) also induce substantial discrepancies in test results due to differing moisture states. Moreover, the coupling effects of these factors further complicate the interpretation of sorptivity behavior. Therefore, during the sorptivity test, it is crucial to adopt a scientific pretreatment protocol to minimize the impact of drying, equilibration, and moisture content on the test results.

The study also explores correlations between sorptivity and other macroscopic performance indicators. Sorptivity demonstrates strong linkages with transport properties such as permeability and electrical flux, as these metrics are fundamentally governed by pore structure. Additionally, sorptivity exhibits predictive potential for mechanical properties and durability outcomes, including carbonation resistance, freeze-thaw durability, and sulfate attack resistance. Compared to other durability indicators, sorptivity not only boasts a relatively straightforward testing process, but its primary advantage lies in its high sensitivity to durability changes in long-age concrete. It also provides highly accurate assessments of the carbonation resistance of cement-based materials. Furthermore, it contributes effectively to evaluating frost resistance and sulfate erosion resistance. When used in conjunction with other durability indicators, sorptivity enables a comprehensive assessment of the durability performance of cement-based materials. As a durability indicator, sorptivity shows promise in forecasting the service life of cement-based materials.

In conclusion, this work underscores the need for refined theoretical models, standardized testing protocols, and comprehensive investigations into the coupling mechanisms governing sorptivity. Addressing these challenges will enhance the reliability of sorptivity as a critical parameter for optimizing material design and durability assessment in cement-based systems.

Cement-based materials exhibit water sensitivity due to their unique physicochemical interactions with water. Current understanding and quantitative characterization of capillary water absorption (sorptivity) require further refinement through integrated theoretical and practical studies. Notably, the two-stage sorptivity mechanism, particularly the physical significance and engineering implications of the secondary sorptivity, demands deeper exploration to enhance its practical application.

Critical anomalies observed in sorptivity evolution highlight the urgent need to develop more scientifically robust testing protocols. A standardized framework for evaluating concrete quality based on sorptivity should subsequently be established.

The measured sorptivity is significantly influenced by material composition and mix design parameters, including cement type, supplementary cementitious materials, water-to-binder ratio, curing conditions (temperature and humidity), aging, and drying pretreatment. Systematic investigations into the correlations between sorptivity and material composition are essential to optimize mix designs for improved durability.

Although sorptivity demonstrates varying degrees of correlation with mechanical properties, other transport indices (e.g., chloride diffusion), and accelerated durability test results, it exhibits unique potential as a complementary indicator for assessing concrete durability. Integrating sorptivity with conventional metrics could enhance the accuracy of service life prediction models for concrete structures. However, further theoretical, experimental, and field studies are imperative to validate its application in engineering practice.

In summary, advancing research—spanning mechanistic interpretation, standardized testing, material optimization, and durability assessment—will provide critical insights into the performance and longevity of cement-based materials in real-world environments.

There are many methods to test the water content, pore water distribution and pore structure characteristics of cement-based materials, and they are different in the measurement principle, accuracy, ease of use and reliability. As a non-destructive testing method, low-field nuclear magnetic resonance (LF-NMR) relaxation technology uses hydrogen and ¹H nuclei in water as a probe. By testing the relaxation signal of ¹H nuclei, it can accurately detect the content of hydrogen protons and the interaction with the pore wall, thereby indirectly obtaining features such as water content, water distribution in pores of different sizes and pore size distribution. They can be tracked and monitored over time and through other physical and chemical processes. Compared to traditional methods such as weighing method for water content and mercury intrusion porosimetry or nitrogen adsorption for porous structure measurement, LF-NMR relaxation technology can perform non-destructive testing without drying. It avoids the destruction of the abundant nanopore structure caused by drying pretreatment, and the pore measurement range covers the nanometer and micron scale pores. These significant technical advantages are very key to the study of the properties of cement-based materials. Because of the undisturbed, non-destructive and accurate characteristics of LF-NMR relaxation technology, it is precise, and thus more and more widely used in the field of cement-based materials, especially the hydration of cement, volume stability, actions of drying-wetting and freezing-thawing cycles, and durability performance. Since almost all these studies are closely related to water content and pore structure, LF-NMR plays an increasingly key role.

Based on the relaxation mechanism of ¹H nuclei and the commonly used relaxation measurement methods, this paper systematically describes the transverse relaxation mechanism of pore fluid and the inversion algorithm of transverse relaxation data. The determination methods of surface relaxivity and quantitative calibration of transverse relaxation signal and continuous/discrete relaxation time spectrum into water content and continuous/discrete pore size distribution are described in detail. In addition, the application status of LF-NMR is summarized and prospected from 5 different perspectives: quantitative characterization of cement hydration process, pore structure at saturation, pore-scale water distribution at unsaturated state, imaging of water spatial distribution, and freezing-thawing damage process analysis.

First of all, by testing relaxation magnetization and transverse relaxation time spectrum, LF-NMR resonance technology can non-destructively monitor the moisture content and its physical constraint state in the hydration process of cement at real time, so as to analyze the time-varying process of pores, especially nanopores, and apprehend the development history of microstructure, so as to realize the undisturbed characterization and analysis of cement hydration process. Secondly, the porosity and pore size distribution of hardened cement-based materials can be obtained according to the relaxation magnetization and relaxation time spectrum, allowing for qualitative and quantitative investigation of the relationship between pore structure and macroscopic properties. Thirdly, the pore classification method based on discrete or distinct peak T₂ spectrum can monitor the distribution of water in pores of different sizes at real time and continuously under unsaturated conditions, which can be used to study the macroscopic properties of cement-based materials such as moisture shrinkage. Fourthly, the water content and relaxation time spectra of different locations can be obtained by one-dimensional imaging, which can directly characterize the internal water migration process of cement-based materials. Finally, since the LF-NMR relaxation technique can only monitor the ¹H nuclear signal in evaporable water other than in solid phase such as ice and calcium hydroxide, it can be used to detect the phase transition of liquid water into ice.

It is found after literature reviewing that the LF-NMR relaxation technique using ¹H nuclei in pore fluid as a probe is especially suitable for characterizing cement hydration, pore structure evolution and pore-scale water allocation and spatial distribution of cement-based materials. On the basis of reasonable selection of hardware platform, test method, test parameters, inversion algorithm and accurate calibration of relaxation magnetization and transverse relaxation time, LF-NMR technology can help accurately measure important information closely related to pore structure and pore-scale water allocation of cement-based materials, such as hydration degree, pore size distribution and unsaturated pore water distribution non-destructively.

Due to the influence of ferromagnetic substances, current LF-NMR analysis is mainly recommended to use white cement as the principal cementitious material. In future, it is necessary to fully consider the influence of ferromagnetic materials on transverse and longitudinal relaxation, to establish LF-NMR relaxation test method suitable for ordinary Portland cement, and to improve the signal-to-noise ratio of relaxation signals, to reduce the dead time of the probe, and to develop an inversion algorithm with good accuracy and precision. The probe diameter (sample chamber size) should be increased to support the test analysis of concrete specimen.

LF-NMR relaxation technology is the promising characterization method that can detect the multi-scale pore structure and unsaturated pore water allocation and other key information of cement-based materials without any pretreatment. Since most of the key properties of cement-based materials, such as volume stability and durability, are closely related to pore structure and pore-scale water allocation, LF-NMR relaxation technology is expected to be able to drive breakthroughs and promote the development of modern concrete science and technology, similar to the encouraging role of magnetic resonance imaging in clinical diagnosis.

The capillary water absorption behavior of cement-based materials deviates progressively from the classical unsaturated flow theories due to specific interactions between water and cement hydrates. Although the impact of supplementary cementitious materials on the initial capillary sorptivity has been widely investigated, their influence on anomalous capillary absorption behavior need further exploration. By taking the technical advantage of low-field nuclear magnetic resonance (LF-NMR), this study investigates the long-term capillary absorption of water and isopropanol (IPA) into white cement mortars with high content of fly ash, slag, and silica fume.

The durability of cement-based materials is significantly influenced by the transport processes of water, gases such as CO₂, and ions like Cl^–. Water transport, particularly capillary water absorption, is crucial for durability performance. In most engineering practices, cement-based materials are non-saturated, making capillary water absorption the primary mode of water transport. This process is driven by capillary pressure arising from the meniscus within pores and is governed by the extended Darcy's law or the Richard's equation. In one-dimensional capillary absorption scenarios, theoretically there exists a linear relationship between the cumulative absorbed volume per unit area and the square root of absorption time, which is known as the square root of time linear law. The slope obtained from linear fitting is defined as the capillary sorptivity, which serves as a quantitative indicator of durability. However, long-term capillary water absorption often deviates from this linear law due to specific physicochemical interactions between water and cement hydrates, especially calcium silicate hydrate (C-S-H) gel. The widespread use of supplementary cementitious materials such as fly ash, slag, and silica fume alters the composition and micro-structure of C-S-H gel through secondary hydration reactions, significantly impacting the properties of cement-based materials. Although the capillary sorptivity has been well recognized as a durability indicator, the anomalous phenomena observed during long-term capillary water absorption are frequently overlooked, leading to incomplete research and inconclusive findings. This study focuses on the long-term capillary absorption processes of water and IPA into cement mortars with representative supplementary cementitious materials, including fly ash, slag, and silica fume. The aim is to elucidate the influence mechanisms of supplementary cementitious materials on the capillary absorption properties of cement mortars.

The experimental section focused on investigating the impact of supplementary cementitious materials on the capillary absorption behavior of cement mortars. Samples were prepared with sands, white cement and high contents of fly ash, slag, and silica fume. These specimens were subjected to both standard curing at 20 ℃ and accelerated curing at 60 ℃ to assess the influence of curing temperature. Testing methods encompassed capillary absorption tests using IPA and water, along with low-field magnetic resonance relaxation technique to analyze pore structure evolution. The capillary sorptivity were determined by fitting the experimental data to a square root of time linear law, and deviations from this law were analyzed to characterize water sensitivity. Data analysis involved examining changes in capillary sorptivity, deviation times, and pore structure characteristics to elucidate the mechanisms by which supplementary cementitious materials affect the capillary absorption properties of cement mortar.

The incorporation of fly ash, slag, and silica fume significantly alters the capillary sorptivity of both IPA and water into cement mortar. Specifically, all three supplementary cementitious materials reduce the capillary sorptivity of IPA, with fly ash exhibiting the most pronounced effect. During the initial stage of capillary absorption of water, slag increases the sorptivity, while silica fume and fly ash decrease it. In the later stage, high silica fume content notably enhances the sorptivity, whereas high slag and fly ash contents decrease it. The water sensitivity of cement-based materials is also affected. High slag and fly ash increases the water sensitivity of cement mortars, leading to earlier deviation from the initial linear law and higher degrees of deviation in later stages. These findings indicate that supplementary cementitious materials modify the composition and microstructure of C-S-H gel, thereby affecting the water sensitivity and durability of cement-based materials.

The addition of three supplementary cementitious materials will reduce the capillary sorptivity of isopropanol, and the reduction effect of fly ash is most significant. In the initial stage of capillary absorption of water, slag significantly increases the initial sorptivity of mortar, while silica fume and fly ash show inhibiting effects. In the later stage, silica fume significantly increases the secondary sorpvitity of water into mortar, while slag and fly ash both decrease their secondary sorptivity. The addition of slag and fly ash will significantly increase the difference between the initial and secondary capillary sorptivity. High content of silica fume can reduce the water sensitivity of cement-based materials, while high content of slag and fly ash can improve the water sensitivity, so as to advance the deviation of capillary absorption of water from the square root of time linear law and to increase the degree of deviation. The improvement of water sensitivity is the most significant for the mortar with high slag substitution. Curing at high temperature slightly reduces the water sensitivity of mortar with high slag content, but almost has no effect on the water sensitivity of mortars with high silica fume and fly ash contents.

The pore structure of cement-based materials significantly influences their compressive strength. Studies have shown that the nanoscale pore structure significantly changes with variations in water content. Traditional pore measurement methods like mercury intrusion porosimetry and gas adsorption require the drying of specimens, potentially altering or even destroying the pore structure. The accuracy and representativeness of the compressive strength-pore structure relationship model established on this basis are therefore questionable. To accurately describe the important correlation between compressive strength and pore structure of cement-based materials, low-field magnetic resonance relaxation technique is utilized to carry out in-situ, nondestructive testing of saturated cement mortar doped with air-entraining agent. This is then combined with compressive strength measurement results to validate and refine the model describing the relationship between compressive strength and pore structure characteristics.

(1) Specimen preparation To avoid the ambiguous effects of ferromagnetic substances when using by low-field nuclear magnetic resonance (LF-NMR) technique, white Portland cement with low Fe₂O₃ content was used to prepare cement mortars with water-to-cement ratio of 0.4. Cement to sand ratio is 1:2. And air-entraining agents (SJ-2 type) were incorporated. Chemical composition of white cement were 0.50% Fe₂O₃, 64.60% CaO, 21.71% SiO₂, 4.60% Al₂O₃, 2.80% SO₃, 2.43% MgO, 0.48% R₂O. The mass ratio of air-entraining agents to cement was 0%, 0.05%, 0.10%, and 0.15%. Considering different mass ratios, they are named as WP-BLK, WP-SJ05, WP-WP-SJ10, and WP-SJ15, respectively.

According to the designed proportions, the white cement, sand, air-entraining agents and water were mixed and casted into prisms of size 20 mm×20 mm×50 mm. After demounting at 24 h, they were all cured in saturated lime water at (20±2) ℃ for 180 d. After reaching the specified age, ten specimens were taken from each group of mortar and put into the high-pressure water saturation equipment for 3 d to ensure that the specimens were completely saturated and set aside.

(2) Pore structure test The pore structure of saturated mortars were tested by LF-NMR technology. Utilising a 2 MHz NMR analyser manufactured by Limecho Ltd., China, transverse relaxation test was performed through the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence at controlled room temperature about 20 ℃. In detail, the echo time τ_e (s) was adopted as short as 60 μs to detect the water confined in nanoscale pores of fast relaxation. Preliminary experimental tests had revealed that echo time τ_e ≤100 μs was short enough to make the contribution of diffusion relaxation negligible due to the controlled low concentration of Fe₂O₃ in white cement pastes. Moreover, A long sequence of N = 40000–60000 echoes generated at time τ=nτ_e (n=1, 2, · · ·, N) (s) was recorded to capture the water stored in large capillary pores and even air voids with long relaxation time. 256–1024 scans with enough long repetition time 5–15 s were averaged. Except for the echo time, the rest of the parameters were flexibly adjusted according to water content of specimen. One can see detailed steps for testing pore structure by low-field magnetic resonance techniques in the relevant literature.

(3) compressive strength test The compressive strength test of specimens were carried out by YAW-300 microcomputer automatic cement folding testing machine, which was produced by Jinan Hengruijin Testing Machine Co., Ltd., with a maximum capacity of 100 kN. The size of the compression surface of the specimen is 20 mm×50 mm, and the loading rate is fixed at 0.2 kN/s.

LF-NMR technology accurately captures the pore structure characteristics of saturated mortar. Empirical models using total porosity or graded porosity as variables are generally effective, but the empirical parameters obtained from model fitting are not stable enough, limiting the model’s applicability. An improved empirical model, based on the pore size distribution curve and assuming a linear relationship between the weights of different pores on compressive strength and their logarithmic pore sizes, offers a concise form, significantly improves fitting accuracy, and accurately reflects the differences within the specimen groups. Furthermore, building on Griffith’s fracture theory, the theoretical model, which uses total porosity and logarithmic mean radius as independent variables, exhibits complex expressions and low predictive accuracy. If pores with a radius above 10 nm are regarded as defects, their influence can be explained by Griffith’s fracture theory. Meanwhile, pores with a radius of 10 nm or less are regarded as internal pores of the gel, and its effect on the strength of the matrix is approximated using the model description proposed by Powers. The resulting modified Griffith fracture theory model is very high in prediction accuracy, stable, and interpretable, accurately reflecting how minor differences in pore structure lead to small changes in compressive strength.

The incorporation of air-entraining agents mainly increases the capillary pores above 10 nm in radius but also decreases the matrix pores below 10 nm in radius. LF-NMR technology can obtain the full-scale pore structure of cement mortar, aiding in the description of the fundamental correlation between pore structure characteristics and compressive strength.