The volumetric deformation of cement-based materials continues over setting, hardening and service periods, as a result of both cement hydration and changes in temperature and humidity. Under restricted conditions, excessive volume shrinkage may lead to cracking in cement-based materials, seriously threatening the long-term performance and durability of structure. As an important admixture to compensate for shrinkage and to control cracking, expansive agents are widely used in engineering construction. The influence of expansive agents on the compensatory effect for drying shrinkage of cement-based materials is closely linked to the evolution of chemically bound water and physically adsorbed water. Since most mesoscopic and microcosmic testing techniques require drying preparations for specimens, it is challenging to analyze the development process of shrinkage-compensating. Low-field nuclear magnetic resonance (LF-NMR) technique has a unique technological advantage for investigating cement-based materials. Taking hydrogen nuclei in water as a probe, LF-NMR technique can non-destructively, rapidly and accurately characterize the pore water content and water distribution in specimens in situ, which promotes to accurately analyze the connection between micro change of pore water and macro volumetric deformation. Therefore the cement paste samples mixed with expansive agents are monitored by non-destructive LF-NMR technique in this work, the weight and length are also measured. From the perspective of water content and its state, the quantitative analysis on evolution of chemically bound water and physically adsorbed water is conducted. Furthermore, the shrinkage-compensating deformation of cement paste is thoroughly analyzed.

White cement, calcium sulfoaluminate and MgO expansive agents (CSA and MEA) are used to prepare cement paste specimens. Water to cement ratio (W/C) is 0.40, and the weight ratio of expansive agent to cement is 5%. The expansion agents are produced by Wuhan Sanyuan Special Building Materials Co. The sizes of cement paste specimens are 20 mm×20 mm×280 mm (size A) and 20 mm×20 mm×50 mm (size B).

Starting from the 3rd day of water-curing, the weight and length of 3 specimens of each sample are measured at regular time intervals, as well as the low-field magnetic resonance tests. In addition, at 56th day of water-curing, another 3 specimens of each mixture proportion and each size are transferred into a high humidity environment (20℃, 98% relative humidity (RH)) for air-curing, and measurements of weight and length, and low-field magnetic resonance tests are conducted periodically. The length test is carried out using a length gauge with an accuracy of 0.001 mm. An electronic balance with an accuracy of 0.001 g is used in the weight test. The low-field magnetic resonance test is performed by a low-field magnetic resonance instrument with a magnetic field strength of 0.047 T and a main frequency of 2 MHz. And the transverse relaxation signal of pore water in specimen is characterized with a CPMG pulse sequence.

After curing in water for 3 d, the weight, expansion deformation and evaporable water content of paste continuously increased during curing process, while the rate of increase gradually decreased. CSA and MEA can significantly increase the increment of chemically bound water and the expansion deformation caused by each unit increment. However, their influences on the evolution of physically adsorbed water and the expansion deformation are both insignificant. During the process of air curing in high humidity (98% RH), starting from the day 0 of air curing (i.e., water curing 56 d), the weight of paste decreases initially and then increases. Similarly, the paste initially shrinks and then expands. Additionally, the evaporable water content decreases rapidly in the first 28 days and then stabilizes. Both CSA and MEA can increase the loss of evaporable water content of paste in the early stage and also enhance the expansion deformation in the later stage to compensate for shrinkage. Considering the complicated expansive process of cement-based materials mixed with expansive agents, the utilization of low-field nuclear magnetic resonance relaxation technique and other testing methods can accurately analyze the expansion deformation, which can improve the expansion mechanism and assist the selection of expansive agents.

The main conclusions of this paper are summarized as following. During the period of 6-month water-curing, all the tested indicators, including weight, expansion deformation, and evaporable water content of paste, continuously increase, while the increasing rate keeps declining. Both CSA and MEA continue to hydrate, resulting in the larger content of chemically bound water. In the meanwhile, they increase the expansion deformation caused by unit increment of chemically bound water. The evolution of chemically bound water significantly influences the effect of expansive agents on compensating for shrinkage. Nevertheless, the evolution of physically adsorbed water and its effect on volumetric deformation is independent of the addition of any expansive agent.

After air-curing at a high humidity (98% RH), the reduction rate of evaporable water content of cement paste with CSA and MEA increases significantly in the early stage, but the difference in its shrinkage deformation is relatively small. And the compensatory effect of expansive agents on shrinkage is still effective in the later stage.