In recent years, due to the increasing demand for concrete, ordinary and high-performance concrete (HPC) may not meet actual engineering needs. Thus, ultra-high-performance concrete (UHPC) is of increasing interest to researchers. Compared with ordinary concrete, UHPC shows a more dense microstructure and excellent compressive strength, because it contains a large amount of cement-based materials, has a low water/binder (w/b) ratio, and uses a large number of superplasticizers [1
]. In addition, the durability of ultra-high-performance concrete has been significantly improved, so UHPC could be used under more severe conditions. However, it must be considered that, due to the influence of a range of factors—such as a high cementitious material content and low w/b ratio—UHPC experiences autogenous shrinkage at an early age [3
]. Compared with ordinary concrete, UHPC is more prone to cracking in the early stage [4
]. The influence of early cracking, caused by restrained autogenous shrinkage, even limits the application of UHPC [5
]. Therefore, solving the problem of the autogenous shrinkage of UHPC is urgently required.
Researchers have studied the following five methods to reduce the occurrence of the autogenous shrinkage of UHPC: (1) the control of the hydration reaction; (2) reduction of the internal restraint; (3) reduction of the surface tension of the pore solution; (4) formation of expansive products; and (5) replenishment of water through internal curing. Of these five methods, internal curing is considered to be the most effective and straightforward method [7
]. Superabsorbent polymer (SAP) [8
] and lightweight aggregates (LWA) [10
] are considered to be two common internal curing materials. While SAP is very effective in reducing autogenous shrinkage, it also introduces additional pores into the concrete structure due to its water swelling effect, resulting in a reduction in the compressive strength of the concrete structure [12
]. It was confirmed by Sun et al. [13
] that the incorporation of SAP reduced the compressive strength, and the compressive strength decreased with the addition of SAP. In the study of Farzanian et al. [14
], it was also concluded that the addition of SAP reduces the compressive strength of cement slurry in cement pastes with a high density of macrovoids. However, in other studies of concrete mixed with SAP, some researchers have come to the opposite conclusion. Bentz et al. [15
] measured compressive strength development in experiments carried out for mortar mixes with w/b = 0.35, with and without SAP (0.4% relative to binder mass). After 28 days of curing, mortar with SAP showed higher compressive strength than the reference mortar; the values were 73 and 61 MPa, respectively. Similarly, Woyciechowski and Kalinowski [16
] studied the influence of the dosing method of SAP on the effectiveness of the concrete. It was found that the 28-day compressive strength of concrete with an activated small particle size (150–850 µm) of SAP was higher than that of the control specimen. In summary, the effect of SAP on strength depends on the system being studied. Compared with SAP, LWA can reduce the autogenous shrinkage of concrete, but whether the compressive strength of the concrete structure is reduced depends on the type of LWA material [17
]. Wang et al. [18
] studied the effects of three different types of pre-wetted LWA (fly ash-clay ceramsite, shale ceramsite, and clay ceramsite) on the compressive strength and shrinkage of concrete and found that LWA will reduce the compressive strength of concrete. Liu et al. [10
] also studied the effect of saturated coral aggregate (SCA) with UHPC and found that, although the autogenous shrinkage is reduced, the mechanical properties are also lost. The most widely used materials for LWA are different types of sand. However, the LWA material used in this paper is zeolite sand. Zeolite sand not only can effectively change the mechanical properties of concrete structures, but is also an environmentally friendly material that can be used for gas purification [19
]. While ordinary natural zeolite sand can absorb a certain amount of water due to its fine pore structure, it has difficulty in completely releasing most of the absorbed water. The crystal structure of zeolite sands can be destroyed by heat treatment to significantly increase their water absorption capacity, while the zeolite sand porosity is changed by particle agglomeration [21
]. According to the literature [22
], increasing the water absorption of zeolite sand by calcination is very effective. Zhang et al. [23
] used calcined zeolite particles with an average size of 0.18 mm as the internal curing agent of high-strength concrete, and it was confirmed that the calcined zeolite increased the internal relative humidity of the concrete and reduced the shrinkage. Zhang et al. [24
] also applied pre-wetted calcined zeolite particles in a high-strength engineered cementitious composite, and more than 60% of autogenous and/or drying shrinkage at 28 days was reduced while the strength of the composite remained as high as the reference specimen. Some zeolite powders were also used to mitigate the autogenous shrinkage of concrete. Pezeshkian et al. [25
] studied the effect of different percentages of silica fume replaced with natural zeolite powder (25%, 50%, 75%, and 100%) on the autogenous shrinkages and mechanical properties of UHPCs. It showed that with UHPC in which 50% in volume of natural zeolite was used as a substitute for silica fume, the 28- and 90-day compressive strengths were only slightly lower than that of reference specimen. Meanwhile, an increasing number of researchers have found that fine lightweight aggregates can react. Li et al. [26
] analyzed the pore solution and found that the expanded shale and clay can reduce the alkalinity, as well as increase the aluminum content, in the pore solution. Suraneni et al. [27
] found that finely ground lightweight aggregate is pozzolanic and participates in the hydration. In this paper, we also found that zeolite sand is not inert.
In practical engineering applications, most construction workers are highly interested in maintaining a similar compressive strength while reducing the autogenous shrinkage of concrete. The aim of this paper is to use pre-wetted zeolite sand to replace a portion of the standard sand in UHPC in order to significantly reduce the autogenous shrinkage of UHPC, without a significant loss of strength caused by adding the zeolite sand. The research methods implemented include water absorption, autogenous shrinkage, compressive strength, X-ray diffraction, and isothermal calorimetry tests.
The innovations of this paper are summarized as follows: Firstly, we find the use of 30 wt.% calcined zeolite sand can reduce the autogenous shrinkage of UHPC without reducing its compressive strength. Secondly, we find zeolite sand is not chemically inert, and the dissolution of fine zeolite sand particles changes the alkali ion concentration of the solution, accelerates the early binder hydration, and promotes the setting of the binder. Finally, the calcination can increase the water-absorbing capacity of zeolite sand and the internal curing effect of zeolite sand is enhanced after calcination.