The influence of two types of alkali activators on the microstructure and performance of supersulfated cement concrete: mitigating the strength and carbonation resistance
Introduction
The traditional Portland cement is criticized for its high energy consumption and carbon dioxide emissions. It accounts for 7%–10% of the total carbon dioxide emissions globally [1]. Hence, it is necessary to find a substitute for Portland cement, called green cementitious materials, apart from prolonging the service life of concrete structures [[2], [3], [4]] [[2], [3], [4]] [[2], [3], [4]]. The supersulfated cement (SSC) is such a kind of green cementitious material. It was developed in 1908 by German Kühl and its raw materials are mainly steelmaking by-product slag and industrial waste gypsum [5]. A small amount of Portland cement or clinker is usually added, which owns a proportion of 5% in supersulfated cement components [6]. In the early stage, the hydration of Portland cement produces calcium hydroxide, which provides an alkaline environment and promotes the dissolution of slag. Further, the calcium, silicon, and aluminum ions dissolved from slag react with sulfate ions from gypsum to form ettringite and calcium silicate hydrate (C–S–H), which are two main hydration products of supersulfated cement [7]. Compared with Portland cement, SSC has a slower hydration rate. Thus, it has an osteoporotic microstructure, which has been reported by Cabrera-Luna [8].
The strength and other properties of supersulfated cement are highly dependent on the features of raw materials. Wang et al. [9] took a comparative study on hydration characteristics of supersulfated cement under three different slag fineness, results indicated that the greater specific surface area of the raw material within a given range contributes to the development of strength. From the point of view of energy, during the grinding process, the mechanical energy will be partially converted into surface energy and internal energy, which will increase the total energy of solid particles and further increase the reactivity. The higher Al2O3 content in the slag is of benefit to the formation of ettringite, thus enhancing the early strength of SSC. Some studies have shown that the Ca/Si of slag suitable for SSC must be greater than 1, and the Al2O3 content must be at least 13% [10,11]. Masoudi et al. [10] studied the effects of two types of granulated blast furnace slag, high aluminum slag (Al2O3 content 13%) and low aluminum slag (Al2O3 content 7%), on the hydration mechanism of supersulfated cement. The results indicated that supersulfated cement with low aluminum slag has a lower hydration degree, but higher compressive strength can also be achieved by adding more alkaline activators. Due to the difference in solubility and dissolution rate of different gypsum, the properties of supersulfated cement are also diverse. Gao et al. [12] investigated the use of various modification methods of phosphogypsum, including washing, calcination, and alkali neutralization. The best result could be obtained after calcinating, with 56d compressive strength over 70 MPa. Further, Liu et al. [13] discussed the calcination modification mechanism of phosphogypsum and its influence on the properties of phosphogypsum based supersulfated cement. Four temperature variables (150 °C, 350 °C, 600 °C, 800 °C) and four time variables (0.5 h, 1 h, 1.5 h, 2 h) were selected. Liu pointed out that calcination pretreatment can reduce the adverse effects of impurities in phosphogypsum and shorten the setting time of supersulfated cement.
In recent years, the main relevant research works were focused on supersulfated cement, such as the best component of cement [14,15], slag composition [6,16], modification of raw materials [13,17], hydration mechanism [18,19]. More attention has been paid to paste or mortar, while minimal works were carried out on supersulfated cement concrete [11,20] [[11], [20], [21], [22]] [[11], [20], [21], [22]]. Generally, although SSC concrete owns many advantages of low heat release, low cost, low pollution, it has slower strength development, poorer carbonation resistance and inferior frost resistance, compared with concrete made from traditional Portland cement. Erdem et al. [23] found that the early age compressive strength of SSC concrete at three days was only approximately 30% of that at 28 days, while that was about 60% for traditional Portland cement. Masoudi [24] made a comparison between SSC and Portland cement concrete, indicating SSC concrete had poor freeze-thaw durability with 90 cycles less than 300 of Portland cement concrete. So far, a few measures have been reported to mitigate those weaknesses and promote the engineering applications of SSC concrete. Ioannou et al. [25] utilized fabric formwork to improve the durability of supersulfated cement concrete, indicating that fabric formwork is one feasible method. Li et al. [26] mixed electric arc furnace reducing slag (EAFRS) to replace granulated blast furnace slag (GBFS) partly, results showed that the hydration of GGBS was considerably accelerated because of the addition of EAFRS. Cabrera-Luna et al. [27] selected pumice instead of slag to produce supersulfated cement concrete, using CaSO4, CaO, and Portland cement as activators, and displayed good overall performance.
However, those measures either employ external equipment or change raw materials, while in this study a high cost-effectiveness matrix enhancement technique will be proposed to promote the hydration of slag and optimize the microstructure of SSC. Two types of alkali activators (lactates-weak alkaline activator, and sodium hydroxyl-potent alkaline activator) were respectively mixed with the SSC raw materials during the concrete sample preparation process, and the mechanical performance and durability of SSC concrete were examined. Besides, the mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), and scanning electron microscope (SEM) were comprehensively utilized to illustrate the microstructure changes of supersulfated cement concrete.
Section snippets
Materials
The cement chosen as alkali activator in this study was P.O. 42.5 Conch Cement, the desulfurized gypsum was employed as sulfate activator with the main component of 1/2H2O·CaSO4, the slag is GBFS with the fineness of only 342 m2/kg, some detailed chemical and physical-mechanical characteristics are shown in Table 1 and Fig. 1, Fig. 2. A sort of polycarboxylic superplasticizer produced by Sobute was added to adjust the workability of concrete during the mixing process with tap water. Besides,
Compressive strength
Fig. 4 gives the compressive strength development of five concrete groups. In contrast to supersulfated cement concrete, the compressive strengths of all ages of group Ctrl are in the leading position. Its compressive strength at 3d, 7d, and 28d reached 36.9 MPa, 52.8 MPa, and 65.8 MPa, respectively, higher than 28.2 MPa, 43.6 MPa, and 51.1 MPa of the reference group SSC-0. On this basis, additional alkaline activators were used to trigger the slag activity, and the content of alkali metal ion
Conclusions
In this work, the strength and durability of SSC concrete with two different kinds of alkali activators (lactates-weak alkaline activator, and sodium hydroxyl-potent alkaline activator) were evaluated, and the microstructure of samples were illustrated by MIP, XRD, FT-IR and SEM measurements. The main findings are as follow:
- (1)
It is feasible to accomplish high strength supersulfated cement concrete by adding lactates, either sodium lactate or potassium lactate, while the incorporation of sodium
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge support from the National Natural Science Foundation of China (Grant Nos: 52050128, 51908119, 6512009004A, and U1706222), the Natural Science Foundation of Jiangsu Province (Grant No: BK20190367), the Fundamental Research Funds for the Central Universities and Shandong High Speed Group Co., Ltd..
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