1. Introduction
In 1940, Stanton [
1] first discovered the alkali-aggregate reaction (AAR), which has attracted the attention of many researchers. As it is extremely destructive and difficult to repair, it has been called the "cancer" of concrete. The AAR is divided into alkali-silicate reactions (ASRs) and alkali-carbonate reactions (ACRs). The ACR is one of the main problems in the long-term durability of concrete.
According to the time sequence, the expansion mechanism of the ACR can be divided into three types. Firstly, Gillott [
2] believed that the expansion is the result of an increase in the solid volume due to water absorption by the clay, where dolomitization only provides a way for clay to absorb water. Secondly, Tang and Tong [
3,
4,
5] believed that, although the absolute volume of the solid phase of the alkali-dolomite reaction is reduced in theory, the rearrangement and crystallization of the reaction products in a restricted space causes the expansion and cracking of the aggregate, respectively, leading to concrete cracking. Thirdly, Katayama [
6,
7,
8] believed that the ACR is the combination of harmful expansion caused by the ASR of microcrystalline quartz and harmless dolomitization. Dolomitization produces the brucite and carbonate reaction ring, but the expansion is caused by the ASR due to microcrystalline quartz. However, Chen, X [
9] and Chen, B [
10] used tetramethylammonium hydroxide (TMAH) as the curing solution to investigate the expansion characteristics caused only by the ACR, as TMAH does not react with SiO
2. Their results showed that the ACR exists separately and contributes to the expansion. Huan Yuan [
11], who also used TMAH as the curing solution, proved (by an expansion stress test and concrete microbar expansion test) that the alkali-carbonate reaction causes expansion.
The concrete durability problem caused by the AAR can be controlled by many methods, such as reducing the available alkali and adding a suitable amount of ash or chemical additives [
12]. Supplementary cementitious materials (SCMS) have different effects in reducing the expansion due to the ACR and ASR [
13]. It is well-known that supplementary cementitious materials have a significant effect in inhibiting the ASR [
14]. Based on the success of these materials, they have been used to prevent the deterioration caused by the ACR. According to the research of Alireza Joshaghani [
15], fly ash and trass can reduce the expansion rate of the ACR. At 56 d, using 10%, 20%, and 30% fly ash can reduce the expansion rate of mortar bars by 47%, 95%, and 73%, respectively, according to the ASTM C1260 standard. According to the experimental results of concrete prisms, the inhibitory effect of trass on the ACR is slightly better than that of fly ash. In a long-term test, the optimal content of fly ash was 20% and the optimal content of trass was 30%. Shehata’s [
16] experiment demonstrated that the expansion of concrete prisms mixed with SCMs exceeded the threshold of 0.040% in two years and no SCM had a complete effect on the ACR in the long-term, although some types were more effective than others at reducing expansion.
Shehata [
16] and Min Deng [
17] have shown that using fly ash to effectively inhibit the ACR expansion of the highly reactive dolomite limestones from Kingston, Canada is difficult. However, whether the ACR of low-reactive dolomite rocks and moderately reactive dolomite rocks can be effectively inhibited has not yet been studied. At present, the related specifications stipulate that concrete works cannot use the ACR reactive rocks as aggregates, limiting the application of dolomite rocks. Research on fly ash on the ACR of low-reactive dolomite rocks and moderately reactive dolomite rocks is therefore necessary. This serves to play a guiding role in the engineering application of dolomitic aggregates.
The main purpose of this study is to examine the inhibition and mechanism of fly ash on the ACR. Although the impact of fly ash on the ASR has been well-documented, there has been little research on the impact of fly ash on the ACR. In addition, most of the studies have focused on highly reactive aggregates, such as the Kingston dolomite limestone of Canada, with no clear research on low-reactive and moderately reactive rocks. In this study, therefore, we focus on low- and moderately reactive dolomite rocks.
4. Conclusions
In this experiment, concrete microbars and concrete prisms made of dolomite aggregates, fly ash, and cement were used to systematically investigate the short- and long-term effects of fly ash on the ACR and to investigate the influence of fly ash on the expansion of dolomite aggregates. From the physical macro-measurements and microstructure analyses, the following main conclusions can be drawn:
Increasing the content of fly ash in concrete microbars and concrete prisms can considerably decrease the expansion rate due to the ACR. In comparison with adding 10% and 20% fly ash, concrete microbars prepared with 30% fly ash exhibited a greatly reduced expansion rate. According to the expansion rates of concrete microbars, when the alkali equivalent (equivalent Na2Oeq) of the cement was 1.0%, for low-reactivity aggregates (such as BFL8), adding 30% fly ash could inhibit the expansion due to the ACR. However, when adding 30% fly ash to moderately reactive aggregates, such as SJW, although the expansion rate decreased, it did not decrease to less than 0.1% and cracking still occurred.
The polarizing microscope and stereomicroscope analysis results indicated that the cracks were expansion cracks, and the expansion was mainly due to the ACR. Adding 30% fly ash could significantly reduce the cracking in concrete microbars.
Through the analysis of the K+ and Na+ concentrations, the pH value, the ionic migration of the cement paste pore solution, and the pore structure, the main mechanism of fly ash inhibiting the ACR is that fly ash refines the pore structure of the cement paste, and the alkali migration rate in the curing solution to the interior of the concrete microbars is reduced. As the content of fly ash increases, the concentration of K+ and Na+ and the pH value in the pore solution gradually decrease. This causes the ACR in the rock to slow, the cracking to reduce, and the ACR expansion to be inhibited.