کارایی و خواص مکانیکی بتن خاكستر بادی فعال شده قلیایی
Abstract
1- Introduction
2- Experimental program
3- Results and discussion
4- Conclusions
References
Abstract
Alkali-activated fly ash-slag (AAFS) concrete is a new blended alkali-activated concrete that has been increasingly studied over the past decades because of its environmental benefits and superior engineering properties. However, there is still a lack of comprehensive studies on the effect of different factors on the fresh and hardened properties of AAFS concrete. This paper aims to provide a thorough understanding of workability and mechanical properties of AAFS concrete cured at ambient temperature and to obtain the optimal mixtures for engineering application. A series of experiments were carried out to measure workability, setting time, compressive strength, splitting tensile strength, flexural strength and dynamic elastic modulus of AAFS concrete. The results showed that workability and setting time decreased with the increase of slag content and molarity of sodium hydroxide solution (SH). Compressive strength increased with the increase of slag content and molarity of SH as well as the decrease of alkaline activator to binder (AL/B) ratio, but it did not have significant relationship with sodium silicate to sodium hydroxide (SS/SH) ratio. In addition, equations provided by ACI code, Eurocode and previous researchers for ordinary Portland cement concrete overestimated the values of splitting tensile strength, flexural strength and dynamic elastic modulus of AAFS concrete. The optimal mixtures of AAFS concrete were set as slag content of 20–30%, AL/B ratio of 0.4, 10 M of SH, and SS/SH ratio of 1.5–2.5 considering the performance criteria of workability, setting time and compressive strength.
Introduction
Alkali-activated materials (AAM) is an inorganic binder derived by the reaction of an alkali metal source (solid or dissolved) with a solid silicate powder such as fly ash (FA) and slag [1]. To date, AAM has been recognized as a promising alternative binder to ordinary Portland cement (OPC) because of its environmental benefits and superior engineering properties [2–5]. The manufacture of OPC is known as a significant contributor to greenhouse gas emissions accounting for around 5% of global CO2 emissions [6,7]. In comparison, there are about 55–75% less greenhouse gas emissions in the production of FA and slag [8]. Thus, the application of AAM as a binder can significantly reduce the CO2 emissions of concrete production. FA has been increasingly considered as a suitable raw material for alkali-activated concrete (AAC) due to its wide availability and adequate composition of silica and alumina. Previous studies [9–19] reported that alkali-activated fly ash (AAF) concrete has excellent mechanical and durability properties when it is cured at elevated temperature. Normally, the curing temperature of 60–85 C is required to activate FA as the reactivity of FA at ambient temperature is too low to be activated by alkali activators [20–22]. Such curing condition may be suitable for manufacturing precast concrete members, but it is not suitable for cast-in-situ concrete in practice. Therefore, it is vital to develop a new type of AAC without curing at elevated temperature, which will widen the practical application of AAC. In addition, the cost and energy consumption associated with the heat curing process will also be reduced. In order to achieve ambient curing, some researchers attempted to improve the reactivity of FA in alkaline environment [23]. In particular, one of the acceptable attempts is to add some calcium containing materials such as slag in AAC [24]. The addition of slag would accelerate FA dissolution and enhance reaction products formation in room curing condition [25]. Both the early and later age properties of AAF concrete are also significantly affected by the additional slag. Until now, an increasing number of studies have been undertaken to investigate the effect of slag on the engineering properties of AAF [2,3,26–30]. Nath and Sarker [2,3,31] studied the influencing factors on the fresh and hardened properties of alkali-activated fly ash-slag (AAFS) concrete. It was found that the dominant influencing factors are the slag replacement level for FA along with the type and content of alkaline activator. One main limitation of this research is that the different activating conditions were not fully considered. For example, the effect of slag content on the properties of AAFS concrete may be affected by the activator with different molarity. Lee [29,32] also explored the mechanical properties of AAFS concrete and suggested a proper slag content of 15–20% of total binder considering the setting time and compressive strength of AAFS concrete. However, it should be noted that the workability of AAFS concrete was not considered in the selection of slag content. In addition, an optimal mixture of AAFS concrete should not only include the slag replacement level but also the alkaline activator to binder (AL/B) ratio, molarity of sodium hydroxide (SH) solution and sodium silicate to sodium hydroxide (SS/SH) ratio, etc. Thus, it is of importance to conduct a comprehensive research focusing on the effects of different factors on the fresh and hardened properties of AAFS concrete and to evaluate the optimal mixtures by taking into account the basic performance criteria of workability, setting time and compressive strength.