**1. Introduction**

Constantly evolving technologies of coal combustion, with the aim of improving energy efficiency, include among others the more efficient fluidized bed combustion (FBC) systems. A well designed boiler can combust coal with relatively high efficiency and an acceptable level of gas emissions. FBC boilers operate at lower temperatures (800–900 ◦C) compared to conventional pulverized coal combustion (1400–1700 ◦C) [1]. The diversity of work conditions of increasingly used fluidized bed boilers makes it possible to use them to burn high-sulfur fuels [2]. The fuel flexibility is an important characteristic of FBC boilers [3]. The best example is circulating fluidized bed combustion (CFBC), which provides low emissions of SO2 and NOx to the atmosphere [3–5] while maintaining the efficiency of coal combustion.

A systematic increase of coal combustion by-products from fluidized boilers has been observed in numerous countries. One of the ways of managing CFBC by-products is to use them as an additive to cement and concrete. Studies on the possibility of the use of CFBC fly ash as a cement or concrete additive have been carried out for several years [1,6–12], but still there are no normative regulations regarding the use of this kind of fly ash as a cement additive.

Some researchers have performed investigations on the material characteristics and mechanical properties of CFBC fly ash in cement-based composites. It has been shown that CFBC fly ash is characterized by high variability of chemical and phase composition [13]. The dominant CFBC particles were coarse and had an irregular flaky shape characterized by a broad particle size range [12] which resulted in high demand of water and admixture [14]. The high content of anhydrite in CFBC resulted in excessive expansion in cement matrix composites due to abundant ettringite formations [15]. Moreover, most of the CFBC fly ash had less pozzolanic activity than the conventional siliceous fly ash [7] and

some of the CFBC fly ashes do not meet the standard requirements to be classified as either Class C or Class F fly ash [11]. The main reason for limiting the use of CFBC fly ash in blended cements is a high content of sulfates (mostly anhydrite), and free calcium oxide, whose presence contribute to creation of secondary ettringite—negative from point of view of the durability of cement matrix composites [16]. The overview of the effect of reactive mineral additives on the microstructure and phase composition of the cement hydration products was presented in [17]. Rajczyk et al. [18] used the DTA method to follow the hydration process of cement containing fluidized bed combustion fly ash. Based on endothermic peaks attributed to the dehydration of phases formed on hydration, the conditions leading to the formation of so-called delayed ettringite were found. Gazdiˇca et al. [1], based on the progress of hydration of blended cements, proved that ettringite formed from anhydrite contained in fluidized bed ash was one of the hydration products built during the formation of the cement stone structure, thus ettringite was not the cause of negative volume changes.

However, despite these observations, it has also been shown that application of CFBC fly ash simultaneously with siliceous fly ash or slag as a binder component makes it possible to obtain a durable cement-based composite. Nguyen et al. [19] used CFBC fly ash as a sulfate activator which significantly improved the mechanical properties of the modified high-volume fly ash cement pastes at early ages. The accelerated hydration of C3S and more precipitated ettringite formation of hardened pastes confirmed the important role of CFBC fly ash to enhance the mechanical properties of the cement pastes. Hlavácek et al. [ ˇ 20] used CFBC fly ash in a ternary binder (plus siliceous fly ash and calcium hydroxide) and they obtained a compressive strength of paste equal to 32 MPa after 28 days of curing. The ternary hydraulic binder containing CFBC fly ash was also analyzed by Škvára et al. [21]. They stated that a ternary binder possessed strength values comparable to those of Portland cement. Lin et al. [22] investigated the properties of controlled low strength material with circulating fluidized bed combustion ash and recycled aggregates. They showed that CFBC hydrated ash resulted in a higher compressive strength when compared with desulfurized slag, but had a lower compressive strength than coal bottom ash. Chi [23] characterized the mortars with CFBC fly ash and ground granulated blast-furnace slag and he found that this kind of fly ash had the potential to partially replace the cementing materials, but the proportion of CFBC fly ash was recommended to be limited to a maximum of 20% due to the decreasing of compressive strength and the increasing of initial setting time of mortars. Dung et al. [24] analyzed the application of raw circulating fluidized bed combustion fly ash and slag as eco-binders for a novel concrete without ordinary Portland cement. They found that this new binder behaves similar to ordinary Portland cement. The pastes showed good workability, proper setting times, and sufficient compressive and tensile strength, which were improved with the decrease of water/binder (w/b) ratio and with the increase of hydration time. Chen et al. [25] tested expansion properties of cement paste with circulating fluidized bed fly ash. They stated that the curing method is of great importance. The expansion development in its early days was dominated by the ettringite, and its quantity and morphology were seriously affected by the inadequateness of water. The application of CFBC fly ash improved concrete durability. Józwiak-Nied ´ ´ zwiedzka [26] analyzed the influence of CFBC fly ash on the chloride and scaling resistance of concrete. She showed that the cement replacement by 15% and 30% CFBC fly ash provided higher chloride resistance, but only replacement by 15% CFBC fly ash can be used to achieve concrete scaling durability. Kubissa et al. [27] showed that use of 25 wt % fluidized fly ash as cement replacement improved concrete water permeability and sorptivity, as well as chloride resistance. On the other hand, Czarnecki et al. [28] found that the depth of concrete carbonation increased with increasing of the content of CFBC fly ash but at the same time they obtained similar results for ordinary siliceous fly ash.

Juenger et al. [29] have recently presented a review paper on emerging supplementary cementitious materials (SCM). They concluded that with the increase in demand for conventional SCM and the simultaneous reduction in supply, there is a great need to find new sources of materials that provide comparable or better properties to highly used fly ash and slag sources. So, the research into new sources of SCMs (like fly ash from bed fluidized coal combustion that does not meet current specifications) and their impact on cement matrix properties has been increasing in recent years, and has great likelihood to further increase as the demand for these materials grows.

Despite many publications on the possible application of the CFBC fly ash in concrete technology, the problem of its proper use remains unresolved. An increasingly technical rigorous regime during the combustion of coal in fluidized bed boilers has contributed to improving the stability of the composition and reducing the amount of undesirable ingredients. Research on the new kinds of fly ash from circulating fluidized bed combustion has considered their specific characteristics of phase composition and paste microstructure. This paper aims to show some relationships between the differences in mineral composition and origin of burning coal. The phase composition as well as microstructure of cement pastes with various content of CFBC fly ash have been studied. A prolonged time of curing has been applied to determined how the replacement of Portland cement by CFBC fly ash would change the microstructure and phase composition of paste stored in high-humidity conditions.
