Recycling of Sustainable Co-Firing Fly Ashes as an Alkali Activator for GGBS in Blended Cements

Abstract

This study investigates the feasibility of co-firing fly ashes from different boilers, circulating fluidized beds (CFB) or stokers as a sustainable material in alkali activators for ground granulated blast-furnace slag (GGBS). The mixture ratio of GGBS and co-firing fly ashes is 1:1 by weight. The results indicate that only CF fly ash of CFB boilers can effectively stimulate the potential characteristics of GGBS and provide strength as an alkali activator. CF fly ash consists of CaO3 (48.5%), SiO2 (21.1%), Al2O3 (13.8%), SO3 (10.06%), Fe2O3 (2.25%) and others (4.29%). SA fly ash consists of Al2O3 (19.7%), SiO2 (36.3%), Fe2O3 (28.4%) and others (15.6%). SB fly ash consists of Al2O3 (15%), SiO2 (25.4%), Zn (20.6%), SO3 (10.9%), Fe2O3 (8.78%) and others (19.32%). The mixtures of SA fly ash and SB fly ash with GGBS, respectively, were damaged in the compressive strength test during seven days of curing. However, the built up strength of the CF fly ash and GGBS mixture can only be maintained for 7–14 days, and the compressive strength achieves 70% of that of a controlled group (cement in hardening cement paste). The strength of blended CF fly ash and GGBS started to decrease after 28 days, and the phenomenon of ettrigite was investigated due to the high levels of sulfur content. The CaO content in sustainable co-firing fly ashes must be higher than a certain percentage in reacting GGBS to ensure the strength of blended cements.

Waste-Based Pervious Concrete for Climate-Resilient Pavements

“…On the whole, the pH values of the mortars containing various proportions of CFFA and BFS and no cement were close to that of the mortar containing only cement. From the previous study [ 11 , 12 ], we can see that the CFFA and BFS in the experiment groups contain compounds such as Ca(OH) 2 , CaO, CaSO 4 , and SiO 2 , among which CaO and CaSO 4 engage in hydration reactions with water: CaSO 4 + 2H 2 O → CaSO 4 ∙2H 2 O, CaO + H 2 O → Ca(OH) 2 , …”

“…Without an additional alkaline activator, the two materials can undergo an autologous alkaline hydration reaction which effectively mimics cement. On the whole, the resulting cement alternative can only achieve approximately 70% to 80% of the compression strength of pure cement mortar; however, it performs well in comparison to other cementless composite materials [ 12 , 13 ].…”

Composite Properties of Non-Cement Blended Fiber Composites without Alkali Activator

“…We speculate that this can be attributed to the mix ratio, which allowed the S and FA to be gelatinized; however, the slowness of the reaction hindered strength development. On the basis of the previous study [29], FA can be used as a sustainable alkali activator for S to activate the alkali-activated or pozzolanic reaction. The tensile strength of the non-cement blended specimens increased with an increase in the proportion of polypropylene fibers, as shown in Figure 5b.…”

“…FA contains large quantities of free lime (f-CaO) and SO 3 , which are beneficial to strength development of hydrated in non-cement blended composites containing S. Note however that f-CaO and/or excessive SO 3 can cause expansion in hardened mortar. Increasing the amount of FA could increase the number of sulphate ions in solution, thereby hindering ettringite formation [29,30]. This can have profound effects on the volume stability and strength of mortar specimens with a high proportion of FA.…”

New additions for eco-efficient cement design. Impact on calorimetric behaviour and comparison of test methods

“…The use of such additions (supplementary cementitious materials, SCMs) in cements and concretes has grown in recent decades to improve concrete mechanical performance and durability, lower the clinker/cement ratio, reduce the CO 2 emissions inherent in cement manufacture, which presently account for 5 % of the total emitted by all industrial activities, and mitigate the construction industry's carbon footprint [7,8].…”