Effect of the by-pass cement-kiln dust and fluidized-bed-combustion fly ash on the properties of fine-grained alkali-activated slag-based composites

Abstract

The aim of this work is to investigate the influence of the by-pass cement-kiln dust (CKD) and two types of the fluidized-bed-combustion (FBC) fly ash on the workability, shrinkage and mechanical properties (compressive and flexural strengths) of the water-glass-activated slag. The utilization of CKD and FBC is very problematic. One of the main reasons for this is a high lime and sulfate content in these wastes, which can lead to the formation of expansive hydration products and, consequently, to the cracking of ordinary Portland-cement (OPC) materials. On the other hand, these products might act against the high shrinkage of the alkali-activated slag (AAS). In order to investigate this assumption and the influence of these admixtures on the other properties mentioned above, fine-grained AAS-based composites were prepared. A ground, granulated blast-furnace slag (the reference binder) was partially replaced (0-50 %) by one type of CKD and two types of the FBC fly ash. A significant reduction in the shrinkage was observed on the samples containing an even lower amount of fly ash, while the composites with CKD did not show any notable shrinkage reduction. When using higher dosages of these admixtures the mechanical properties were reduced, but lower dosages did not have such effects, especially as regards the compressive strength. The workability was also strongly dependent on the admixture dosage. Its improvement was observed especially in the case when slag was replaced by up to 30 % of CKD. Keywords: alkali-activated slag, by-pass cement-kiln dust, FBC fly ash, workability, strength, shrinkage Namen tega dela je preiskati vpliv prahu iz pe~i za cement (CKD) in dveh vrst lete~ega pepela (FBC) iz zgorevanja v vrtin~asti plasti na obdelovalnost, kr~enje in mehanske lastnosti (tla~na in upogibna trdnost)`lindre, aktivirane z vodnim steklom. Uporaba CKD in FBC je zelo problemati~na. Eden od glavnih razlogov za to je visoka vsebnost apna in sulfatov v teh odpadkih, kar lahko povzro~i nastanek ekspanzijskih hidracijskih produktov in posledi~no pokanje portlandskih cementnih materialov (OPC). Po drugi strani pa ti produkti lahko zavirajo veliko kr~enje`lindre, aktivirane z alkalijami (AAS). Da bi preverili te predpostavke in vpliv teh dodatkov na druge, zgoraj omenjene lastnosti, so bili pripravljeni na osnovi AAS drobnozrnati kompoziti. Grobozrnata plav`na`lindra (referen~no vezivo) je bila delno nadome{~ena (0-50 %) z eno vrsto CKD in dvema vrstama FBC lete~ega pepela. Opa`eno je bilo ob~utno zmanj{anje kr~enja pri vzorcih s celo majhnim dele`em lete~ega pepela, medtem ko kompozit s CKD ni pokazal opaznega zmanj{anja kr~enja. Pri uporabi ve~jih dele`ev obeh dodatkov so se poslab{ale mehanske lastnosti, pri majhnem dodatku pa ni bilo takega vpliva na tla~no trdnost. Obdelovalnost je bila mo~no odvisna od koli~ine dodatkov. Njeno izbolj{anje je bilo posebno opazno v primeru, ko je bila`lindra nadome{~ena z do 30 % CKD. Klju~ne besede: z alkalijami aktivirana`lindra, prah iz pe~i za cement, FBC lete~i pepel, obdelovalno...

Utilization of Fly Ashes from Fluidized Bed Combustion: A Review

“…It can be stated that FBCFA could be suitable for an alkali-activated material precursor, but often co-binders or pre-treatment of fly ash is needed. The studies acknowledged the low reactivity of FBCFAs in alkali-activation due to their crystalline nature, but compressive strengths of 10-30 MPa were often achieved [130][131][132][133][134][135][136][137][138][139], which is enough for most mortar-and paste-type applications. Even up to 50 MPa was reached [140] when most of the binder was slag-based and fly ash amount was 10-20 wt %.…”

“…Even up to 50 MPa was reached [140] when most of the binder was slag-based and fly ash amount was 10-20 wt %. The most common co-binder was metakaolin [33,37,138,141,142], but also GGBFS, PCC, OPC and FBC bottom ashes were used [37,130,132,135,136,[141][142][143][144]. Pre-treatment by grinding [132,133,136,137,139,145,146] or alkali fusion [143,146] was used in some papers to increase the reactivity of the ashes.…”

Some Issues of Shrinkage-Reducing Admixtures Application in Alkali-Activated Slag Systems

“…Recently, many various efforts to reduce the shrinkage of AAS were made, such as the partial replacement of BFS by mineral additives such as FA [ 21 , 23 , 24 , 25 ] and silica fume [ 23 ] or a combination of these [ 23 ], initial curing at elevated temperature [ 26 , 27 , 28 , 29 ], internal curing [ 30 ], use of fibers [ 31 , 32 , 33 ] and utilization of some expanding admixtures [ 34 , 35 , 36 ]. The most effective seem to be the partial replacement of slag by silica fume and heat curing prior to dry air exposure.…”

Experimental Determination of Early Shrinkage of Alkali-Activated Slag

“…The production of PC is rather demanding, economic-wise and even more so environmentally-wise, which has led to many efforts to partially replace it. The main AAM components are aluminosilicates, which are present for example in blast furnace slag, formerly used in the construction industry only as aggregate and for blended cements [1]. They harden due to inorganic polycondensation reactions during which the dissolved aluminate and silicate anions merge via oxygen atoms and form an amorphous or semicrystalline phase (C-A-S-H gel), wherein its structure varies depending on the used raw materials [2,3].…”