Sodium silicate activated slag‐fly ash binders: Part I – Processing, microstructure, and mechanical properties

Abstract

Alkali silicate activated slag and class F fly ash‐based binders are ambient curing, structural materials that are feasible replacements for ordinary Portland cement (OPC). They exhibit advantageous mechanical properties and less environmental impact than OPC. In this work, five sodium silicate activated slag‐fly ash binder mixtures were developed and their compressive and flexural strengths were studied as a function of curing temperature and time. It was found that the strongest mixture sets at ambient temperature and had a Weibull average flexural strength of 5.7 ± 1.5 MPa and Weibull average compressive strength of 60 ± 8 MPa at 28 days. While increasing the slag/fly ash ratio accelerated the strength development, the cure time was decreased due to the formation of calcium silicate hydrate (C–S–H), calcium aluminum silicate hydrate (C–A–S–H), and (Ca,Na) based geopolymer. The density, microstructure, and phase evolution of ambient‐cured, heat‐cured, and heat‐treated binders were studied using pycnometry, scanning electron microscopy, energy dispersive X‐ray spectroscopy (SEM‐EDS), and X‐ray diffraction (XRD). Heat‐cured binders were more dense than ambient‐cured binder. No new crystalline phases evolved through 28 days in ambient‐ or heat‐cured binders.

Slag‐fly ash and slag‐metakaolin binders: Part II—Properties of precursors and NMR study of poorly ordered phases

“…The mixing protocol discussed in Part I was adopted here with minor variations . As‐received, sodium silicate solution (BW‐50 from PQ Corporation, Valley Forge, PA) having a molar SiO 2 :Na 2 O ratio (modulus) of 1.65:1 was mixed with class F fly ash (Boral, Jacksonville, FL) using an IKA high shear mixer (Model RW20DZM, Wilmington, NC) for 5 minutes.…”

“…If slag and fly ash are sourced properly, the binders can lead to significant reductions in carbon emissions . Additionally, the binders have excellent compressive and flexural strengths, as well as reasonable set times . Previous literature reports many SFB mixture formulations .…”

Sodium silicate activated slag‐fly ash binders: Part III—Composition of soft gel and calorimetry

“…These peaks corresponded to gehlenite and akermanite . XRD of heat‐treated slag from Part I revealed crystalline peaks corresponding to a solid solution of gehlenite and akermanite.…”

“…Grade 100 ground granulated blast furnace slag (LaFarge, Chicago, IL) was added and further mixed for another 5 minutes. The mixing procedure and raw material properties are discussed in detail in Part I . All of the SFB mixtures are listed in Table below.…”

“…In Part I of this series, slag‐fly ash binder mixtures were created, the raw materials were selected and their compressive strengths and microstructures, were investigated after the binder hardened . From Part I, it was hypothesized that most of the reaction took place shortly after mixing the raw materials . Isothermal calorimetry was performed on these binders using in situ mixing in order to capture early reaction behavior.…”