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

Abstract

Sodium silicate activated, slag‐fly ash binders are potential alternative binders to Portland cement. In this study, the early age properties of slag‐fly ash binders namely, set time, and heats of reaction were investigated. Set time was investigated using a combination of two methods namely, the ASTM C403 penetration testing, and s‐wave ultrasonic wave reflectometry (SUWR). The discrepancy in set time identified by these two methods suggested the presence of a soft gel which eventually hardened with time. The composition of this soft gel was analyzed by suspending the chemical reaction of the binder after the soft gel formed, but before it hardened. In order to analyze the composition of the soft gel, selective chemical extractions were performed on the binder. 29Si Magic Angle Spinning‐Nuclear Magnetic Resonance (MAS‐NMR), and FTIR spectroscopy were performed on binders and extraction residues. The soft gel contained a modified calcium silicate hydrate gel (C–N–S–H where N=Na), with a short mean chain length and no observable Al incorporation. Orthosilicate units were also found to be present in relatively high proportions when compared to hardened binders at later ages.

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

“…The presence of Al(V) might be due to the presence of a small amount of aluminum atoms in strained fivefold coordination in the glassy gehlenite portion of both grade 100 and grade 120 slags. FTIR and XRD of slags can be found in Part I, Part III, and in the supplemental information.…”

“…However, it dissolved very little fly ash and metakaolin . FTIR and MAS‐NMR spectra of HCl extraction residues exhibited little or no change when compared to starting fly ash (see and ) …”

Bone ash reinforced geopolymer composites

“…Alkali‐activated cement containing large amounts of calcium was initially developed by Glukhovsky in Ukraine during the 1950 s 13 . It can be made from waste materials of Class F fly ash and ground granulated blast furnace slag 13‐18 . These materials contain a cementitious binder phase of calcium silicate hydrate (CSH), which is the binder phase in cements.…”

“…In nuclear magnetic resonance, the silicate tetrahedra share linkages with two or sometimes three other tetrahedra, denoted as Q 2 or Q 3 (mAl) in Magic Angle Spinning nuclear magnetic resonance (MAS NMR). Alkali‐activated cements are often confused with geopolymers originally pioneered by Davidovits using calcium‐free systems based on metakaolin and possessing an X‐ray amorphous structure of tetrahedral alkali aluminosilicate frameworks (Q 4 (mAl) where m = 1‐4) 1,7,12,14‐17 . In geopolymer, the Al is four‐coordinated, creating a negative charge imbalance which is compensated for by the presence of Group I monovalent cations such as Na + or K + 1,7,12,14‐17 .…”

“…1,7,11 Alkali-activated cement containing large amounts of calcium was initially developed by Glukhovsky in Ukraine during the 1950 s. 13 It can be made from waste materials of Class F fly ash and ground granulated blast furnace slag. [13][14][15][16][17][18] These materials contain a cementitious binder phase of calcium silicate hydrate (CSH), which is the binder phase in cements. In nuclear magnetic resonance, the silicate tetrahedra share linkages with two or sometimes three other tetrahedra, denoted as Q 2 or Q 3 (mAl) in Magic Angle Spinning nuclear magnetic resonance (MAS NMR).…”