Karen Luke

Selective dissolution of hydrated blast furnace slag cements

Abstract Methods for determining the content of blast furnace slag (BFS) in blended cements composed of BFS and OPC have been briefly reviewed. A variety of dissolution procedures were investigated and the results compared critically. Extractions with methanol-salicyclic and/or acetic acid show that these solvents attack the BFS but fail to dissolve certain components of the cement which have been identified by X-ray diffraction. A recent method involving EDTA solvent extraction appeared more promising and additional work was undertaken to verify and improve it. The method was used to determine BFS content of blended cements hydrated for periods up to 12 months.

Modification of cement pore fluid compositions by pozzolanic additives

Abstract The impact of blending agents on the internal environment of cement systems is assessed by chemical analysis of the pore fluid in cured blends. The short-term behaviour of PFA blends is complicated by the presence of soluble alkali. In general, pore fluid sodium levels are not much altered by PFA while potassium is reduced. SiO 2 fume has a more immediate impact on the pore fluid chemistry: 10–20% additions can lead to order-of-magnitude reductions, especially in caesium content. The potential of cement to uptake chloride anions is assessed: the correlation with the sum of tricalcium aluminate (C 3 A) and ferrite phases is more significant than with C 3 A alone. Blast furnace slags can markedly affect the internal redox potential, which decreased from +100mV in Portland cement to minus 200–225mV in slag-rich blends.

Internal chemical evolution of the constitution of blended cements

Blends comprising 30% of either fly ash or blast furnace slag and 70% ordinary Portland cement have been aged up to two years at 25°, 40° and 55° in CO2-free conditions. Changes in the mineralogy were determined by selective dissolution, TG, XRD, analytical electron microscopy and Ca(OH)2 determinations on both whole samples and dissolution residues. Portland cements converge on a steady-state but blended cements, especially those with slags, show much evidence of persistent internal inhomogeneity. Higher cure temperatures accelerate reaction but not necessarily in the same direction as at 25°C.

Equilibria and non-equilibria in the formation of xonotlite and truscottite

Abstract The conclusions of Roy and Harker (1960) regarding phase equilibria in the silica-rich part of the CaO-SiO 2 -H 2 O system are substantially confirmed; in particular, truscottite (C 7 S 12 H ≈3 ) coexists stably with silica and aqueous solution from below 200°C to approximately 350°C at saturated steam pressures. However, non-equilibrium situations readily occur in which xonotlite (C 6 S 6 H) is formed when truscottite would be expected. These situations are attributed to incongruent solubility of truscottite, coupled with extreme reluctancr or inability of xonotlite to react with silica and aqueous solution to give truscottite. As a result, if xonotlite forms before truscottite it persists, and any process in which silica is effectively removed from the system, however temporarily, can cause a conversion of truscottite into xonotlite that is for practical purposes irreversible and potentially complete. Such loss of silica could occur through liquid or vapor transport, and may be brought about by temperature gradients.

Characterization of Fly Ash and its Reactions in Concrete

Fly ashes are currently being produced that are much more widely different from each other in composition and other characteristics than had been previously experienced, owing to the widespread use of low rank subbituminous and lignitic coals. The current ASTM classifications into Class F and Class C pozzolan categories are not adequate to describe all their important properties. Current characterization methods are reviewed, including physical characterization by particle size distribution, shape, apparent specific gravity, content of hollow grains and of residual coal fragments, etc., chemical procedures of various kinds, and SEM, EDXA, XRD, and other methods for the determination of mineralogical content and glass character. Etching and chemical dissolution procedures are particularly important. The state of these various methods, current results of their use in rly ash characterizations, and the relations of these to reactivity and performance of fly ashes in cement and concrete are discussed.

Some factors affecting formation of truscottite and xonotlite at 300–350°C

Abstract Formation of truscottite and xonotlite was studied at Ca/(Si + Al) = 0.6, 300–350°C and saturated steam pressures. Truscottite is favored by use of silicic acid rather than quartz, or of β-C2S rather than Ca(OH)2, and by the presence of small amounts of alkali. Partial replacement of Si by Al has no apparent influence on whether truscottite or xonotlite is formed. In runs at 350°C and saturated steam pressure using commercial Class G or Class J oil-well cements, truscottite was slowly replaced by xonotlite, but wollastonite may be the ultimately stable product under these conditions.

Ca7Si16O40H2, a new calcium silicate hydrate phase of the truscottite group

A new phase, of probable composition Ca7Si16O40H2, has bee prepared hydrothermally from CaO and silicic acid in silica rich mixes at temperatures around 375†C. It is structurally closely related to truscottite, from which it is derived by taking one of the two types of silicate layer without the other. Analytical and unit cell data are presented, together with the probable crystal structure and calculated X-ray powder diffraction patterns.