L. S. Dent Glasser

The chemistry of ‘alkali-aggregate’ reaction

Abstract To clarify ideas about the alkali aggregate reaction, the reaction between sodium hydroxide and various forms of silica has been studied. Variations with time in the sodium, hydroxyl and silica concentrations in solution have been followed and the volume change of solid silica immersed in solution has been measured. If the silica is an absorbant and reactive form such as silica gel, there is an immediate drop in both sodium and hydroxyl concentrations. Subsequently, as the silica dissolves, sodium concentration rises again. Both the final concentration of dissolved silica and the volume expansion of solid silicate pass through a maximum at an intermediate total Si0 2 /Na 2 0 mole ratio.

Excess oxygen and the stability of “12CaO.7A12O3”

Abstract At temperatures between 1200°C and the melting point, “12CaO.7A12O3” contains 0.07 - 0.10% by weight excess oxygen. This can be detected by ordinary wet-chemical analysis for peroxide. The excess oxygen required for the synthesis of “12CaO.7A12O3” is readily obtained from any oxygen-containing atmosphere or from traces of moisture in inert atmospheres.

A multi-method study of C3S hydration

Abstract C3S pastes hydrated at 25°C have been studied using QXRD (to determine uncreacted C3S), TG (to determine CH and water), and trimethylsilylation (to determinemonomeric and dimeric silicate) and the results compared with ones obtained with analytical electron microscopy. Monomeric silicate is accounted for by unreacted C3S. The silicate in the C-S-H formed during the first few days is entirely dimeric, but at later ages dimer and polymer are both present. A new hypothesis for the reaction mechanism is tentatively proposed.

On the role of calcium in the alkali-aggregate reaction

Model systems for the alkali-aggregate reaction, using sodium hydroxide, calcium hydroxide and silica gel, have been studied. In one series of experiments, powdered silica gel was agitated with the alkaline solutions, and the changes in solution concentrations with time were followed; some of the solid products were analyzed. C-S-H forms so long as appreciable quantities of calcium ion remain in solution; thereafter the system behaves in the same way as model systems with no calcium introduced. Data are presented to show how [Ca2+] and [SiO2] in solutions in contact with C-S-H vary with pH. In a second series of experiments, the behaviour of lumps of reactive silica gel was observed; in some cases growths similar to those in a ‘silicate garden’ were noted, and the cause of this phenomenon is discussed. Analysis of the residual solid gel shows that the amount of calcium that penetrates into the interior is negligible.

Identification of some of the polysilicate components of trimethylsilylated cement paste

Abstract Trimethylsilyl derivatives from polymerised silicate anions in an eight-year-old cement paste have been fractionated by high-pressure liquid chromatography (hplc) and the fractions examined by thin layer chromatography, mass spectrometry and infrared spectrometry. The hplc separates linear from cyclic species. The major low molecular weight silicate anion in the C-S-H larger than dimer appears to be the linear pentamer. The presence in small quantity, of both linear tetramer and cyclic pentamer has been confirmed and the identification of the two high-temperature gas-liquid chromatography peaks with those species is supported.

Some observations on the rapid chemical test for potentially reactive aggregate

Abstract The chemistry of the ‘rapid chemical test’ is discussed. The choice of phenolphthalein as indicator is a poor one, and the difference in the end-point to phenolphthalein and methyl orange is a function of dissolved silica, as well as of any CO 2 content. Suggestions for possible improvements are made.

Hydrothermal study in the system Na2O-CaO-SiO2-H2O at 300°C

Abstract Using silicic acid, fused quartz, CaO, NaOH, α-CS, NS, NC 2 S 3 , N 2 CS 3 (2) as starting materials, reaction products are pectolite, β-NS 2 , xonotlite, foshagite, NC 2 S 2 H, CH, C 6 S 2 H 3 , quartz and NS. Pectolite is the predominent phase; its formation requires a minimum amount of water as solvent. Pectolite and NC 2 S 2 H are the only two quaternary products in the system. The reactivity of the starting materials, conditions of formation and the importance of the aqueous phase, including the way it is added, are discussed.

Refractive index, density and the coordination number of aluminum

Abstract Comparison of observed and calculated values of the molar refractivities of calcium aluminates and calcium aluminate hydrates gives information about the coordination numbers of the aluminum atoms.