Effect of microsilica and blast furnace slag on pore solution composition and alkali-silica reaction

Abstract

Pore fluid analysis and 6-month expansion tests were carried out on plain, 10 and 20% microsilica, and 60 and 70% blast furnace slag cements. The objective was to study the correlation between expansions and hydroxide concentrations in the pore solution, alkali-silica reaction (ASR) behaviour of microsilica- and slag-blended cements, optimisation of microsilica addition, and alkali-removing capacity of blast furnace slag. Incorporation of 10–20% microsilica and 60–70% slag reduced expansions from nine times the permissible expansion to safe values ranging from one-tenth to one-half the allowable expansion. The data developed in this study confirm a broad correlation between the hydroxide ion concentration and ASR-generated expansions. However, there are obvious deviations which are strongly indicative of other concurrently occurring mechanisms in addition to the alkali removal action. Blast furnace slag is shown to be an active remover of alkalis and is especially effective in medium-alkali cements, where for equal alkali contents, the performance of 60% slag cement is comparable with that of 10% microsilica cement. However, the effectiveness of the slag decreases as alkali content of the cement increases. Addition of 20% microsilica sweeps virtually all the hydroxide ions from the pore solution. Ten per cent microsilica significantly reduces OH − concentration, although in 1·2% and 1·5% alkali cements the hydroxide ion concentrations remain above the postulated threshold of 250 mM/l. However, the expansions for 10% microsilica cement with OH − concentration of 368 mM/l is well below the allowable limit. This suggests that, for blended cements, the 250 mM/l value is excessively conservative and a value close to 400 mM/l would be more realistic. This is possibly a result of the fact that the incorporation of reactive silica in the form of microsilica or slag, in addition to chemical factors, brings about a marked refinement and segmentation of the pore structure, thereby effectively retarding the transportation of alkalis to reaction sites. From both the standpoint of expansion and the 400 mM/l threshold value, it seems that for most cases of medium- to high-alkali cements, 10% cement replacement by microsilica is adequate for ASR control.