In this paper, which describes part of a major investigation, the role of cement content in specifications for concrete durability is examined. The main cements considered were (a) Portland cement (PC; CEM I), (b) PC/30% PFA (CEM II/B-V) and (c) PC/50% GGBS (CEM III/A), while other BS EN 197-1 cements were tested selectively. Concretes of fixed w/c ratio (w/(PC + addition) ratio), but with cement contents varying around the minimum values given in standards were evaluated. The workability of concrete with low cement and water contents was controlled with a superplasticiser and the fines content was maintained by the use of ground limestone as a filler. Tests were carried out to examine these effects on the fresh, engineering, permeation and durability properties of concrete. The results show that at fixed w/c ratio, reduction in cement content by up to 22% had no adverse effect on most concrete properties and, if anything, gave some improvement. The various cement types and combinations tested had little effect on the trends observed. Overall, it was concluded that, in addition to minimum strength class and maximum w/c ratio (and in some cases cover depth), specifying minimum cement content for concrete durability was not necessary.

This paper considers the influence of moisture addition (conditioning) to PFA of up to 40% by mass on its physical and chemical properties and performance as a cement component in concrete. The work examined conditioned PFA, covering a range of fineness levels and lime contents, stored for up to 18 months, either in the laboratory or in stockpiles established at four UK power stations. Data for laboratory-conditioned PFA indicated progressive particle agglomeration (mainly in the finer fractions) with storage period, which was greatest at moisture levels of 10–20% and for material with total and free lime contents in excess of 3·0% and 0·1%, respectively. Corresponding increases in water demand and reductions in strength factor were also noted with conditioned material. Other changes, including increased LOI, reduced free lime content and formation of hydration products such as bassanite and gypsum, indicated that the effects were due to chemical activity. However, no significant changes to bulk oxide, modal composition and glass contents of PFA were obtained, suggesting that conditioning effects are mainly localized at particle surfaces. In general, similar behaviour to that in the laboratory was noted for PFA stored in power station stockpiles. The workability and strength development to 28 days of conditioned PFA concrete were comparable to that containing dry PFA, with only slight reductions in both properties with long-term conditioned PFA storage.

In this paper, the second of the series, examining the role of cement content in specifying concrete durability, the influences of aggregate characteristics are considered. As for the first paper, the reported work was carried out within a framework of mix adjustments, required to produce practical concretes, that is, with the inclusion of filler and superplasticising admixture. A total of five UK normalweight aggregates and a lightweight aggregate were examined. It is shown that, for a given w/c ratio and aggregate type, reduction in cement and water contents (and the associated mix adjustments) did not adversely affect concrete properties (fresh, bulk engineering, permeation and durability). Indeed, many aspects of concrete performance were enhanced, with the improvements with cement reduction generally being greatest for aggregates having low absorption. Furthermore, the influence of cement content at a given w/c ratio on durability was found to be less significant than that of a change in aggregate type for the range of UK materials tested. The implications of the study for concrete construction practice are discussed.

No abstract

A B S T R A C T R I~ S U M I ~This paper describes a study carried out to examine the influence of storing pulverized-fuel ash (PFA) in an excess of water, or lagoon, on the properties of the material itself and its suitability for use as a component of the binder in concrete. The work considered PFA slurried and stored in the laboratory (simulated lagoon PFA), and material recovered from power station lagoons (lagoon PFA). The results indicate that simulated lagoon PFA tends to agglomerate and this appears to relate to chemical changes taking place within the material, as indicated by increasing loss-on-ignition, the formation of gypsum and bassanite, and leaching of solubles from particle surfaces into the simulated lagoon solution. Despite these effects, the use of this material, as part of the binder, has only a minor influence on concrete properties, with small changes in workability, bleeding and compressive strength measured. There was general agreement between the results for laboratory and site lagoon PFA. If the effects noted are accommodated within the concrete mix design, there is potential for using lagoon PFA as a binder component in concrete construction.