J.E. Gillott

Mechanism of alkali-silica reaction and the significance of calcium hydroxide

Experiments indicated that Ca(OH)2 [CH] aggravates alkali-silica reaction causing increased expansion of mortars bars. Ca(OH)2 has two major functions: firstly it acts a “buffer” to maintain a high pH, i.e. a high concentration of hydroxyl ions in pore solutions; secondly, Ca++ ions may exchange for alkali ions on silica gel leading to further production of swelling alkali-silica silica complex. A mechanism of alkali-silica reaction is proposed which emphasizes the effect of Ca(OH)2 on reaction and expansion.

Microstructural investigation of innovative UHPC

The production of ultra high performance concrete (UHPC) with target strengths greater than 200 MPa has recently been considered for specific structural applications that need this enhanced mechanical performance. The main purpose of developing these innovative UHPC mixtures is to produce high-strength precast concrete elements with excellent durability to serve as both the inner wedges and the outer barrel of a new nonmetallic anchorage system. The anchorage is for post-tensioning applications using carbon fibre-reinforced polymer tendons. The UHPC mixtures examined show very dense microstructures with some unique characteristics. The bond between the micro carbon fibres and the cement paste seems to be very good and the cement paste observed in the vicinity of the fibres was shown to be very dense and homogeneous. The micro carbon fibres seem to govern the strength and postcracking behaviour of these materials.

Alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR) in activated blast furnace slag cement (ABFSC) concrete

This paper examines the dimensional change of concrete prisms that were fabricated using concrete made with either sodium silicate or sodium carbonate ABFSC binder incorporating either an innocuous control aggregate, an aggregate that is known to be reactive by way of ASR or ACR in ordinary Portland cement (OPC) concrete or an aggregate that contains reactive silica although a good service record has been exhibited. The concrete prism test procedures detailed in the Canadian Standards Association (CSA A23.2-14A-94) specifications were used. The results of a parallel study of the early dimensional stability of ABFSC concrete which involved petrographic examination of concrete thin sections are discussed. It was found that, although ABFSC concrete is more vulnerable to ACR, it is less susceptible to expansion due to ASR than OPC concrete, however, expansions after one year can still be unacceptable. Unlike OPC, many large cavities form within the paste structure of ABFSC mixtures at early ages which is accompanied by shrinkage. This behaviour must be considered in the analysis.

The influence of mortar-aggregate bond strength on the behaviour of concrete in uniaxial compression

Abstract Modification of aggregate surfaces by ball mill treatment with various grades of abrasive grit allowed concretes to be prepared which differed only in the surface texture of the coarse aggregate. Due to the dependence of mortar-aggregate bond strength on the texture of the aggregate surface, an assessment could thus be made of its influence on the behaviour of concrete in compression. Substantial changes in the stress and longitudinal strain at ultimate load were observed. These appeared to be caused mainly by the retardation of initial bond failure at Initiation Stress due to the increased bond strength of the rougher aggregates. The occurrence of mortar-aggregate columns and cone-like deposits of mortar at the “poles” of aggregate particles was observed in failed concrete specimens and failure of the mortar-aggregate bond in both tension and compression-shear was confirmed.

Effect of replacement of silica flour with silica fume on engineering properties of oilwell cements at normal and elevated temperatures and pressures

Abstract Substitution of silica flour with varying proportions of silica fume affects strength and permeability of hardened cements. At ambient conditions changes are proportional to the amount of silica fume. After hydrothermal treatment (H.T.) at 230°C and 2.75 MPa compressive strength decreases markedly for the 1:1 fume:flour mix; a higher proportion (3:1) results in acceptable strengths which steadily increase with period of fog-room precuring. Mixes with different proportions of flour:fume show a strength minimum between 14 and 28 days of total age. Water permeability after H.T. decreases with increasing amount of fume; however, each mix showed a maximum after about 14 days. Property changes depend on the amount of free silica, CH and amount and type of C-S-H at the onset of hydrothermal reaction. The fabric of the fog-room cured material is largely retained after H.T. evidently due to epitactic growth of xonotlite.

Improved control of alkali-silica reaction by combined use of admixtures

Abstract Expansion of mortar or concrete caused by alkali silica reaction (ASR) is known to be reduced by certain pozzolans, by air-entrainment, by some retarders and by decrease in the alkali content of the material. Those methods of controlling expansion have been used individually and in combination in mortar bars made with 2% highly expansive opal. Their effectiveness has been evaluated by comparing the expansion of these bars with one another and with that of controls containing opal but no mineral or chemical admixtures. The bars containing both a retarder and silica fume showed essentially the same reduction in expansion as bars containing silica fume only. The bars containing a combination of a retarder and an air-entraining agent showed greater reduction in expansion than the bars containing either retarder or air-entraining agent when used separately. It is concluded that combined admixtures supplement one another and have the best chance of improving performance significantly only when in each case reduction in expansion depends upon a different mechanism.

The behavior of silicocarbonatite aggregates from the Montreal area

Aggregates in the Montreal area of Canada have been made from crushed Ordovician limestones (which include silicocarbonatite rock), which have been associated with durability problems in Portland cement concrete which, in precast units, included large numbers of rail ties. Concrete made with silicocarbonatite aggregates contained over 1.5 percent more Na2O than similar bars made with Exchaw limestone aggregates. A reaction involving the rare mineral dawsonite in the silicocarbonatite is thought to be responsible for the higher Na2O content. This article reports on a study undertaken to confirm the increase of NaOH content of concrete containing silicocarbonatite and the associated increase in expansion due to alkali-aggregate reaction (AAR). The authors also considered whether the dawsonite reaction itself is expansive, investigated other effects of the dawsonite reactions, and reexamine the later-age expansion of bars made by accelerated curing, which in previous research was attributed to delayed ettringite formation (DEF). Results indicate that damage from AAR was aggravated by the dawsonite reaction and DEF was associated with high temperature curing. However, a lack of expansion attributable to DEF in heat-cured concrete and mortar made with silicocarbonatite aggregates and recent literature suggest that the release of alkali, carbonates, and aluminate ions by the dawsonite reaction may have potentially beneficial side effects if properly exploited; these could help to prevent impaired durability performance by Portland cement concrete.

The effect of initial curing temperature on the performance of oilwell cements made with different types of silica

Abstract Oilwell cement blended with silica subjected to hydrothermal conditions (2.75 MPa, 230°C) shows changes in engineering properties which depend on the type and proportions of silica. The most significant changes occur after shorter periods of pre-curing. Thus the phase composition and fabric at the onset of hydrothermal reactions has a very significant effect on post-hydrothermal strength and permeability. Variation of the initial curing period and temperature, which influences the rate of both hydration and pozzolanic reactions, may be used to change the characteristics of hardened cement. Cements cured at lower temperatures showed decreased strength and increased permeability due to a much slower rate of the pozzolanic reaction between silica fume and calcium hydroxide and a reduced rate of cement hydration. After hydrothermal treatment, however, samples pre-cured at lower temperatures for less than 3 to 4 weeks, showed increased compressive strength and slightly decreased permeability.

Modification of engineering behaviour of thermal cement blends containing silica fume and silica flour by replacing flour with silica sand

Abstract Replacement of silica flour with coarser particles of sand in cement blends containing silica fume affected the compressive strength and permeability of hardened mixes at ambient and hydrothermal conditions. At ambient conditions the major effect was a marked increase in water permeability due to larger and/or better interconnected pores. Early compressive strength also increased. After 7 days of hydrothermal treatment at 230°C and 2.75 MPa, strength increased markedly. Values were practically independent of period of precuring for two proportions of fume to sand used in this work. Higher strength values resulted for mixes with higher fume:sand ratio. Water permeability was not significantly affected, however, the lowest values were obtained for blends with fume:sand ratio of 3:1. Replacement of silica flour by sand was beneficial particularly in regard to the compressive strength of hydrothermally treated cement-silica blends.

Effects of Hydrothermal Treatment on the Engineering Properties, Microstructure, and Composition of Oilwell Cements

Under hydrothermal conditions normal portland cements show loss of strength and increase in permeability due to changes in phase equilibria and in the chemistry of hydration (Lea, 1971; Eilers et al., 1980). Cement blends with 40% silica flour or sand show improved hydrothermal characteristics; at ambient temperature, however, an inert filler degrades engineering properties. Improved ambient properties are achieved when silica fume, a highly reactive pozzolan (Buck and Burkes, 1980; Malhotra and Carette, 1982; Malhotra, 1984), replaces part of the cement. This is attributed to changes in composition and microstructure due to the pozzolanic reaction (Gallus and Pyle, 1978; Cheng-yi and Feldman, 1985a,b). In the case of oilwell cement-fume blends ambient properties are thus improved but hydrothermal behavior is more complex (Grabowski and Gillott, 1989).