Karen L. Scrivener

Hydration products of alkali activated slag cement

This paper presents some recent research on the microstructural development during alkaline activation of slag pastes. Pastes of different slags activated with a range of activators have been prepared and some preliminary results from the study of the pastes activated with sodium hydroxide and waterglass solutions are presented. X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron (BSE) imaging of polished samples in the SEM coupled with X-ray microanalysis, and secondary electron imaging of fracture surfaces were used to study the pastes. This study indicates: (i) that the products form by a dissolution and precipitation mechanism during the early stages of reaction, but at later stages the reaction may continue by a solid state mechanism; (ii) regardless of the activator used, the main hydration product is calcium silicate hydrate with low C S ratio and varying degrees of crystallinity; (iii) a crystalline phase of hydrotalcite type is formed in slag activated with either NaOH or waterglass; (iv) a crystalline phase of AFm type is also formed in slag activated with NaOH; (v) no hydrates of zeolite group or mica group were formed in slag activated with either NaOH or waterglass solution after wet curing at 20 ± 2 °C up to 15 months or at 80 °C for 14 days.

Delayed ettringite formation

Abstract Delayed ettringite formation (DEF) can damage concrete that has experienced a temperature above about 70°C. Claims that slow release of sulfate from the clinker can have a similar effect in concrete not thus heated are unsupported. Chemical and microstructural aspects of DEF are reviewed. Expansion results from formation of ettringite crystals of submicrometre size in the paste, the larger crystals readily observed in cracks and voids being recrystallisation products. The rate and ultimate extent of expansion are influenced by factors of three types: chemistry, which determines how much ettringite can be formed; paste microstructure, which determines the stresses produced by its formation; and concrete or mortar microstructure, which determines the response of the material to those stresses. Alkali present before the end of the heat treatment can increase expansion, but when present subsequently, it decreases expansion by inhibiting ettringite formation. Leaching therefore promotes expansion.

Factors affecting the strength of alkali-activated slag

The effect of several factors on the strength of alkali activated slags has been investigated. The most important factors were found to be: the type of alkaline activator, the means of adding activator, the dosage of alkali, the type and fineness of slag, SiO2/Na2O ratio (modulus, Ms) when using waterglass solution, curing temperature, liquid/slag or water/slag ratio and additive. Some of these factors are interdependent and the effect of changing more than one is usually not additive. The optimum range for each factor is suggested through reviewing previous work and our recent results of a full factorial range strength study. The interaction of factors is considered and discussed throughout the paper, hoping to gain a better understanding of the processing of alkali-activated slag (AAS) cement and concrete.

The percolation of pore space in the cement paste/aggregate interfacial zone of concrete

The cement paste in the interfacial transition zone (ITZ) around aggregate particles has a significantly higher porosity than bulk cement paste. This elevated porosity will effect the transport properties of concrete. The penetration of Woods metal into concrete samples indicates that the porosity in the interfacial zone is permeated preferentially to the bulk paste. This technique also indicates the width of the interfacial zone in which the porosity is interconnected.

Development of vegetable fibre-mortar composites of improved durability

The primary concern for vegetable fibre reinforced mortar composites (VFRMC) is the durability of the fibres in the alkaline environment of cement. The composites may undergo a reduction in strength and toughness as a result of weakening of the fibres by a combination of alkali attack and mineralisation through the migration of hydration products to lumens and spaces. This paper presents several approaches used to improve the durability performance of VFRMCs incorporating sisal and coconut fibres. These include carbonation of the matrix in a CO2-rich environment; the immersion of fibres in slurried silica fume prior to incorporation in the ordinary Portland cement (OPC) matrix; partial replacement of OPC matrix by undensified silica fume or blast-furnace slag and a combination of fibre immersion in slurried silica fume and cement replacement. The durability of the modified VFRMC was studied by determining the effects of ageing in water, exposure to cycles of wetting and drying and open air weathering on the microstructures and flexural behaviour of the composites. Immersion of natural fibres in a silica fume slurry before their addition to cement-based composites was found to be an effective means of reducing embrittlement of the composite in the environments studied. Early cure of composites in a CO2-rich environment and the partial replacement of OPC by undensified silica fume were also efficient approaches in obtaining a composite of improved durability. The use of slag as a partial cement replacement had no effect on reducing the embrittlement of the composite.

29Si and 27Al NMR study of alkali-activated slag

This paper presents the results of the investigation of the hydration of alkali-activated slag (AAS) by nuclear magnetic resonance spectroscopy (NMR). The cross-polarization (CP) technique was used in combination with magic-angle spinning (MAS). This research was part of a systematic study of alkaline activation of slag by several different techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with X-ray microanalysis of energy dispersive spectra (EDS), differential thermal analysis (DTA) and calorimetry. This NMR study provides information on the polymerization of silicates, the role of aluminates in cement hydration and the nanostructure of C-S-H gel.

High-performance concretes from calcium aluminate cements

Calcium aluminate cements have a radically different chemistry to Portland cements. Due principally to their higher cost, they do not compete directly with Portland cements. Nevertheless, concretes based on these cements have very high performance in specific applications. Two of these are discussed in this article: resistance to acid attack and particularly biogenic corrosion and abrasion resistance in hydraulic structures. Such applications extend the range of applications for cementitious materials.

Analysis of Phases in Cement Paste using Backscattered Electron Images, Methanol Adsorption and Thermogravimetric Analysis

In backscattered electron (bse) images of polished cement sections, anhydrous material, calcium hydroxide, other hydration products (mainly C-S-H) and porosity can be distinguished on the basis of their grey level in the image. Using an image analyzer connected directly to the SEM, it is possible to resolve these four components and so measure their relative proportions and distributions. The effects of magnification and the number of fields measured on the accuracy of bse image analysis are examined. The volume fractions of anhydrous material, porosity and calcium hydroxide derived from bse image analysis are compared with those obtained by other techniques and good correlation was found for the measurement of anhydrous material and porosity.

What causes differences of C-S-H gel grey levels in backscattered electron images?

Backscattered electron (BSE) images of heat-cured concretes show alite grains surrounded by inner C-S-H gel of two distinct grey levels (referred to as two-tone inner C-S-H gel). The lighter rim forms at elevated temperature whereas the darker rim develops during subsequent exposure to moisture at 20 °C. This microstructural feature can potentially be used as an indicator to assess the curing history of a concrete. However, microstructural examinations of room-temperature concretes containing silica fume or which have been exposed to severe conditions (external sulfate, carbonation) also show distinct rims of two-tone inner C-S-H gel. The chemical compositions of the rims were determined by EDX microanalysis in the scanning electron microscope (SEM). Our results show that for heat-cured samples, the different grey levels of the two-tone inner C-S-H are caused by relative differences in microporosity and water content and not by ones in chemical composition. However, in silica-fume blended concrete, sulfate attacked or carbonated specimens the different grey levels of the two-tone inner C-S-H gel were associated with significant differences in chemical composition. This difference allows two-tone inner C-S-H gel arising from heat curing to be distinguished from that arising from these other causes.

ANALYSIS OF COMPRESSIVE STRESS-INDUCED CRACKS IN CONCRETE

This paper presents the results of experimental studies of the micromechanical behavior of concrete under different loading conditions. Cylindrical specimens of normal- and high-strength concrete were tested under uniaxial and confined compression. Cracks and pores in the concrete specimens were impregnated with an alloy that has a low melting point. At the stress of interest, this alloy was solidified to preserve the stress-induced microcracks as they exist under load and images from the cross sections of the concrete specimens obtained using scanning electron microscopy (SEM). Stereological analysis that interprets three-dimensional structures by means of two-dimensional sections was used on the computerized images to determine the density, orientation, and branching of the compressive stress-induced microcracks and the effect of confinement on microcrack behavior. The density and branching of the microcracks decreased as the confining stress increased. The confining stress had a pronounced influence on microcracks in the interfacial transition zone (ITZ) between the cement paste and aggregate. The amount of interfacial cracking decreased significantly as the confining stress was increased. Under uniaxial compression there were significant differences in the crack patterns observed in normal- and high-strength concretes. Under confined conditions the two types of concrete had similar microcrack patterns.