Alite Grains Research Articles

Feasibility Studies for Use of Copper Slag in Clinker Manufacture

This study investigated the feasibility of using copper slag (CS-C) as a raw material in the production of Portland cement (PC). The effect of addition of 0%-3.5% CS-C on the clinkerization process, mineralogy, microstructure, and compressive strength development was studied. The Presence of CS-C in the raw mix resulted in the reduced temperature of clinkerization by about 50 °C. The quality of clinker prepared from the raw mix containing 3% CS-C and burned at 1400 °C was comparable to that of clinker prepared from the raw mix containing no CS-C and burned at 1450 °C. Gradual polymorphic modification of alite phase from rhombohedral to monoclinic upon the addition of CS-C was confirmed by X-ray diffraction (XRD) study. XRD and optical microscopy studies revealed the formation of more alite in the clinker prepared, at 1400 °C, from the raw mix containing 3% CS-C compared with the clinker prepared, at 1450 °C, from the raw mix containing no CS-C. This finding indicated the mineralizing effect of CS-C on clinkerization. Well-developed and large-sized alite grains were observed in the clinker samples prepared using CS-C. Portland clinker, prepared using CS-C as a raw mix component showed better mechanical performance characteristics than the control clinker. These results clearly establish the beneficial role of CS-C as a raw mix component in clinkerization and can replace conventional sources of iron such as laterite.

Micro‐reactors to Study Alite Hydration

A new approach to study the early hydration reaction of alite is presented. It relies on milling micron‐sized gaps into alite grains using a Focused Ion Beam. This provides for a “micro‐reactor” in which hydration in confined conditions can be followed after introducing aqueous solutions of any desired composition. Hydration is followed by stopping the reaction at desired ages using solvent exchange and imaging the gaps using a Scanning Electron Microscope. These micro‐reactors offer several advantages over conventional methods to monitor alite hydration. In particular, we can study the time evolution of dissolution in a reproducible manner using a setup that mimics particles in close contact. This paper presents this new methodology and applies it to studying the effects of solution composition and selected chemical admixtures on the dissolution of alite. New insights into the role of these factors are obtained and discussed.

A reply to the discussion “Accelerated growth of calcium silicate hydrates: Experiments and simulations” by S. Bishnoi and K. Scrivener

We sincerely thank Dr. Bishnoi and Prof. Scrivener for their com-ments [1] on Nicoleau's paper[2]. This discussion contributessigni cantly in clarifying the current debate about alternatives tomodel the cement hydration kinetics. In spite of our initial intereston the accelerating part of the hydration[2], the main point discussedby Bishnoi and Scrivener concerns the limitation of growth kinetics ofC-S-H during the alite hydration, leading to the post-peak decelera-tion in isothermal calorimetry. Our reply will therefore also focus onthis part of the hydration process, where different models are todayin opposition. Doing so, we answer the ve different points claimedby Bishnoi et al. in[1].1. On factors limiting the growthThe cornerstone condition for a proper description of the alite hy-dration kinetics is to take into account (1) the nucleation and growthof C-S-H and (2) the dissolution of alite[3]. The hydration kineticscannot be described only by a nucleation and growth process as inBishnoi's model[4] and in other models based on“Avrami-like” equa-tions discussed later in this reply. Besides in the original Garrault andNonat's model[5], the dissolution was also not considered since onlythe accelerating part of the hydration was addressed in this paper. Inthis part, the limitation by the dissolution is negligible. An extensionof this model including also the dissolution rate and the grain sizedistribution was recently proposed[6].In order to generate a limitation of the hydration rate, modelsbased on Avrami-like equations introduce some impingements tothe C-S-H growth. In particular, the Bishnoi's model asserts that thedeceleration of the growth originates with“impingements with C-S-H originating from different particles will affect nucleation and growthkinetics” as it has been rst proposed in[4] and also questionably as-sumed in[1]. This con guration is schematically depicted by the case1inFig. 1. This hindrance undergone by the C-S-H growth is termedas an inter-layers impingement. On the contrary, in the Garrault andNonat's model[5], there is also a hindrance but due to the collisionof growing clusters located on same grains. This can be termed asan intra-layer impingement (case 2 inFig. 1). Nevertheless, thehindrance is not suf cient to account for the complete deceleration;it is actually considered that the main contribution to this decelera-tion is the decrease of the dissolution rate due to the coverage ofthe alite surface by C-S-H[2,3,5]. The limitation of the dissolution isdescribed in Nicoleau's paper through a diffusion barrier resultingfrom the coverage of alite grains by C-S-H and parameterized by apermeability coef cient.The free space between cement grains is strongly affected by thegrain size distribution, as the water to alite ratio also does. Thegrain size distribution has therefore a considerable effect in case 1(Bishnoi's model) since the C-S-H growth and its limitation isdescribed by the lling of the free space around the cement grainsas it has been proposed in[4] and mentioned in [1]. As far as earlyhydration times are considered, the particle size distribution is notimportant in case 2 since the growth is described by clusters growingonto a single grain. The rst point debated by Bishnoi et al. is there-fore strictly dependent to their model. The question is now toconsider which model is more relevant for a proper description ofalite hydration. We recall in the following section several experimen-tal facts in favor of case 2.2. Effect of the water to cement ratioMore than 20 years ago, it was experimentally demonstrated rstby Damidot [7 9] in stirred and diluted suspensions and con rmedlater [10] by others that the water to cement (or the water to alite/C

Tobermorite/jennite- and tobermorite/calcium hydroxide-based models for the structure of C-S-H: applicability to hardened pastes of tricalcium silicate, β-dicalcium silicate, Portland cement, and blends of Portland cement with blast-furnace slag, metakaolin, or silica fume

The purpose of this article is to discuss the applicability of the tobermorite–jennite (T/J) and tobermorite–‘solid-solution’ calcium hydroxide (T/CH) viewpoints for the nanostructure of C-S-H present in real cement pastes. The discussion is facilitated by a consideration of the author's 1992 model, which includes formulations for both structural viewpoints; its relationship to other recent models is outlined. The structural details of the model are clearly illustrated with a number of schematic diagrams. Experimental observations on the nature of C-S-H present in a diverse range of cementitious systems are considered. In some systems, the data can only be accounted for on the T/CH structural viewpoint, whilst in others, both the T/CH and T/J viewpoints could apply. New data from transmission electron microscopy (TEM) are presented. The ‘inner product’ (Ip) C-S-H in relatively large grains of C 3S or alite appears to consist of small globular particles, which are ≈4–8 nm in size in pastes hydrated at 20 °C but smaller at elevated temperatures, ≈3–4 nm. Fibrils of ‘outer product’ (Op) C-S-H in C 3S or β-C 2S pastes appear to consist of aggregations of long thin particles that are about 3 nm in their smallest dimension and of variable length, ranging from a few nanometers to many tens of nanometers. The small size of these particles of C-S-H is likely to result in significant edge effects, which would seem to offer a reasonable explanation for the persistence of Q 0(H) species. This would also explain why there is more Q 0(H) at elevated temperatures, where the particles seem to be smaller, and apparently less in KOH-activated pastes, where the C-S-H has foil-like morphology. In blended cements, a reduction in the mean Ca/Si ratio of the C-S-H results in a change from fibrillar to a crumpled-foil morphology, which suggests strongly that as the Ca/Si ratio is reduced, a transition occurs from essentially one-dimensional growth of the C-S-H particles to two-dimensional; i.e., long thin particles to foils. Foil-like morphology is associated with T-based structure. The C-S-H present in small fully hydrated alite grains, which has high Ca/Si ratio, contains a less dense product with substantial porosity; its morphology is quite similar to the fine foil-like Op C-S-H that forms in water-activated neat slag pastes, which has a low Ca/Si ratio. It is thus plausible that the C-S-H in small alite grains is essentially T-based (and largely dimeric). Since entirely T-based C-S-H is likely to have different properties to C-S-H consisting largely of J-based structure, it is possible that the C-S-H in small fully reacted grains will have different properties to the C-S-H formed elsewhere in a paste; this could have important implications.

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.

Electron probe microanalysis of gasifier slag cement pastes

The widespread use of cement replacement materials in concrete has generated interest in the potential use of gasifier slag (GS), a new replacement material with a chemical composition intermediate between that of ground granulated blastfurnace slag (GGBS) and pulverized fuel ash (PFA). A partial understanding of how the material behaves in concrete can be obtained through a study of the hydration of GS cements. New information on the hydration products has been gained by an examination of the composition of hydrates in GS cement pastes. Electron probe microanalysis has been used to study the composition of hydration rims around alite and slag grains in two GS cement pastes. The results are expressed in terms of atomic ratios, depicted graphically and compared with published probe data for GGBS and PFA cement pastes. The presence of GS modifies the composition of the hydrates in the rims around alite grains, particularly by the incorporation of aluminium. The composition of the hydrate around the GS grains is shown to be more similar to that around PFA than to that around GGBS grains.

The electron microprobe analysis of the CSH phases in a 136 year old cement paste

Electron probe microanalysis of a meso-Portland cement paste, 136 years old, shows the formation of inner and outer CSH around alite grains. The compositions of these hydrates are identical to the compositions of inner and outer hydrates from modern OPC's.

Effect of Age on Diffusion in Hydrated Alite Cement

Hydration of alite cement paste of 0.59 water/cement ratio caused the larger pores (>50 nm) to progressively fill with calcium hydroxide and a porous calcium silicate hydrate gel. At early stages of hydration, the larger pores were directly interconnected and diffusion rates were consequently rapid. At later times, the hydrate shells around the adjacent alite grains began to intergrow, continuity of the larger pores was reduced, and diffusion rates were reduced. Beyond 70% hydration, diffusion rates diminished very rapidly with only small amounts of additional hydration. The reduction in continuity of the larger pores was monitored directly by microscopic examination of resin replicas of the pores.

The composition of the C-S-H phases in portland cement pastes

The composition of the calcium silicate hydrate (C-S-H) present in Portland cement pastes from eight days to five years old, and in concrete samples ten years old, has been determined by electron probe micro-analysis. The dependence of C-S-H composition on water/cement ratios has also been studied. When the cement is only partially hydrated inner and outer hydrate layers are seen around alite grains. These hydrates have different chemical compositions. With progressive hydration it becomes impossible to distinguish the inner from outer hydrates without a reference alite grain. At this stage two distinct C-S-H compositions can still be found by microanalysis. After further hydration a single CSH composition predominates. The calcium content of C-S-H varies significantly with water/cement ratio and there is new evidence to suggest that in the C-S-H structure Mg replaces Ca, and that Al, S and Fe replace Si.

The effect of pulverised-fuel ash on the c/s molar ratio and alkali content of calcium silicate hydrates in cement

The effect of pulverised-fuel ash on the C/S molar ratio and alkali content of calcium silicate hydrates in cement has been investigated by electron microprobe analysis. It has been found that for a cement that has been hydrated for 8 days pfa caused the C/S ratio of the inner hydrates around the alite grains to be lowered from 1.71 to 1.55. More potassium was found in the lower C/S ratio hydrates. It is not clear whether the resulting reduction in available alkali is sufficient to explain the effectiveness of pfa in reducing the expansion due to the alkali-silica reaction.