Multi-phase hydration model for prediction of hydration-heat release of blended cements

Recently, the heat release during cement hydration and the so-caused temperature rise was exploited for (i) identification of material properties of early-age cement-based materials (stiffness, strength), and (ii) determination of the diameter and the cement content of jet-grouted structures. In this paper, the underlying hydration model for determination of the heat release and its rate is refined for Ordinary Portland Cements (OPC) and extended towards blended cements. Hereby, the overall degree of hydration with one kinetic law is replaced by a multi-phase hydration model, taking the hydration kinetics of the main clinker phases into account. As regards blended cements, which are commonly used in engineering practice, the effect of slag hydration is incorporated into the presented multi-phase model. The developed hydration model for both plain and blended cement is validated by means of differential-calorimetry (DC) experiments.     

Early age strength enhancement of blended cement systems by CaCl2 and diethanol-isopropanolamine

The enhancement of the 1 day strength of cementitious systems by a combination of calcium chloride (CaCl₂) and diethanol-isopropanolamine (DEIPA) was studied, particularly in blended cement systems. A combination of quantitative X-ray diffraction with Rietveld refinement (QXRD), scanning electron microscopy (SEM)/backscattered electron image analysis, thermogravimetric analysis (TGA), and isothermal calorimetry were used to investigate the mechanism of strength enhancement by the additives. The additives were found to increase the early age mortar strength by enhancing the cement hydration, with the DEIPA enhancing primarily the aluminate hydration. DEIPA also affected the morphology of portlandite which was formed as thin plates. In parallel, the calcium-to-silica ratio of the C-S-H was found to increase with the use of DEIPA, possibly because of the inclusion of microcrystalline portlandite. After 48 h DEIPA was found to directly enhance the rate of reaction of granulated blast-furnace slag and fly ash.     

Strength optimization of “tailor-made cement” with limestone filler and blast furnace slag

The use of cements made with portland clinker and two or three additions has grown because they present several advantages over binary cements. Production of composite cements has produced a necessary shift in the manufacture process used in the cement industry. Now, it is known that the separate grinding and mixing technology is more convenient in order to produce these cements, called market-oriented or tailor-made cements. However, their optimum formulations require the help of methods of experimental design to obtain an appropriate performance for a given property with the least experimental effort.In this study, the interaction between limestone filler (LF) and blast-furnace slag (BFS) is analyzed in mortars in which portland cement (PC) was replaced by up to 22% LF and BFS. For this proposition, a two-level factorial design was used permitting to draw the isoresponse curves. Results show that compressive and flexural strength evaluated at 2, 7, 14, 28, 90 and 360 days are affected in different ways by the presence of mineral additions.     

The encapsulation of Mg(OH)2 sludge in composite cement

A range of magnesium hydroxide waste sludges arising from the re-processing of nuclear fuel exist in the UK and require safe long-term disposal. Similar wastes undergo a cementation process in order to immobilise radioactive material prior to disposal. Simulant magnesium hydroxide sludges have been prepared and their subsequent interactions with composite cement systems based on the partial replacement of ordinary Portland cement with blastfurnace slag and pulverised fuel ash have been studied. This work has concluded that there was little reaction between the sludge and any of the composite cements during hydration. Apart from a small quantity of a hydrotalcite-type phase containing magnesium from the sludge, the main phases detected were C-S-H and unreacted brucite. This indicates that the magnesium in the sludges is encapsulated by the cement, rather than being immobilised or chemically bound within the hardened matrix.     

Activation of blast furnace slag by a new method

Blast furnace slag is used as supplementary cementing material for the production of blended cement and slag cement. Its latently hydraulic properties can be activated by several methods. Most applications employ the use of high pH values in the pore solution (> 13.0) to accelerate the corrosion of the glass network of the slag.It is shown in this work that activation is also possible by lowering the pH to a range between 11.8 and 12.2 by the addition of calcium hydroxide and soluble calcium salts. Among the salts investigated in this study are calcium chloride, calcium bromide, calcium nitrate, calcium formate, and calcium acetate. Other salts can be used alternatively as long as they are able to increase the calcium ion concentration and thus reduce the pH in the pore solution via the calcium hydroxide equilibrium. Complex formation of organic anions with calcium ions in the pore solution is a serious handicap when using organic calcium salts.This concept was tested on a particular slag improving its early compressive strength. It was possible to increase the strength of mortar bars produced from the pure slag from 3 MPa to 25 MPa after seven days by adding calcium hydroxide, calcium carbonate and calcium acetate. The early strength of slag cement containing 80% slag was increased from 6 to 16 MPa after two days by adding calcium chloride. The final strength was increased from 36 to 53 MPa after 28 days (water/cement-ratio = 0.40, 20 °C).Analytical data is included to demonstrate that application of the aforementioned concept is able to increase heat liberation and degree of slag consumption.     

Compressive strength and hydration of wastepaper sludge ash-ground granulated blastfurnace slag blended pastes

Compressive strength and hydration characteristics of wastepaper sludge ash-ground granulated blastfurnace slag (WSA-GGBS) blended pastes were investigated at a water to binder (w/b) ratio of 0.5. The strength results are compared to those of normal Portland cement (PC) paste and relative strengths are reported. Early relative strengths (1 day) of WSA-GGBS pastes were very low but a marked gain in relative strength occurred between 1 and 7 days and this increased further after 28 and 90 days. For the 50% WSA-50% GGBS blended paste, the strength achieved at 90 days was nearly 50% of that of the PC control paste. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric (TG) analysis were carried out to identify the mineral components in the WSA and the hydration products of WSA and WSA-GGBS pastes. The principal crystalline components in the WSA are gehlenite, calcium oxide, bredigite and α′-C₂S (stabilised with Al and Mg) together with small amounts of anorthite and calcium carbonate and traces of calcium hydroxide and quartz. The α′-C₂S and bredigite, which phase separate from liquid phase that forms a glass on cooling, are difficult to distinguish by XRD. The hydration products identified in WSA paste are CH, C₄AH₁₃, C₃A.0.5CC?.0.5CH.H_11.5 and C-S-H gel plus possible evidence of small amounts of C₂ASH₈ and C₃A.3CS?.H₃₂. Based upon the findings, a hydration mechanism is presented, and a model is proposed to explain the observed strength development.     

Modeling the hydration of concrete incorporating fly ash or slag

Granulated slag from metal industries and fly ash from the combustion of coal are industrial by-products that have been widely used as mineral admixtures in normal and high strength concrete. Due to the reaction between calcium hydroxide and fly ash or slag, the hydration of concrete containing fly ash or slag is much more complex compared with that of Portland cement. In this paper, the production of calcium hydroxide in cement hydration and its consumption in the reaction of mineral admixtures is considered in order to develop a numerical model that simulates the hydration of concrete containing fly ash or slag. The heat evolution rates of fly ash- or slag-blended concrete is determined by the contribution of both cement hydration and the reaction of the mineral admixtures. The proposed model is verified through experimental data on concrete with different water-to-cement ratios and mineral admixture substitution ratios.     

Hydration mechanisms of super sulphated slag cement

The hydration and the strength evolution of supersulphated cements (SSC) produced by the activation of two different ground granulated blast furnace slags with anhydrite and small amounts of an alkaline activator have been investigated. The main differences between the two mixtures are found to be the strength development, the dissolution rate of the slags, the amount and volume of the individual hydration products formed and the growth mechanisms of the ettringite. The chemical composition of the slag had a large influence on the amount of the hydrates formed and thus on the volume of the hydrated slag.Advancement of the amount of hydrates of a slag with low reactivity by adding aluminium sulphate and calcium hydroxide did increase the amount of ettringite. Nevertheless, the early compressive strength was not increased, but late strength shows a slow increase. It was concluded that the early compressive strength of an SSC using low reactive slag cannot be overcome by adding stoichiometric amounts of constituents which are used for the formation of a specific hydration product. The best way to increase early compressive strength is to increase the intrinsic dissolution rate.     

Chloride diffusion in ordinary, blended, and alkali-activated cement pastes and its relation to other properties

Chloride transport into cementitious materials is critical from the viewpoint of protection of reinforcement. This paper is part of a larger study of the characteristics and performance of alkali-activated cementitious materials (AAC) whose properties equal or exceed those of normal Portland cement-based materials. Steady state chloride diffusion studies have been made of pastes of Type I Portland cement, and its blends with different proportions of ground granulated blast-furnace slag. Very substantial reductions in diffusion rates have been found with increased proportion of slag. In addition, alkali activation has been shown to reduce the diffusion rate by at least a factor of two. Other properties determined include: density, porosity, pore size distribution (Hg), BET (N₂) surface area, shrinkage, compressive and flexural strength, leaching, alkali-aggregate reaction, and freezing and thawing resistance. Comparisons with results of previous studies and with other blending components are discussed.     

Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

Effect of treatment temperature on the early hydration characteristics of superplasticized silica fume blended cement pastes

In this investigation, two mixes were used: ordinary Portland cement (OPC) and a blended cement prepared with the partial substitution of OPC by 10 mass% silica fume (SF). The setting and hardening characteristics were monitored by the aid of electrical conductivity as a function of curing time. The shear stress and electrical conductivity were studied at different temperatures, namely, 20, 35, 45 and 55 °C. As the temperature increases, the shear stresses decrease with the increase of shear rate. The height of electrical conductivity peaks of superplasticized cement pastes increases due to the increase of the paste fluidity. In the presence of 1.0% polycarboxylate (PC), the electrical conductivity of cement pastes decreases from 1 to 28 days. PC retards the hydration of cement pastes. The presence of PC extended the setting times of cement pastes at 35 °C than at 20 °C due to the increase in the adsorption capacity at this temperature. PC extends the dormant stage of the hydration process and delays the onset of the accelerating stage, without affecting its rate.     

The use of ferro-silicate slag from ISF zinc production as a sand replacement in concrete

Technical studies have shown that ferro-silicate slag from the Imperial Smelting Furnace (ISF slag) production of zinc can be used as a replacement for sand in cementitious mixes. The ISF slag contains trace quantities of zinc and lead, which are known to cause retardation of concrete set. Testing of experimental concrete mixes proves this retardation affect, although the delay in set does not appear deleterious to the eventual concrete hydration. If a gelatinous layer containing lead and zinc ions is formed around the cement grains in the concrete mix, set begins when this layer is disrupted and then continues as it would as if no zinc or lead had been present. Leaching studies demonstrate that pulverized-fuel ash and ground granulated blastfurnace slag have the potential to reduce the leaching of lead and zinc ions from the ISF slag, even in highly alkaline solutions.     

Effect of curing temperature and type of cement on early-age shrinkage of high-performance concrete

This paper presents the results of an experimental study on the influence of curing temperature and type of cement [Portland cement and blast-furnace slag (BFS) cement] on the autogenous deformations and self-induced stresses in early-age concrete. It was found that higher temperatures do not lead to higher deformations in the observed period, but generally cause a faster shrinkage and a faster development of self-induced stresses. Another experimental finding is that, at the temperatures tested, concrete made with BFS cement shows higher shrinkage in the first days than concrete made with Portland cement.     

Carbonation rates of concretes containing high volume of pozzolanic materials

The project studies the influence of fly ash and slag replacement on the carbonation rate of the concrete. The experimental work includes samples of pure Portland cement concrete (CEM I 42,5 R), blast-furnace slag concrete (CEM III-B), and fly ash blended concrete. To reveal the effect of curing on carbonation rate, the concretes were exposed to various submerged curing periods during their early ages. After that, the samples were subsequently exposed in the climate room controlling 20 °C and 50% RH until the testing date when the samples had an age of 5 months. Then, the accelerated carbonation test controlling the carbon dioxide concentration of 3% by volume, with 65% relative humidity were started to perform. The depth of carbonation can be observed by spraying a phenolphthalein solution on the fresh broken concrete surface. Finally, according to Fick's law of diffusion theoretical equations are proposed as a guild for estimating the carbonation rate of fly ash and blast-furnace slag concretes exposed under natural conditions from the results from accelerated carbonation tests.     

Mechanism for performance of energetically modified cement versus corresponding blended cement

The microstructure of cement paste of 50/50 mixes of cement/quartz and cement/fly ash, both ground in a special mill [energetically modified cement (EMC) process] and simply blended, have been studied under sealed curing conditions. The grinding process reduced the size of both cement grains and quartz/fly ash markedly and created flaky agglomerates of high inner surface for the finer particles. EMCs had much higher degree of hydration at 1 day, but similar as blends at 28 days. The pores were much finer for EMC paste due to smaller particles as also reflected in the strength. The morphology of calcium hydroxide in EMC paste appeared more mass like. Pozzolanic reaction was insignificant for quartz in EMC, but increased for fly ash. Thus, improved performance of EMC versus OPC can be explained by increased early hydration and extensive pore size refinement of the hardened binder resulting in reduced permeability and diffusivity for concrete.     

Properties and microstructure of the hardened alkali-activated red mud-slag cementitious material

A new kind of alkali-slag-red mud cementitious (ASRC) material, with both high early and ultimate strength and excellent resistance against chemical attacks, has been developed by the introduction of composite solid alkali activator into slag-red mud mixture system. Tests on strength development and other properties such as resistance against carbonation, simulated seawater, diluted acid, sulfate solution and freeze and thaw cycles of the ASRC cement were carried out and the results were reported in this paper. Meanwhile, the microstructure of the hardened ASRC cement paste, such as porosity and pore size distribution, and morphological characteristics of the resultant cement stone were also analyzed with the aid of MIP, SEM, etc. The results showed that the hardened cement paste had almost integrated and very compacted structure, more appropriate pore structure and less coarse crystallized products, which were believed to be the physical reasons for its high early and ultimate strength and good resistance against chemical attacks.     

Effect of shrinkage-reducing admixtures on the properties of alkali-activated slag mortars and pastes

The effect of a shrinkage-reducing admixture (SRA) based on polypropylenglycol on the dimensional stability of waterglass-activated slag mortars was studied. The analysis also showed the effect of the admixture on pore structure of the mortars as well as on the mineralogical composition and microstructure of the alkali-activated slag pastes.The SRA reduced the shrinkage by up to 85 and 50% when the alkali-activated slag mortar specimens were cured at relative humidities of 99 and 50%, respectively. The mechanism primarily involved in shrinkage reduction is the decrease in the surface tension of pore water prompted by the admixture. The SRA also modified the pore structure - under both curing conditions - increasing the percentage of pores with diameters ranging from 1.0 to 0.1 μm. Capillary stress is much lower in these pores than in the smaller capillaries prevailing in mortars prepared without admixtures.Microstructurally, the SRA occasioned a slight increase in the proportion of Si units Q² in the CSH gel and a decrease in the percentage of Al replacing the Si in the gel structure. The admixture did not, however, modify the mineralogical composition of the pastes.Finally, the SRA admixture retarded the alkaline activation of the slag, more intensely at higher admixture dosages. While the admixture did not significantly alter the degree of reaction in pastes cured for 7 days at RH = 99%, the value of this parameter dropped by 7% in the presence of the admixture in pastes cured at 50% relative humidity.     

Analysis of mechanism on water-reducing effect of fine ground slag, high-calcium fly ash, and low-calcium fly ash

The addition of ultrafine powder (UFP) to concrete can improve the fluidity of concrete, showing a water-reducing effect. The aim of this article was to analyze the water-reducing mechanism of UFP both experimentally and theoretically. Three UFPs—fine ground slag, high-calcium fly ash, and low-calcium fly ash—were chosen for the study. The contrastive experiments were done to investigate the fluidity of mortars with 30%, 45%, 60%, and 75% equivalent cement replaced by fine ground slag, high-calcium fly ash, and low-calcium fly ash, respectively. The results showed the physical and chemical characteristic of the powders, such as their grain morphology, glass phase activities, densities, specific areas, and their grain cumulating conditions, can strongly affect their water-reducing effect.     

An experimental study on the water-purification properties of porous concrete

The results of an experiment on the compressive strength and water purification properties of porous concrete are reported in this paper. Two sizes of coarse aggregate were used, namely 5 to 10 mm, and 10 to 20 mm. Three absolute volume ratios of paste-aggregate were used, namely 30%, 40% and 50% for a given size of aggregate. The compressive strength is found to be higher when the size of the aggregate is smaller, and when the paste-aggregate ratio (P/G, vol.%) is smaller. In the water purification experiment, the amount of organisms attached on the porous concrete surface is indirectly examined by the consumption of the dissolved oxygen (DO, mg/l). Water purification of the porous concrete is evaluated by the removal amount of the total phosphorus (T-P, mg/l) and total nitrogen (T-N, mg/l). A porous concrete with a smaller size of aggregate and a higher void content was found to have superior ability of the removal of the T-N and T-P in the test water. This is due to the large specific surface area of the porous concrete. Results from this study show that porous concrete using industrial by-products is able to purify water efficiently.     

The hydration phase and pore structure formation in the blends of sulfoaluminate-belite cement with Portland cement

Sulfoaluminate-belite (SAB) cements are an attractive class of low-energy cements from the viewpoint of saving energy and releasing less CO₂ into the atmosphere during their production. Their hydraulic activity, however, does not match that of the ordinary Portland cement (PC) and needs improvement before they can be used on their own. However, SAB cements when blended with PC have the potential to be used effectively in traditional applications as shown by this study. Mortars made with blends of SAB cements and PC, and a cement-to-sand ratio of 1:3 by weight and a water-to-cement ratio of 0.5, indicate a superior protection against corrosion of steel to those made with blends of PC and blast-furnace slag (BFSPC). The prepared mortars were stored at 20 °C for 90 days under either a 60% relative humidity (RH)-dry air, or 100% RH-wet air conditions. With further improvement in the SAB cement quality through better understanding of their characteristics, a genuine competition between SAB/PC and BFSPC can be expected in practice.