Investigation on mechanical properties, durability and micro-structural development of steel slag blended cements

To improve the properties of steel slag blended cements, a chemical activator was added into blended cements, the mechanical properties and durability of steel slag blended cements were investigated. The results show that steel slag in blended cement pastes presents low hydraulic activity and makes practically no contribution to strength development. After the addition of chemical activator, the mechanical properties and durability of ternary blended cements are increased significantly. The hydration process and micro-structural development of blended cement was investigated by isothermal calorimeter and scanning electric microscope, respectively. Steel slag started hydration in the first 3?days in the presence of chemical activator, steel slag and granulate blast furnace slag reacted with Ca(OH)₂ to form a dense microstructure as curing proceeded. Therefore, both early and late compressive strengths of steel slag blended cement with 35% cement clinker and 30% steel slag can be comparable with those of Portland cement.     

A comparison of early hydration properties of cement–steel slag binder and cement–limestone powder binder

The early hydration properties of cement–steel slag composite binder and cement–limestone powder composite binder were compared in this study by determining the hydration heat of binder within 3 days, the pore structure of paste and the compressive strength of mortar at the age of 3 days. Results show that at the curing temperature of 25 °C, the early hydration heat of the binder containing steel slag is smaller, and the early pore structure of the paste containing steel slag is coarser, but the early compressive strength of the mortar containing steel slag is higher compared with the mix containing limestone powder. Though the early reaction degree of steel slag is low, its chemical contribution to the strength of mortar cannot be neglected. At the curing temperature of 50 °C, the early hydration heat of the binder containing steel slag is larger, and the early pore structure of the paste containing steel slag is finer, and the early compressive strength of the mortar containing steel slag is even higher compared with the mix containing limestone powder. Raising curing temperature can enhance the role played by steel slag more significantly than that played by limestone powder in the hydration and hardening of the composite binder.     

The difference among the effects of high-temperature curing on the early hydration properties of different cementitious systems

The difference among the effects of high-temperature curing on the early hydration properties of the pure cement, the binder containing fly ash, the binder containing GGBS, and the binder containing steel slag was investigated by determining the compressive strength, non-evaporable water content, hydration heat, and Ca(OH)₂ content. Results show that the order of the influence degrees of high-temperature on the early hydration of different binders is the binder containing GGBS > the binder containing steel slag > the binder containing fly ash > the pure cement. In the case of short period of high-temperature curing (only 1 day), the strength growth rate of the concrete containing GGBS is the greatest. Though the influence of increasing high-temperature curing period on the hydration degree of the binder containing fly ash is not the most significant, the strength growth rate of the concrete containing fly ash is the most significant due to the excessive consumption of Ca(OH)₂ by reaction of fly ash. In the case of high-temperature curing, the Ca(OH)₂ content of the paste containing steel slag is much higher than those of the paste containing GGBS and the paste containing fly ash, so though high-temperature curing promotes the hydration of the binder containing steel slag significantly, its influence on the strength growth rate of the concrete containing steel slag is not so significant.     

Study on optimization of hydration process of blended cement

To optimize the hydration process of blended cement, cement clinker and supplementary cementitious materials (SCMs) were ground and classified into several fractions. Early hydration process of each cementitious materials fraction was investigated by isothermal calorimeter. The results show fine cement clinker fractions show very high hydration rate, which leads to high water requirement, while fine SCMs fractions present relatively high hydration (or pozzolanic reaction) rate. Cement clinker fractions in the range of 8–24 μm show proper hydration rate in early ages and continue to hydrate rapidly afterward. Coarse cement clinker fractions largely play “filling effect” and make little contribution to the properties of blended cement regardless of their hydration activity (or pozzolanic activity). The hydration process of blended cement can be optimized by arranging high activity SCMs, cement clinker, and low activity SCMs in fine, middle, and coarse fractions, respectively, which not only results in reduced water requirement, high packing density, and homogeneous, dense microstructure, but also in high early and late mechanical properties.     

Comparing study on hydration properties of various cementitious systems

The hydration properties of slag sulfate cement (SSC), slag Portland cement (PSC), and ordinary Portland cement (POC) were compared in this study by determining the compressive strength of pastes, the hydration heat of binders within 72 h, the pore structure, the hydration products, and the hydration degree. The results indicated that main hydration products of PSC paste and POC paste are calcium hydroxide and C–S–H gel, while those of SSC paste are ettringite and C–S–H gel from the analyses of XRD, TG–DTA, and SEM. At the early curing age, the compressive strength depends on the clinker content in the cementitious system, while at the late curing age, which is related to the potential reactivity of slag. From hydration heat analysis, the cumulative hydration heat of PSC is lower than that of POC, but higher than that of SSC. Slag can limit chemical reaction and the delayed coagulation of gypsum, which also plays a role in the early hydration. So SSC shows the lowest heat release and slag can’t be simulated without a suitable alkaline solution. Based on MIP analysis, the porosity of POC paste is the smallest while the average pore size is the biggest. At the age of 90 days, the compressive strength of SSC can get higher development because of its relative smaller pore size than that of PSC and POC paste.     

Use of Aminosilane Coupling Agents in Cementitious Materials

The work presented in this paper has been focused on the evaluation of the influence of the addition of aminosilanes like γ-aminopropyl-triethoxysilane (3-AMPS) on physical and mechanical properties of fresh and hardened polymer modified cement mortars (PMCMs), which contained also mineral fillers like fumed silica and blast furnace slag. The role of the aminosilane in cementitious system was to produce surface modification and covalent bonding among different inorganic phases of inorganic matrix (cement, aggregate, fumed silica) and strong chemical interaction between inorganic matrix and organic (polymer) phase of the mortar. The strong chemical bonding of inorganic matrix and polymer phase transformed composite mortar into cementitious material with more like monolithic structure. Mortars with the addition of aminosilanes showed improved physical-mechanical properties like bending strength, adhesion to concrete, and lower dry-shrinkage.     

Improvement of surface cementitious properties of coarse fly ash by dehydration and rehydration processes

Owing to poor bonding between coarse fly ash particles and hydration products, gap-graded blended cements with fly ash usually show lower compressive strengths than Portland cement. Surface cementitious properties of coarse fly ash were improved by dehydration and rehydration processes in the present study. The results show that during the calcination at 750?°C, C?CS?CH gel is mainly transformed into a new nesosilicate, which is similar to a less crystalline C₂S. The formation of melilite from hydration products is also noticed at 900?°C, however, this will not contribute to rehydration of calcined fly ash. Rehydration of new generated nesosilicate on the surface of coarse fly ash leads to a better bonding between coarse fly ash particles and hydration products. As a result, both early and late mechanical properties of gap-graded blended cements containing 25% cement clinker and 39% calcined coarse fly ash are higher than those of 100% Portland cements.     

Hydration,mechanical properties and durability of high-strength concrete under different curing conditions

The properties of high-strength concrete under standard curing condition (20 °C, 95% RH), high-temperature curing condition (50 °C) and temperature match curing condition were comparatively investigated. The cumulative hydration heat of composite binder containing fly ash and silica fume is lower than that of composite binder containing the same amount of slag. Addition of fly ash and silica fume clearly reduces the adiabatic temperature rise of concrete, but adding slag leads to higher adiabatic temperature rise than Portland cement concrete. High-temperature curing condition and temperature match curing condition lead to the sustainable increase in compressive strength of concrete containing mineral admixture, but they hinder the later-age strength development of Portland cement concrete. For cement–slag paste and cement–fly ash–silica fume paste, the non-evaporable water contents increase significantly and the pore structures are much finer under high-temperature curing condition and temperature match curing condition, which negatively affect the pore structure of Portland cement paste. The differences in properties of concrete among three curing conditions become smaller with time. The properties obtained under standard curing condition can approximately reflect the long-term properties of high-strength concrete in the real structure. The concrete prepared with cement–fly ash–silica fume composite binder has the highest compressive strength, finest pore structure and best resistance to chloride permeability under any curing condition. This composite binder is very suitable to prepare the high-strength concrete with large volume.     

Comparison between the effects of superfine steel slag and superfine phosphorus slag on the long-term performances and durability of concrete

Superfine particles have been used as mineral admixtures to enhance physical properties, mechanical properties, and durability of concrete in a lot of research. In this study, superfine steel slag (FSS) and superfine phosphorus slag (FPS) were ground to 643 and 657 m² kg^?1, respectively. The water-to-binder (W/B) ratios were set as 0.45 as well as 0.35, and the cement replacements adopted were 15 and 30%. The effects of FSS and FPS on long-term performance and durability of concrete were investigated. The results show that the increase amplitude of reaction degree of FPS is higher than that of FSS at late age (after 90 days). FPS can improve the pore structure of concrete which is beneficial to the resistance to carbonation and chloride ion penetration for concrete at late age while FSS cannot. FPS is also more advantageous to the development of compressive strength and splitting tensile strength of concrete when compared to FSS at late age. FPS is much more beneficial to the resistance to sulfate attack of concrete while FSS is more disadvantageous to the resistance to sulfate attack of concrete as the replacement ratio increases.     

Heat resistance of portland cements

Recent fire cases indicated again the importance of fire research. Fast development of construction technology requires new materials. Initiation and development of fire are strongly influenced by the choice of construction materials. In addition to their mechanical properties, their behaviour in elevated temperature is also of high importance. Residual compressive strength of concrete exposed to high temperatures is influenced by the following factors: water-to-cement ratio, cement-to-aggregate ratio, type of aggregate and water content of concrete before exposing it to high temperatures and the fire process. Therefore, mix design and composition of concrete are of high importance for high temperatures. Based on the literature, the fire resistance of concrete is influenced by the used cement type. As regards the cement type, considerable importance has been attached to the various auxiliary materials, such as slag, fly ash, trass, metakaolines and silica fume. There has been no special research devoted to the fire behaviour of pure portland cements. Pure portland cements can be made with various oxide compositions or with different grinding fineness, which increases the resistance of cements to fire. The question arises what effects grinding fineness and oxide composition have on fire resistance of cements. In my experiments, the resistance of portland cements of different composition and grinding fineness to fire (high temperature) were examined. For the test of the solidified cement paste, cement paste cubes of 30-mm edge length were prepared. The specimens were stored in water for 7 days and then in laboratory conditions for 21 days. The cubes of more than 28 days were heated to the given temperature in the furnace and then kept at the given temperature for 2 h (50, 150, 300, 500, 800 °C). Following the 2 h of thermal load, the specimens were examined once their temperature cooled down to room temperature. I have experimentally demonstrated that in case of portland cements, the grinding fineness and aluminate modulus of the cement (i.e. the oxide composition of the cement) have a significant effect on its fire resistance.     

Deterioration of hardened cement paste under combined sulphate-chloride attack investigated by synchrotron XRD

The exact mechanisms of the phase transitions caused by a combined sulphate-chloride attack are discussed controversially. The main points concern the mutual influences of sulphate and chloride ions during the secondary binding processes of these anions within cement hydrate phases. We simulated combined sulphate-chloride attack under laboratory conditions using solutions containing NaCl and Na₂SO₄ in different concentrations. Three sample compositions were used for the preparation of the specimens. In two of them, 30% of Portland cement was replaced by supplementary cementitious materials (fly ash, slag).The phase distribution in the samples was determined using synchrotron X-ray diffraction. The analysis with high spatial resolution allows the localisation of the secondary phase formation in the microstructural profile of the sample. A mechanism of the phase developments under combined sulphate-chloride attack is derived.     

Influence of high-volume electric furnace nickel slag and phosphorous slag on the properties of massive concrete

This study applied high-volume electric furnace nickel slag (FS), phosphorous slag (PS) and a mixture of the two (FP) to massive concrete, and using fly ash (FA) as the control admixture, investigated the effects of FS and PS on the hydration and hardening process of the cementitious materials, the mechanical properties and the durability of the concrete. Two curing conditions were set, namely the standard curing condition and temperature-matched curing condition (or constant 25 and 50 °C). The hydration heat, hydration products, pore size distribution, mechanical properties and ability of the concrete to resist chloride ion penetration were tested. The results show that the activity of PS and FP is higher than that of FA, while that of FS is lower than that of FA; the improvement of FP on the pore structure of the hardened paste is close to that of FA at late ages under the standard curing condition but better than that of FA at all ages under the temperature-matched curing condition; high-volume FP concrete shows similar or even superior mechanical properties and permeability to chloride ions of concrete to high-volume FA concrete at late ages under both curing conditions.     

Sulphate resistance and passivation ability of the mortar made from pozzolan cement with zeolite

Sulphate resistance and passivation ability of the mortars made from pozzolan cement of CEM IV/A (P) type according to European Standard EN 197-1 (zeolite blended cement with 60.82 mass% of PC clinker, 35.09 mass% of zeolite and 4.09 mass% of gypsum abbreviated as ZBC) and ordinary Portland cement (abbreviated as PC) are introduced. Resistance tests were performed in water and 5% sodium sulphate solution (both 20°C) for 720 days. The increased sulphate resistance of pozzolan cement relative to that of PC was found. The key quantitative insight into the hydrate phase behaviour is given by thermal analysis. This is due to pozzolanic reaction of zeolite with PC resulting in reduction of the formed Ca(OH)₂ opposite to the reference PC. Ability of pozzolan cements with 15 to 50 mass% of zeolite to protect steel against corrosion was verified in 20°C/85% RH-wet air within 180-day cure. Steel was not corroded in the mortars made with pozzolan cement containing up to 35 mass% of zeolite. Pozzolan cement of CEM IV/A (P) type containing 35 mass% of zeolite is a suitable cementitious material for concrete structures exposed to sulphate attack. Steel is protected against corrosion by this pozzolan cement in the same measure as the reference PC.     

Effect of water/binder ratio and temperature on the hydration heat and properties of ternary blended cement containing slag and iron tailing powder

Journal of Thermal Analysis and Calorimetry - The effects of water/binder ratio and temperature on hydration heat and properties of ternary blended cement containing slag and iron tailing powder...     

Application of calorimetry as a main tool in evaluation of the effect of carbonate additives on cement hydration

Calorimetry was applied to follow the hydration in the Portland cement–dolomite–limestone mixtures. In the experiments the limestone additive of various fineness (standard component of various common cements), as well as the dolomite additive (not a standard component) were used. The rate of hydration versus time for common cements reflects the proper setting and early hardening during the first days after mixing with water (two or three peaks and the induction period between them). The aim of measurements presented in this work was to show the course of heat evolution curve and the heat evolved values, equivalent to the acceleration/retardation of hydration, in case of the pastes produced from Portland cement and the carbonate additives mixed in variable proportions, as well as to verify the results by other methods. The rate of heat evolution accompanying cement paste hydration, total heat evolved, conductivity of hydrating suspension and rheological (flow) properties versus time are modified by the fine grained carbonate additives. This is due to the hypothetical nucleating effect of limestone and dolomite.     

Optimization of cementitious composite for heavyweight concrete preparation using conduction calorimetry

The present work investigates the hydration heat of different cement composites by means of conduction calorimetry to optimize the composition of binder in the design of heavyweight concrete as biological shielding. For this purpose, Portland cement CEM I 42.5 R was replaced by a different portion of supplementary cementitious materials (blast furnace slag, metakaolin, silica fume/limestone) at 75%, 65%, 60%, 55%, and 50% levels to obtain low hydration heat lower than 250 j g^?1. All ingredients were analyzed by energy dispersive X-ray fluorescence (EDXRF) and nuclear activation analysis (NAA) to assess the content of major elements and isotopes. A mixture of two high-density aggregates (barite and magnetite) was used to prepare three heavyweights concretes with compressive strength exceeding 45 MPa and bulk density ranging between 3400 and 3500 kg m^?3. After a short period of volume expansion (up to 4 h), a slight shrinkage (max. 0.3°/°°) has been observed. Also, thermophysical properties (thermal conductivity, volumetric specific heat, thermal diffusivity) and other properties were determined. The results showed that aggregate content and not binder is the main factor influencing the engineering properties of heavyweight concretes.

     

Influence of pre-curing time on the hydration of binder and the properties of concrete under steam curing condition

There is a pre-curing period before the freshly made concrete elements were exposed to steam curing in the steam curing process. In this paper, the influence of pre-curing time on the hydration of binder and the properties of concrete under steam curing condition was investigated. Three binders were used: the pure cement, the binder containing high content of GGBS, and the binder containing high content of fly ash. Three pre-curing times (1, 3, and 6 h) and one steam curing period at 60 °C (over 8 h) were adopted. Results show that pre-curing time has limited influence on the hydration degree of binder, and compressive strength and pore structure of paste. The influence of pre-curing time has limited influence on the compressive strength and chloride permeability of the pure cement concrete and the concrete containing high content of GGBS at whether early or late ages, indicating that the proper pre-curing time can be as short as 1 h for these two concretes. Increasing pre-curing time enhances the late-age compressive strength of the concrete containing high content of fly ash significantly, but it has limited influence on the late-age permeability.     

Reactivity of cement mixtures containing waste glass using thermal analysis

A laboratory study was undertaken to compare the performance of waste glass as a supplementary cementitious material (SCM) to traditional SCMs at the same particle size and level of replacement in both high and low alkali cement paste. The consumption of Ca(OH)₂ as measured by differential thermal analysis (DTA) is used as an indicator of reactivity. The DTA results of the pastes aged to 150 days are presented, and indicate that glass reactivity is similar to ground-granulated blast furnace slag (GGBFS) and lower than silica fume (SF) at comparable particle sizes. Alkali–silica reaction (ASR) is not present for particle sizes below 100 μm, but is induced by agglomeration of the glass particles and is observed by fluorescence in optical microscopy images. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) are used to compare the microstructural properties of the SCMs and measure the chemical composition of the reaction products. The alkalinity of the cement was found to influence the nature of composition as observed by thermal analysis, and the temperatures at which their reactions occurred.     

Structure of white cement mortars with high content of marble powder

In this study, three types of cementitious composites based on (i) white Portland cement and sand (cement-to-aggregate 1:3, and water-to-cement 0.50), (ii) white Portland cement and marble powder (cement-to-aggregate 1:2, and water-to-cement 0.60), and (iii) white Portland cement and marble powder with polycarboxylate-based admixture (HRWR) (cement-to-aggregate 1:2, and water-to-cement 0.40?+?HRWR) were studied. Their states after 28 and 120?days of water curing were evaluated by measurement of physical?Cmechanical properties, such as density, compressive strength and porosity. Thermal analysis, X-ray diffraction analysis and scanning electron microscopy were used to identify the crystal phases and their morphology. The experimental data show that the white cement mortars with higher water content exhibit larder variety of newly formed phases, like hydration products of the C?CS?CH type. The structure of mortars with polycarboxylate-based admixture is so dense that there is no possibility of crystal hydrates development at late curing ages. The use of marble as filler leads to a partial inclusion of carbonate ions in the newly formed hydrated phases (carbo-aluminates).     

The Role of Titanium Dioxide on the Hydration of Portland Cement: A Combined NMR and Ultrasonic Study

Titanium dioxide (TiO₂) is an excellent photocatalytic material that imparts biocidal, self-cleaning and smog-abating functionalities when added to cement-based materials. The presence of TiO₂ influences the hydration process of cement and the development of its internal structure. In this article, the hydration process and development of a pore network of cement pastes containing different ratios of TiO₂ were studied using two noninvasive techniques (ultrasonic and NMR). Ultrasonic results show that the addition of TiO₂ enhances the mechanical properties of cement paste during early-age hydration, while an opposite behavior is observed at later hydration stages. Calorimetry and NMR spin–lattice relaxation time T₁ results indicated an enhancement of the early hydration reaction. Two pore size distributions were identified to evolve separately from each other during hydration: small gel pores exhibiting short T₁ values and large capillary pores with long T₁ values. During early hydration times, TiO₂ is shown to accelerate the formation of cement gel and reduce capillary porosity. At late hydration times, TiO₂ appears to hamper hydration, presumably by hindering the transfer of water molecules to access unhydrated cement grains. The percolation thresholds were calculated from both NMR and ultrasonic data with a good agreement between both results.