Effect of calcium sulfate source on the hydration of calcium sulfoaluminate eco-cement

The availability of cements, including eco-cements, with tailored mechanical properties is very important for special applications in the building industry. Here we report a full study of the hydration of calcium sulfoaluminate eco-cements with different sulfate sources (gypsum, bassanite and anhydrite) and two water/cement ratios (0.50 and 0.65). These parameters have been chosen because they are known to strongly modify the mechanical properties of the resulting mortars and concretes. The applied multi-technique characterization includes: phase assemblage by Rietveld method, evolved heat, conductivity, rheology, compressive strength and expansion/retraction measurements. The dissolution rate of the sulfate sources is key to control the hydration reactions. Bassanite dissolves very fast and hence the initial setting time of the pastes and mortars is too short (20 min) to produce homogeneous samples. Anhydrite dissolves slowly so, at 1 hydration-day, the amount of ettringite formed (20 wt%) is lower than that in gypsum pastes (26 wt%) (w/c = 0.50), producing mortars with lower compressive strengths. After 3 hydration-days, anhydrite pastes showed slightly larger ettringite contents and hence, mortars with slightly higher compressive strengths. Ettringite content is the chief parameter to explain the strength development in these eco-cements.     

Formulation of mortars with nano-SiO2 and nano-TiO2 for degradation of pollutants in buildings

This paper reports on the design of cement mortars that use nano-SiO₂ (nS) and nano-TiO₂ (nT) particles, aiming to improve the durability of traditional building materials while giving new functionalities (aerial decontamination of pollutants). Samples with 0–2 wt.% nS, 0–20 wt.% nT, 0.45–7 wt.% superplasticizer (SP) and 0.45–0.58 water/binder weight ratio were prepared. The formulations of mortars were defined according to rheology and flow table measurements, then showing suitable workability. The temperature of hydration, compressive strength, water absorption, and photocatalytic degradation of pollutants (NO_x and Orange II dye) were also evaluated. In general, the rheological behavior and the temperature of hydration changed in distinct levels, depending on the dosage and type of nanoadditives, but nT influenced more significantly the results. However, such differences were not identified on the compressive strength and water absorption. In addition, NO_x photocatalytic degradation up to 1 h under solar light ranged from 65% to 80%, while Orange II degradation after 9 h under visible light changed from 18% to 50%.     

A study on the effect of nano silica on compressive strength of high volume fly ash mortars and concretes

This paper presents the effect of nano silica (NS) on the compressive strength of mortars and concretes containing different high volume fly ash (HVFA) contents ranging from 40% to 70% (by weight) as partial replacement of cement. The compressive strength of mortars is measured at 7 and 28 days and that for concretes is measured at 3, 7, 28, 56 and 90 days. The effects of NS in microstructure development and pozzolanic reaction of pastes containing above HVFA contents are also studied through backscattered electron (BSE) image and X-ray diffraction (XRD) analysis. Results show that among different NS contents ranging from 1% to 6%, cement mortar containing 2% NS exhibited highest 7 and 28 days compressive strength. This NS content (2%) is then added to the HVFA mortars and concretes and the results show that the addition of 2% NS improved the early age (7 days) compressive strength of mortars containing 40% and 50% fly ash by 5% and 7%, respectively. However, this improvement is not observed at high fly ash contents beyond 50%. On the other hand, all HVFA mortars exhibited improvement in 28 days compressive strength due to addition of 2% NS and the most significant improvement is noticed in mortars containing more than 50% fly ash. In HVFA concretes, the improvement of early age (3 days) compressive strength is also noticed due to addition of 2% NS. The BSE and XRD analysis results also support the above findings.     

Synthesis,characterization and properties of calcium ferroaluminate belite cements produced with electric arc furnace steel slag as raw material

This study investigated the use of 10 (M1), 17 (M2) and 27 wt.% (M3) electric arc furnace steel slag (EAFS) as a raw material in the production of calcium ferroaluminate belite cement clinker, after firing at 1320 °C. The thermal behavior of the raw meals was studied by TG/DSC and XRD whereas for the analysis of the clinkers, XRD/QXRD, SEM/EDS and EPMA were employed. The resulting clinker was co-grinded with 5 and 20 wt.% Flue Gas Desulfurization (FGD) gypsum and the properties were determined by a series of tests in accordance to EN standards. The evolution of hydration was investigated by SEM and the development of compressive strength. The results revealed that the formed phases in the clinkers were C₂S, C₄AF and C₄A₃Ŝ. The main hydration products were ettringite, AFm and hydrogarnet. The leached Cr^VI was below 1 ppm in M3. Compressive strength in cements with 5 wt.% FGD gypsum was (in MPa): 18.3 for M1, 14.3 for M2 and 7.8 for M3 at 28 days, whereas for 20 wt.% FGD gypsum, the values were almost doubled.     

Contribution of limestone to the hydration of calcium sulfoaluminate cement

Calcium sulfoaluminate (CSA) cements can be blended with mineral additions such as limestone for properties and cost optimization. This study investigates the contribution of limestone to the hydration of a commercial CSA clinker regarding the hydration kinetics, hydrate assemblage and compressive strength. Nine formulations were defined at M-values of 0, 1.1 and 2.1 (M = molar ratio of anhydrite to ye’elimite) without and with medium and high limestone contents.Calorimetric results indicate that limestone accelerates the hydration reaction especially at M = 1.1, probably due to the filler effect. The phase assemblages were calculated by thermodynamic modeling using Gibbs Energy Minimization Software (GEMS). With increasing limestone content the formation of ettringite and calcium monocarboaluminate is predicted at the expense of calcium monosulfoaluminate. With increasing M-value more ettringite is predicted at the expense of the monocarbonate and less calcite takes part in the hydration reactions.The modeled results compare well with the experimental data after 90 d of hydration, except that calcium hemicarboaluminate was found instead of monocarbonate, which is assumed to be due to kinetics considerations.The lowest compressive strength occurs in ternary formulations, whereas in the absence of calcium sulfate, strength is significantly higher.The results presented here indicate that in CSA cements, limestone accelerates early hydration kinetics, takes part in the hydration reactions at M < 2, and has a positive effect on strength development in systems without anhydrite.     

The effect of zinc oxide additions on the performance of calcined sodium montmorillonite and illite shale supplementary cementitious materials

The objectives of this study were to use activation treatments on sodium montmorillonite and illite shale, to alter early hydration or later pozzolanic reactivity when used as supplementary cementitious materials (SCMs). For comparison purposes, treatment methods were also applied to the highly reactive pozzolan, metakaolin, and the inert filler, quartz. Activation treatment strategies included the addition of 0.15 wt% zinc oxide and the use of thermal treatments to the SCMs at temperatures of 650 °C, 830 °C and 930 °C. The use of zinc oxide additions increased the early hydration rate of SCM-containing pastes, yet introduced a chemical retardation and negatively impacted early compressive strengths. Moreover, the results suggest that retardation was inversely correlated with the pozzolanic reactivity of the SCM used. Thermal treatment methods were effective at influencing the SCM pozzolanic reactivity, with montmorillonite calcined at 830 °C and illite calcined at 930 °C behaving as late-reacting pozzolans. SCMs calcined at these temperatures resulted in higher 90 day compressive strengths compared to mortars containing the quartz filler. Overall, this study provides insight into different strategies that maybe used to enhance the reactivity of impure calcined clays in order to facilitate their acceptance into the concrete industry.     

Jet mill grinding of portland cement,limestone, and fly ash: Impact on particle size,hydration rate,and strength

While the majority of commercial ordinary portland cement (OPC) is ground using a ball mill or a vertical roller mill, other industries have shown that jet mill grinding can be an alternative approach for grinding materials. This paper investigates the potential application of jet mill grinding for two systems. The first system is a blend of OPC and 15% limestone, and the second system is a blend of OPC and 40% fly ash. It was observed that when jet mill grinding is used, the average particle size of the powders is decreased to approximately 4 μm or less with a narrower particle size distribution than that achieved using ball milling. In addition to evaluating the size and shape of the particles obtained from the jet mill grinding process, this paper focuses on evaluating, using isothermal calorimetry, the effect these changes in particle size and distribution have on the extent and rate of hydration as well as their effect on the compressive strength of cement pastes or mortars.This study also investigated differences between inter-grinding and blending separately ground materials to form an OPC/limestone mixture. Both inter-ground and separately ground OPC/limestone mortars demonstrated an accelerated hydration at early ages accompanied by an increase in early age strength. This appears to be primarily due to the increased surface area of the finer particles that provides more available surface for the hydration reaction. The inter-grinding appeared to be more effective than grinding the materials separately because an improved graded particle size distribution was obtained. The inter-ground OPC/limestone mixture shows accelerated initial hydration at water to powder ratios (w/p, where powder = cement + limestone) of 0.50 and 0.35 when compared with the samples before grinding. At the lower w/p of 0.35, the OPC/limestone mixture appears much more efficient. In the OPC/fly ash mixture, jet mill grinding also accelerates the rate of hydration and strength development.     

Bentonite as a natural additive for lime and lime–metakaolin mortars used for restoration of adobe buildings

This work estimates the behaviour of mortars based on lime, seeking their application as renders of adobe walls. Mortars with binder:aggregate 1:3 volumetric ratio were prepared as is traditionally used in old buildings in central parts of Portugal.Due to specificity of the support, two clays, natural clay bentonite (5 wt.%) and artificial clay metakaolin (20 wt.%) were used as additives to lime mortar to prepare 3 types of blended mortars, besides the air lime reference mortar. Mortar prisms 4 × 4 × 16 cm were analysed to assess mechanical properties and salt resistance. Moreover, the mortars were placed in three ways on old adobes taken from demolished houses and their behaviour was verified by artificial accelerated ageing test. Lastly, mortars were applied on a wall made from traditional adobes, where panels were monitored and trials with adhesion strength and Karsten tubes have been conducted. The results obtained by comparison of the characteristics from all the experimental procedures reveal that mortar containing air lime and 5 wt.% of bentonite fulfils in the best way the requirements in its use as render of adobe buildings.     

Influence of polyether polyol on the hydration and engineering properties of calcium sulfoaluminate cement

The aim of the present paper was to investigate the efficiency of polyether polyol as shrinkage-reducing admixture on pastes and mortars prepared with calcium sulfoaluminate cement (CSA). CSA was prepared by mixing CSA clinker and re-crystallized gypsum in different proportions. Three types of polyether polyol were added at a dosage of 1.5 wt% of CSA when hydrating pure pastes and standard mortars. The engineering properties of mortars (compressive strength, drying shrinkage) and the microstructure of pastes were investigated. The results show that polyol reduces drying shrinkage of CSA-based mortars without affecting the nature of hydrates formed. The effect of polyol mainly depends on its molecular weight.     

A study on the effects of high temperature on mechanical properties of fiber reinforced cementitious composites

The flexural strength and ductility properties of cementitious composites (mortar) under high temperature may be significantly improved by incorporating different types of fibers. In this study, four different types of fibers are added to cement mortars with the aim to investigate their mechanical contributions to mortars under high temperature, comparatively. Polypropylene (PP), carbon (CF), glass (GF) and polyvinyl alcohol (PVA) fibers are chosen for research. These fibers are added into mortars in five different ratios (0.0%, 0.5%, 1.0%, 1.50% and 2.0%) by volume. The mortars are subjected to the following temperatures: 21 °C (normal conditions), 100 °C (oven dry), 450 °C and 650 °C. The mechanical properties investigated are flexural strength, deflection and compressive strength of the cement mortars. In addition, thin sections of mortars are investigated to obtain changes in mortar because of high temperature. It is concluded that all fiber types contribute to the flexural strengths of mortars under high temperature. However, this contribution decreases with an increase in temperature. The samples with PVA show the best flexural performance (75–150%) under high temperature. CF which does not melt under high temperature also gives high flexural strength (11–85%). The compressive strengths of the mortars reduce under high temperature or with fiber addition. The highest increase in flexural strength and the lowest decrease in compressive strength is at 0.5–1.5% for CF if all temperature conditions are taken into consideration. The optimum fiber addition ratios of the samples containing PP and GF are 0.5% by volume. And for PVA, it is between 0.5% and 1.5% by volume.     

Tailored setting times with high compressive strengths in bassanite calcium sulfoaluminate eco-cements

This work deals with the hydration of a calcium sulfoaluminate (CSA) eco-cement prepared with bassanite and different additives (type and content) at a fixed water/CSA ratio of 0.5. Pastes prepared with bassanite show high water demands, high viscosity values and short initial setting times which are related to the fast dissolution rate of bassanite and the subsequent precipitation of gypsum. These facts have a dramatic effect onto the mechanical strength values, and make necessary the addition of additives.Here, the addition of different amounts of specific retarders (polycarboxylate, tartaric acid and phosphonic acid) not only improved the workability of pastes and mortars, but also delayed the setting time by modifying the dissolution rates of the phase(s), and improved mechanical strengths. Finally, mortars with high compressive strengths (46 and 84 MPa at 1 and 7 days of hydration, respectively) and, chiefly, tailored setting times with high strengths have been prepared.     

Synthesis of partial stabilized cement–gypsum as new dental retrograde filling material

The study describes the sol–gel synthesis of a new dental retrograde filling material partial stabilized cement (PSC)–gypsum by adding different weight percentage of gypsum (25% PSC + 75% gypsum, 50% PSC + 50% gypsum and 75% PSC + 25% gypsum) to the PSC. The crystalline phase and hydration products of PSC–gypsum were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analysis. The handling properties such as setting time, viscosity, tensile strength, porosity and pH, were also studied. The XRD and microstructure analysis demonstrated the formation of hydroxyapatite and removal of calcium dihydrate during its immersion in simulated body fluid (SBF) on day 10 for 75% PSC + 25% gypsum. The developed PSC–gypsum not only improved the setting time but also greatly reduced the viscosity, which is very essential for endodontic surgery. The cytotoxic and cell proliferation studies indicated that the synthesized material is highly biocompatible. The increased alkaline pH of the PSC–gypsum also had a remarkable antibacterial activity.     

Alkali-activated cements and mortars based on blast furnace slag and red clay brick waste

This study investigates the effects of adding various concentrations, sources and compositions of ground red clay brick waste (RCBW) on the properties of fresh and hardened pastes and mortars of alkali-activated slag. The method used to grind the granulated blast furnace slag (GBFS) and RCBW (separate and conjoint) is also assessed, along with the fineness (300–900 m²/kg) of the blended alkali-activated GBFS-RCBW cement, the alkali activator (sodium carbonate or sodium silicate) and the curing conditions (normal conditions or steam curing). The water requirement and setting time for the fresh pastes are also considered; and in the case of the hardened paste and mortar, the water absorption, density and compressive/flexural strength are measured after 1, 3, 7 and 28 days of aging. From the results obtained, it is demonstrated that the addition of 40% RCBW improves the 7- and 28-day strength of blended alkali-activated slag pastes and mortars, and can replace up to 60% of the slag without losing strength.     

Influence of fired clay brick waste additions on the durability of mortars

The use of metakaolin is known to help improve properties of Portland cement-based mortars. The presumed similarities between the characteristics of metakaolin and those of a powdered (<45 μm) fired clay brick clean waste (CBW) led to the investigation of the effect on the durability of mortars of partial replacement (10, 25 and 40 wt.%) of Portland cement by CBW. Properties such as 28 and 90 days-compressive strength, water absorption, apparent porosity, absorption by capillarity, chloride retention, carbonation depth and sulphate resistance were evaluated. The CBW-containing cured mortars showed improved strength and density, as the result of combined physical and pozzolanic pore filling effect of added CBW. However, CBW-free mortar exhibited larger spreading and, being more porous, higher sulphate resistance and ability to absorb chlorides. Optimum performance was found for the 40 wt.% CBW mortar whose compressive strength can be up to 130% higher than that of the CBW-free mortar.     

Compressive strength and drying shrinkage of fly ash-bottom ash-silica fume multi-blended cement mortars

This paper studies the physical properties, compressive strength and drying shrinkage of multi-blended cement under different curing methods. Fly ash, ground bottom ash and undensified silica fume were used to replace part of cement up to 50% by weight. Specimens were cured in air at ambient temperature, water at 25, 40 and 60 °C, sealed with plastic sheeting for 28 days. The results show that absorption and volume of permeable pore space (voids) of blended cement mortars at 28 day under all curing methods tend to increase with increasing silica fume replacement. The compressive strength of blended cement with fly ash and bottom ash was lower than that of Portland cement control at all curing condition while blended cement with silica fume shows higher compressive strength. In addition, the compressive strength of specimens cured with water increased with increasing curing temperature. The drying shrinkage of all blended cement mortar cured in air was lower than that of Portland cement control while the drying shrinkage of blended cement mortar containing silica fume, cured with plastic sealed and water at 25 °C was higher than Portland cement control due to pore refinement and high autogenous shrinkage. However, the drying shrinkage of blended cement mortar containing SF cured with water at 60 °C was lower than that of Portland cement control due to lower autogenous shrinkage and the reduced microporosity of C–S–H.     

Strength development of alkali-activated fly ash produced with combined NaOH and Ca(OH)2 activators

Alkali-activated matrices can attain compressive strengths of the order of 30–120 MPa, primarily depending on the mix formulation. The objective of this study is to evaluate compressive strength development by testing, at different ages, fly-ash-based mortars alkali-activated with activating solutions containing varying proportions of a combination of NaOH and Ca(OH)₂. The NaO/SiO₂ ratio was constant for all samples (N/S = 0.3). Three different CaO/SiO₂ molar ratios of the total quantity of CaO to the total quantity of SiO₂ present in the mix were studied; specifically: 0.05 (C/N = 0.033) – M5; 0.15 (C/N = 0.370) – M15; and 0.25 (C/N = 0.700) – M25. The M25 mix attained compressive strength of 30 MPa at 7 days. However, after periods of 28 and 91 days, M25 compressive strength had decreased to 22 MPa and 16 MPa respectively. The M15 matrices exhibited similar compressive strength results. In contrast, the M5 mix exhibited increasing compressive strength over time. The SEM micrographs of M5 and M25 matrices showed the presence of two different aluminosilicate gels: the M5 sample developed a massive aluminosilicate gel over time, while the M25 sample began to exhibit a spongy gel at 28 days, resulting in a weaker material. Therefore, the reduction in compressive strength appears to be related to increasing amounts of CaO (higher C/S and C/N) for the alkali-activated matrices tested in this study.     

An investigation of the synthesis,consolidation and mechanical behaviour of Al 6061 nanocomposites reinforced by TiC via mechanical alloying

Nanostructured Al 6061–x wt.% TiC (x = 0.5, 1.0, 1.5 and 2.0 wt.%) composites were synthesised by mechanical alloying with a milling time of 30 h. The milled powders were consolidated by cold uniaxial compaction followed by sintering at various temperatures (723, 798 and 873 K). The uniform distribution and dispersion of TiC particles in the Al 6061 matrix was confirmed by characterising these nanocomposite powders by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The mechanical properties, specifically the green compressive strength and hardness, were tested. A maximum hardness of 1180 MPa was obtained for the Al 6061–2 wt.% TiC nanocomposite sintered at 873 K, which was approximately four times higher than that of the Al 6061 microcrystalline material. A maximum green compressive strength of 233 MPa was obtained when 2 wt.% TiC was added. The effect of reinforcement on the densification was studied and reported in terms of the relative density, sinterability, green compressive strength, compressibility and Vickers hardness of the nanocomposites. The compressibility curves of the developed nanocomposite powders were also plotted and investigated using the Heckel, Panelli and Ambrosio Filho and Ge equations.     

Effect of activated coal mining wastes on the properties of blended cement

The large volumes of coal waste generated world-wide in mining operations are mostly deposited in refuse dumps, to the severe detriment of the surrounding groundwater and soil. After calcination under controlled conditions, this waste has been shown to exhibit high pozzolanicity, making it apt for use as an addition in the manufacture of blended cements.The present paper describes the first detailed study designed to evaluate the behavior of coal tailings from different sources. After activation at 650 °C for 2 h, this waste was used to manufacture blended cements containing 10 and 20 wt.% of the addition. Inclusion of this pozzolan did not affect the initial setting time, although the compressive strength of the blended mortars declined, by 4.7–8.3% in the 10% and by 9.76–14.9% in the 20% material. Nonetheless, the activated carbon waste (ACW) blends complied with all the requirements for Type II/A cement in the existing European legislation.     

Electromagnetic wave absorbing characteristics of carbon black cement-based composites

Electrical resistivity, compressive strength, and the electromagnetic absorbing effectiveness of carbon black (CB) cement-based composites (CBCC) with different contents of high-structure CB were studied in this paper. The results indicate that the resistivity of CBCC versus the concentration of CB curves has typical features of percolation phenomena: CBCC in the percolation threshold zone contains 0.36–1.34 vol.% of CB. Thus, the conductive network can be formed in CBCC by using small amount of high-structure CB. Compressive strength of CBCC decreases with CB content increasing. Especially, compressive strength decreases substantially when CB content is more than 3.0 wt.%. CBCC exhibits good performance of absorbing electromagnetic waves in the frequency range of 8–26.5 GHz. For CBCC containing 2.5 wt.% of CB, the minimum reflectivity reaches ?20.30 dB. The frequency bandwidth in which the reflectivity is less than ?10 dB was from 14.9 GHz to 26.5 GHz. The filling of CB has improved the dielectric constant and the loss factor of the cement material remarkably. The loss factor of CBCC increases with the CB content increasing.     

Properties of binary and ternary reactive MgO mortar blends subjected to CO2 curing

Research and development of low CO₂ binders for building material applications is warranted in efforts to reduce the negative environmental impacts associated with the cement and concrete industry. The purpose of this study is to investigate the effect of carbonation curing on the mineralogy, morphology, microstructure and evolution of compressive strength of mortars comprised of general use (GU) cement, ground granulated blast furnace slag (GGBFS), and reactive MgO used as cement replacement. This study investigates binary (GU–MgO) and ternary (GU–GGBFS–MgO) blends exposed to atmosphere curing (0.0038%CO₂) and carbonation curing (99.9%CO₂). Carbonation-cured mortars exhibited greater compressive strengths than atmosphere mortars at all ages (7 d, 28 d, and 56 d). Increasing percentages of reactive MgO decreased the compressive strength markedly less for carbonation-cured mortars than atmosphere-cured mortars particularly due to magnesium calcite formations. Magnesium calcite influenced the morphology of carbonates and promoted the carbonate agglomeration resulting in a dense and interconnected microstructure.