Early hydration of quick cements in the system C11A7 · CaF2C2SC2(A,F)

To explain the initial hydration of quick cements of the system C₁₁A₇ · CaF₂C₂SC₂(A,F), their SO₃/Al₂O₃ ratio was varied, Portland cement was added to increase the alcalinity of the pore solution in the cement stone, and setting retarded was used during the tests. The process of hydration was investigated with the help of X-ray diffraction and scanning electron microscopy as well as via the determination of the chemically bound water. The quantity of rapidly growing ettringite crystals increases with a rising SO₃/ Al₂O₃ value. The optimal SO₃/Al₂O₃ value of the system C₁₁A₇ · CaF₂C₂SC₂(A,F) is found between 0.26 and 0.38. The sole addition of setting retarder or Portland cement can distinctly reduce the formation of monocarbonate or even stop it completely. However, the simultaneous addition of both additives (setting retarded and a Portland cement) supports the formation of ettringite and monocarbonate crystals and through this also supports the increase of initial strength of quick cement.     

Early carbonation curing of concrete masonry units with Portland limestone cement

The effect of carbonation curing on the mechanical properties and microstructure of concrete masonry units (CMU) with Portland limestone cement (PLC) as binder was examined. Slab samples, representing the web of a CMU, were initially cured at 25 °C and 50% relative humidity for durations up to 18 h. Carbonation was then carried out for 4 h in a chamber at a pressure of 0.1 MPa. Based on Portland limestone cement content, CO₂ uptake of PLC concrete after 18 h of initial curing reached 18%. Carbonated and hydrated concretes showed comparable compressive strength at both early and late ages due to the 18-h initial curing. Carbonation reaction converted early hydration products to a crystalline microstructure and subsequent hydration transformed amorphous carbonates into more crystalline calcite. Portland limestone cement could replace Ordinary Portland Cement (OPC) in making equivalent CMUs which have shown similar carbon sequestration potential.     

Performance of Portland limestone cements in mortar prisms immersed in sulfate solutions at 5 °C

The results of a test programme to investigate the sulfate resistance of mortars, immersed up to 12 months at 5 °C in magnesium sulfate and sodium sulfate solutions, is described. The mortars were prepared from four cements; a Portland cement, a sulfate-resisting Portland cement and two Portland limestone cements containing 15% by mass of an oolitic limestone and a carboniferous limestone. The mortar specimens were subject to BS 5328 Class 4A and 4B sulfate exposure conditions. These are the highest classes for concretes prepared using sulfate-resisting Portland cement (SRPC) before surface protection is required and are two and three classes higher than those recommended for concretes prepared using Portland cement (PC) and Portland limestone cement (PLC), respectively. Two free water-cement ratios were used, 0.5 and 0.75. Performance was monitored by visual assessment, expansion and changes in flexural and compressive strengths.At a free water-cement ratio of 0.75, the PC mortars and PLC mortars exhibited visually very severe attack with the former showing expansion and reductions in strength, and the latter mainly reductions in strength. At a free water-cement ratio of 0.50 both the PC mortars and PLC mortars showed slight/moderate to severe visual attack, the degree of deterioration appearing slightly greater in the PLC mortars, more especially those made with oolitic limestone. The PLC mortars also exhibited reductions in compressive failure load. The SRPC mortars exhibited little visual deterioration, no expansion, a small increase in flexural strength and no significant reductions in compressive strength. At a free water-cement ratio of 0.75 substantial amounts of thaumasite, together with ettringite was present in the surface layers of the deteriorated PLC mortars whilst ettringite was present in the surface layers of the deteriorated PC mortars. It is concluded that mortars made with a PC with a C₃A content of about 10% by mass were broadly similar in their vulnerability to sulfate attack at 5 °C as PLC mortars containing 15% limestone by mass, although the mode of attack was different.     

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.     

Influence of pozzolana on sulfate attack of cement stone affected by chloride ions

This study investigated the influence of natural pozzolana (opoka) additive on the hydration of Portland cement and the effects of pozzolana on sulfate attack of cement stone affected by chloride ions. In the samples, 25 % (by weight) of the Portland cement was replaced with pozzolana. The specimens were hardened for 28 days in water, and then one batch was soaked in a saturated NaCl solution and another in a 5 % Na₂SO₄ solution for 3 months at 20 °C. After being kept for 3 months in a saturated NaCl solution, samples were transferred to a 5 % Na₂SO₄ solution and kept under these conditions for 3 months. It was estimated that under normal conditions, pozzolana additive accelerated the hydration of calcium silicates and initiated the formation of CO₃ ^2?–AFm; opoka also decreased the threshold pore diameter of hardened Portland cement paste. It was found that Cl^– ions penetrate to monosulfoaluminate, form Friedel’s salt, and release SO₄ ^2? ions, which react with unaffected monosulfoaluminate and form extra ettringite; when samples were transferred to the 5 % Na₂SO₄ solution, a greater quantity of new ettringite was formed. Meanwhile, pozzolana additive reduced the penetration of chloride and sulfate ions into the structure of Portland cement hydrates and inhibited sulfate attack of cement stone treated in a saturated NaCl solution.     

Optimization of calcium chloride content on bioactivity and mechanical properties of white Portland cement

This research investigates the optimization of calcium chloride content on the bioactivity and mechanical properties of white Portland cement. Calcium chloride was used as an addition of White Portland cement at 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10% by weight. Calcium chloride was dissolved in sterile distilled water and blended with White Portland cement using a water to cement ratio of 0.5. Analysis of the bioactivity and pH of white Portland cement pastes with calcium chloride added at various amounts was carried out in simulated body fluid. Setting time, density, compressive strength and volume of permeable voids were also investigated. The characteristics of cement pastes were examined by X-ray diffractometer and scanning electron microscope linked to an energy-dispersive X-ray analyzer. The result indicated that the addition of calcium chloride could accelerate the hydration of white Portland cement, resulting in a decrease in setting time and an increase in early strength of the pastes. The compressive strength of all cement pastes with added calcium chloride was higher than that of the pure cement paste, and the addition of calcium chloride at 8 wt.% led to achieving the highest strength. Furthermore, white Portland cement pastes both with and without calcium chloride showed well-established bioactivity with respect to the formation of a hydroxyapatite layer on the material within 7 days following immersion in simulated body fluid; white Portland cement paste with added 3%CaCl₂ exhibited the best bioactivity.     

Hydration of quaternary Portland cement blends containing blast-furnace slag,siliceous fly ash and limestone powder

In this study the hydration of quaternary Portland cements containing blast-furnace slag, type V fly ash and limestone and the relationship between the types and contents of supplementary cementitious materials and the hydrate assemblage were investigated at ages of up to 182 days using X-ray diffraction and thermogravimetric analysis. In addition thermodynamic modeling was used to calculate the total volume of hydrates. Two blast-furnace slag contents of 20 and 30 wt.% were studied in blends containing fly ash and/or limestone at a cement replacement of 50 wt.%. In all cases the experiments showed the presence of C–S–H, portlandite and ettringite. In samples without limestone, monosulfate was formed; in the presence of limestone monocarbonate was present instead. The addition of 5 wt.% of limestone resulted in a higher compressive strength after 28 days than observed for cements with lower or higher limestone content. Overall the presence of fly ash exerts little influence on the hydrate assemblage. The strength development reveals that amounts of up to 30 wt.% fly ash can be used in quaternary cements without significant loss in compressive strength.     

Analysis of carbonate and sulphate attack on concrete structures

The current study was designed to explain the progress and causes of damage to structural concrete exposed to atmospheric CO₂ and rainwater (23-year old slab) and interactions of atmospheric CO₂, rainwater and gypsum (50-year old columns). The testing programme included SEM with EDXA and XRD analysis, and tests determining compressive strength and water absorption. During the carbonation process the basic product identified in the concrete microstructure was calcite or “secondary” calcite, which appeared at the surface of concrete, in the cracks or in the form of dripstone. The formation of ettringite and calcite was observed in the concrete columns exposed to interactions of gypsum, carbon dioxide and rainwater for 50 years. Ultimately, the only product present on the exterior of the concrete was calcite. This is attributed to the conversion of ettringite to calcite due to a direct reaction with CO₂. The test results were compared to the theoretical hypotheses.     

Calcium silicate cements obtained from rice hull ash: A comparative study

This work describes the utilization of rice hull as raw-material for the preparation of two calcium silicates namely, β-Ca_1.91Ba_0.04SiO₄ and β-Ca_1.96Ba_0.04SiO₄. The synthesis was completed at 800°C. Hydration rate and compressive strength of mortars prepared with the two calcium silicates were studied and compared to mortars prepared with commercial Portland cement. Hydration rates for both silicates, studied by thermogravimetric and FTIR analysis are very similar; after 60 days the hydration rates are around 42–43% and reaches 75% after 270 days. Compressive strength experiments were performed using test specimen prepared with commercial Portland cement as reference, and blends of Portland cement and the two calcium silicates, at replacement levels of 10 and 20%. Results have shown that after a 90 days curing period, the compressive strength of the reference and the blends containing 10% of each of the calcium silicates show the same behavior. Using a replacement level of 20% there is a small decrease in compressive strength. This behavior is attributed to the lower hydration rate of these calcium silicates.     

Effect of further water curing on compressive strength and microstructure of CO2-cured concrete

This study aims to investigate the effects of further water curing on the compressive strength and microstructure of CO₂-cured concrete. The results showed that concrete with a residual w/c ratio of 0.25 showed the most rapid strength development rate upon further water curing due to hydration of uncarbonated cement particles. Thermogravimetric, IR-spectrophotometric and scanning electron microscope examinations indicated that further hydration of the cement particles could form C-S-H gel and ettringite crystals. The results showed that the calcite formed during the initial CO₂ curing was consumed during the further hydration of C₃A, and produced calcium monocarbonaluminate hydrate. Also, Ca(OH)₂ was not detected due to its reaction with the formed silica gel. Mercury intrusion porosimetry test results indicated that the porosity and pore size of the CO₂ cured mortar decreased further after water curing.     

The sulphate resistance of cements containing red brick dust and ground basaltic pumice with sub-microscopic evidence of intra-pore gypsum and ettringite as strengtheners

This paper presents a laboratory study on the deterioration of blended cement combinations of plain Portland cement (PPC) with red brick dust (RBD) and ground basaltic pumice (GBP). One type of clinker, same Blaine values and two different proportions of additive by mass of clinker, were employed. In addition to these blends, Portland cements without additives were prepared as control specimens.The compressive strength and the sulphate resistance of cements have been experimentally studied in this paper. A series of laboratory tests were undertaken on all specimens. A large quantity of sheet-like C-S-H was found in the mortars incorporating RBD and GBP. The results indicated that the increase in the additive content caused a significant increase in the sulphate resistance of the mortars. Hence, the studied RBD and GBP can be recommended for use as admixtures in cement production. The development of the particular microstructure including the secondary minerals in the plain and blended cements were studied via SEM analysis. SEM images revealed the presence of ettringite and Portlandite minerals, where the former was most probably responsible for the increase (together with the gypsum roses) as well as a decrease of strength based on its formation at different sites and crystal form. Portlandite was responsible for an increase in the specimen strength.     

Belite cements obtained from ceramic wastes and the mineral pair CaF2/CaSO4

The cement industry is seeking alternative approaches to reduce the high energy and environmental costs of Portland cement manufacture. One such alternative is belite cement. In the present study clinkers with high (36–60%) belite contents were obtained at 1350 °C from raw mixes consisting of ceramic waste and the fluxing/mineralised pair CaF₂/CaSO₄. The factors found to affect the mineralogical composition and the clinker phase polymorphs obtained were the lime saturation factor (LSF), the presence of ceramic waste and the addition of CaF₂ and CaSO₄.The reactivity of these belite clinkers with water was analysed with isothermal conduction calorimetry. A statistical study was then conducted on the findings to determine the effect of each variable when the response signals were peak heat flow rate and the time needed to reach that peak. The statistical analysis identified the optimal experimental conditions to be a LSF of 90%, a CaSO₄ content of 2.60%, and the absence of both ceramic waste and CaF₂.     

Anti-corrosion performance and microstructure analysis on a marine concrete utilizing coal combustion byproducts and blast furnace slag

Marine concrete structures are exposed to serious attack through a number of physical and chemical deterioration processes, which deserve special attention from worldwide scientists and policy makers. This paper presents a new approach to enhance the anti-corrosion ability for concrete composed of coal combustion byproducts and blast furnace slag. Based on the experimental results, it has been concluded that the concrete composed of pozzolanic material not only provides a great chance to utilize a huge amount of industrial solid waste but also performs much better in tests of compressive strength, anti-corrosion test, and chloride penetration rather than the ordinary Portland cement concrete. The microanalysis found that needle-shaped ettringite or Ca(OH)₂ crystals were presented dominantly in the microstructure of the early age hydration product, however, the amorphous C–S–H or C–A–S–H plays a more significant role in the middle to late curing age on mechanical properties and anti-corrosion abilities.     

Dehydration and rehydration processes of cement paste exposed to high temperature environments

Microstructural changes of an OPC cement paste after being exposed at various elevated temperatures and further rehydration have been evaluated using ²⁹Si MAS-NMR. Thermogravimetry and XRD are also employed to complement the information. NMR studies of cement paste exposed to high temperatures demonstrate a progressive transformation of C-S-H gel that leads at 450°C, to a modified C-S-H gel. For temperatures above 200°C to a progressive formation of a new nesosilicate. At 750°C, the transformation of C-S-H is complete into the nesosilicate form with a C₂S stoichiometry close to larnite, but less crystalline. Also is observed an increase of portlandite that takes place up to temperatures of 200°C. A progressive increase of calcite formation up to 450°C is noticed. The ettringite disappearance below 100°C is confirmed and the portlandite and calcite are converted to lime at 750°C. The initial anhydrous phases as larnite and brownmillerite remain unaltered during heating. Rehydration of the heated samples (450 and 750°C) shows recrystallization of calcite, portlandite and ettringite, and the C-S-H reformation from the new nesosilicate. The larnite and brownmillerite remain unaltered during rehydration. The developing of damaged due to the formation of microcracking is detected and improved because of rehydration phenomena.     

Sisal fiber-reinforced cement composite with Portland cement substitution by a combination of metakaolin and nanoclay

This paper reports the partial replacement of Portland cement (PC) by combination of metakaolin (MK) and nanoclay (NC) in sisal fiber-reinforced cement composites by studying the microstructure, mechanical behavior, and the interfacial properties between fiber and cement matrices. The mechanical properties of cement matrix and natural fiber-reinforced composites are studied using compressive strength development and flexural behavior, respectively. The tensile behavior of the natural fiber was also investigated and analyzed by Weibull distribution model. The characteristics of hydration products were analyzed by scanning electron microscope, X-ray diffraction, and thermogravimetry analysis. Our results show that the combination of MK and NC can improve the hydration of cement more effectively, with better microstructure and enhanced mechanical properties, than mixes without them. The calcium hydroxide (CH) contents of matrixes with 50 wt% combined substitutions, containing 1, 3, and 5 wt% of nanoclay, were 58.12, 60.16, and 64.25 % less than that of PC, respectively. The ettringite phase is also effectively removed due to the substitution of MK and NC, which improve both Al/Ca and Si/Ca ratios of calcium silicate hydrates (C–S–H) due to the high content of SiO₂ and Al₂O₃. The interfacial bond between fiber and cement matrix and flexural properties of sisal fiber-reinforced cement composites are also significantly improved. The optimum interface adhesion between sisal fiber and matrix was achieved by replacing cement by 27 % MK and 3 % NC, which increased the bond strength and pull-out energy by 131.46 and 196.35 %, respectively.     

Advance treatment by nanographite for Portland pulverised fly ash cement (the class F) systems

This is a full article that overcomes such some negative side effects as rapid coagulation and reduced early strength in class F fly ash-substituted cement (FFA-SC) by serving nano graphite particle (nG). This study uses class F fly ash (FFA), nG, and ASTM type I cement as constituent materials to prepare proper pulverized fly ash–Portland cement combinations (35% FFA + 65% ASTM I + 1.1% nG i.e.). Pastes include lime and/or lime + nG, and tap water to examine ups and downs in Ca(OH)₂ content. Mortars also contain these prepared cements, tap water and/or tap water + super plasticizer (SP), and mortar sand in order to measure fluidity, flexural strength, and compressive strength according to present standard methods. Results indicate for FFA-SC system that the nano graphite particle increases the reduced early strength gain at early age, and the SP reduces the rapid coagulation. The use of nG also shows to be favorable in terms of the Ca(OH)₂ content, the fluidity and the flexural strength gain, and compressive strength gain in FFA-SC system when compared to the pure Portland cement with and without nano graphite particle.     

Mathematical model for the prediction of cement compressive strength at the ages of 7 and 28 days within 24 hours

In this study 450 cement mortar cubes were cast from 50 different cement samples taken from 9 different cement factories, to develop a mathematical model that can predict Portland cement compressive strength at ages 7 and 28 days within 24 hours only. This is in order to save time and expense, that is lost in waiting for such a long period, and for quality control assurance for both produced cement (in cement factories), and concrete mixes in constructions. In addition, attention has been made on the right choice of variables of the cement itself (phase composition and fineness). In addition, an attempt has been made to use other variables that are believed to affect compressive strength of Portland cement as the minor oxides MgO, SO₃ and soundness. Other variables obtained from chemical analysis of the cement as LOI, IR, and LSF were also included in the model. The most important thing in this study is to get use of the concept of using early age strength to predict Portland cement strength at later ages for the first time. An attempt was made to combine both accelerated strength testing (as an early strength and UPV of cement mortar specimens), with the characteristics of the cement mentioned above, in predicting the compressive strength of cement. It was found that the accelerated strength yields good and high correlation with the compressive strength of cement, especially at the age of 28 days. In this work too, the importance of the ultrasonic pulse velocity (UPV) and mortar density were evident and the usefulness of using these variables in predicting the compressive strength of the cement was proved (because of fixing most of the factors affecting this property). Thus, it is possible to have good results that can be used in the prediction of compressive strength of cement. It was found that using each of the accelerated compressive strength f_acc, UPV and density of the mortar cubes yielded high correlation with the compressive strength than any of the other variables. Different combinations of variables were introduced into the model, in order to choose the variables that can significantly predict the cement compressive strength. In this work, it was possible to obtain a model that can predict the cement strength with standard errors of only 1.887 and 1.904 MPa and coefficients of correlation of 0.903 and 0.928, for cement strengths at 7 and 28 days respectively.     

Effects of sucrose and sorbitol on cement-based stabilization/solidification of toxic metal waste

The effects of sucrose or sorbitol addition on the hydration, unconfined compressive strength and leachability of Portland cement pastes containing 1% Pb and 1% Zn were studied as a function of time. Whereas Pb and Zn were found to shorten the time to achieve maximum hydration of Portland cement, the combination of these metals with 0.15 wt% sucrose or 0.40 wt% sorbitol retarded the setting of cement by at least 7 and 28 days, respectively, without affecting the strength at 56 days. The leachability of Pb and Zn evaluated by the TCLP 1311 protocol at 56 and 71 days was slightly reduced or unchanged by the addition of sucrose or sorbitol. SEM-EDS and XRD analyses revealed that ettringite precipitation was favored whereas the formation of CSH gel, which accounts for most of the strength of hydrated cement, was delayed in cement pastes containing both metals and sucrose or sorbitol. These results indicate that controlled additions of sucrose or sorbitol can add flexibility to the handling of cement-treated metal waste, particularly when it needs to be transported by truck or pipeline between the treatment plant and the disposal site, without affecting its long-term performance.     

Mechanical properties and durability of mortar and concrete containing natural pozzolana and limestone blended cements

The benefits of limestone filler (LF) and natural pozzolana (NP) as partial replacement of Portland cement are well established. Economic and environmental advantages by reducing CO₂ emission are well known. However, both supplementary materials have certain shortfalls. LF addition to Portland cement causes an increase of hydration at early ages inducing a high early strength, but it can reduce the later strength due to the dilution effect. On the other hand, NP contributes to hydration after 28 days improving the strength at medium and later ages. Hence, ternary blended cement (OPC–LF–NP) with better performance could be produced. In this paper, mortar prisms in which Portland cement was replaced by up to 20%LF and 30%NP were tested in flexure and compressive strength at 2, 7, 28 and 90 days. Some samples were tested under sulfate and acid solutions and for chloride ions permeability. Results show that the use of ternary blended cement improves the early age and the long-term compressive and flexural strengths. Durability was also enhanced as better sulfate, acid and chloride ions penetration resistances were proved.     

Hydration characteristics of municipal solid waste incinerator bottom ash slag as a pozzolanic material for use in cement

This study investigated the pozzolonic reactions and engineering properties of municipal solid waste incinerator (MSWI) bottom ash slag blended cements (SBC) with various replacement ratios. The 90-day compressive strengths developed by SBC pastes with 10% and 20% cement replacement by slags generated from the bottom ash were similar to that developed by ordinary Portland cement pastes. Thermal analyses indicated that the hydrates in the SBC pastes were mainly portlandite (Ca(OH)₂) and calcium silicate hydrate (C–S–H) gels, similar to those found in ordinary Portland cement paste. It is also indicated that the slag reacted with Ca(OH)₂ to form C–S–H. The average length (in terms of the number of Si molecules) of linear polysilicate anions in C–S–H gel, as determined by ²⁹Si nuclear magnetic resonance, increased in all the SBC pastes with increasing curing age, which outperformed that of ordinary Portland cement at 90 days. It can thus be concluded from the study results, that municipal solid waste incinerator bottom ash can be processed by melting to obtain reactive pozzolanic slag, which may be used in SBC to partially replace the cement.