Compressive strength and hydration with age of cement pastes containing dune sand powder

This experimental work has focused on studying the possibility of using dune sand powder (DSP) as a part mass addition to Portland cement. Studying the effect of addition dune sand powder on development of compressive strength and hydration with age of cement pastes as a function of water/binder ratio, was varied, on the one hand, the percentage of the dune sand powder (physico-chemical and chemical effect) and on the other, the fineness of dune sand powder (physical effect). In order to understand better the chemical effect (pozzolanic effect) of dune sand powder in cement pastes, we followed the mixtures hydration (50% pure lime + 50% DSP) by X-ray diffraction. These mixtures pastes present a hydraulic setting which is due to the formation of a C–S–H phase (calcium silicate hydrate). The latter is semi-crystallized. This study is a simplified approach to that of the mixtures (80% ordinary Portland cement + 20% DSP), in which the main reaction is the fixing of the lime coming from the cement hydration in the presence of the dune sand powder (pozzolanic reaction), to form calcium silicate hydrate C–S–H semi-crystallized of second generation. The results proved that up to 20% of dune sand powder as Portland cement replacement could be used with a fineness of 4000 cm²/g without affecting adversely the compressive strength. The dune sand powder, despite its crystalline nature, presents a partial pozzolanic reactivity.     

Performance of limestone cement mortars in a high sulfates environment

With the aim of studying the influence of cement composition on resistance in high sulfates environment, standard mortars have been produced using ordinary Portland cement (CEM I – 32.5) and limestone cement with 35% limestone (CEM II/B-LL – 32.5). The pore size distribution of the cement pastes was measured. The mortars were immersed in a 5% Na₂SO₄ solution at 20 °C for 1.5 years and the caused deterioration was been visually observed at a regular basis. Furthermore, the mortars expansion was being estimated by measuring the change of length. At the end of the experiment the compressive strength of the mortars was measured. The deterioration products of the mortars have been identified by means of X-ray diffraction, optical microscopy and environmental scanning electron microscopy. The limestone cement based mortar presented cracking that started at the age of 6 months and continued throughout the experiment. It also displayed high expansion after 250 days of immersion in a 5% Na₂SO₄ caused, as proved using the analytical techniques, by the formation of gypsum and ettringite. Concluding, the cement with 35% limestone did not perform as well as ordinary Portland cement under the most aggressive laboratory conditions. Hence, it is obvious that the addition of limestone in the cement leads to a totally different behaviour than Portland cement with respect to the resistance in high sulfates environment.     

Effects of size fraction on composition and fundamental properties of Portland cement

To characterize roles of size fraction of Portland cement to its properties, Portland cement was classified into several fractions by air classifier. Then composition and hydration related properties of each Portland cement fraction were investigated. The results show deviation of chemical and mineral compositions occurs during classification. Both fine and coarse Portland cement fractions have undesirable strength contribution. Portland cement fractions falling in the range of 8–24 μm have low water requirement, high hydration rate and highest 28 days compressive strength. Thus Portland cement is recommended to be arranged in middle fraction (8–24 μm), while high activity supplementary cementitious materials (SCMs) and low activity SCMs (or inert fillers) are suggested to be arranged in fine and coarse fractions, respectively, by which Portland cement can be replaced by SCMs (or inert fillers) in larger extent without or with little performance loss.     

Stabilization of Cu (II) wastes by C3S hydrated matrix

Cement is the most adaptable binder currently available for the immobilization of heavy metals. C₃S, tricalcium silicate, is one of the main phases in OPC and many of its properties were related to C₃S. In the present work the stabilization of Cu (II) wastes by C₃S during its early hydration (from 3 h – 7 days) was studied. Copper ions here either doped inside C₃S during its preparation or present in the water of hydration. Levels of Cu (II) used were 1 and 3 wt.% of C₃S. The chemically combined water and free Ca(OH)₂ contents were determined after 3, 6 h and 1, 3 and 7 days of hydration. X-ray diffraction examination was performed for some selected samples. The results showed that the presence of Cu (II) ions retard the early hydration of C₃S. This is due to the precipitation of the less soluble Cu(OH)₂ which retards the precipitation of Ca(OH)₂ as a result of the reduction in pH. Immobilization percentage of Cu (II) ions inside the C₃S hydrated matrix was examined by the determination of the leached copper by using atomic absorption spectroscopy. Most of the investigated samples showed high degree of immobilization of Cu (II) ions and the doped mixes showed better immobilization results than mixes hydrated in water containing wastes. The rate of leached Cu (II) ions from the matrix of hydrated C₃S was investigated by the application of diffusion equation derived for a plane source model.     

Hydration heat kinetics of concrete with silica fume

The objective of this study was to evaluate the influence of silica fume on the hydration heat of concrete. Portland cement was replaced by silica fume in amounts from 10 % to 30 % by mass in concrete with w/(c+sf) ratios varying between 0.25 and 0.45. A superplasticizer was used to maintain a fluid consistency. The heat of hydration was monitored continuously by a semi-adiabatic calorimetric method for 10 days at 20 °C. The calorimetric study indicated that the hydration was modified by the presence of silica fume. In the early stages, the silica fume showed a high activity and accelerated the hydration rate as compared to that of the reference concrete. The fine silica fume particled provided nucleation sites for hydrates growth. Then the pozzolanic activity took over and increased both strength and the hydration heat. A substitution of Portland cement by 10% with silica fume produced greater strength and cumulative heat of hydration as compared to that of the reference concrete.     

The effect of curing temperature on white cement hydration

Cement manufacture has undergone extensive change in its attempts to rise to the successive challenges posed by society. The hydration of Portland cement is a complex phenomenon that depends on both reagent characteristics and reaction conditions. It is a well-known fact that the curing temperature plays an important role on the hydration kinetics. The present study is the first to be undertaken on the effect of curing temperature on white Portland cement paste hydration over long hydration times (365 days at 20 °C and 124 days at 60 °C). The technique used, ²⁹Si and ²⁷Al NMR spectroscopy, is particularly well adapted to the study of cement hydration. White cement hydration generates a C–S–H gel in which the aluminium taken up forms bridge bonds. After nine days at 60 °C, the degree of reaction expressed in terms of the Al(IV)/Al(VI) ratio nearly doubles the value found after 90 days at 20 °C.     

The effect of activator concentration on the residual strength of alkali-activated fly ash pastes subjected to thermal load

This investigation reports on a comparative study of the residual compressive strength at different temperatures of alkali-activated fly ash (AAFA) pastes chemically activated using sodium silicate with three different concentrations named 20, 30 and 40 (wt.%). The behaviour of different mixtures in conditions of rapid temperature changes was studied. Water quench test was applied to determine thermal shock resistance. The traditional Portland cement pastes were used as a reference. The temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C has been examined. Pore solution pH and compressive strengths before and after exposure to elevated temperatures were determined. The various decomposition phases formed were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTG) and scanning electron microscopy (SEM). The results indicated that fly ash activated by sodium silicate is more able to resist degradation caused by exposure to elevated temperature than Portland cement system as its relative strengths are superior. In the hardened AAFA pastes, as activator concentration dosage increased as the relative compressive strengths and thermal shock resistance decreased. The AAFA system is able to maintain a sufficient high pH to retain the passive protective layer on the surface of any reinforcing steel present.     

Hybrid effect of carbon nanotube and nano-clay on physico-mechanical properties of cement mortar

In this work, several nanomaterials have been used in cementitious matrices: multi wall carbon nanotubes (MWCNTs) and nano-clays. The physico-mechanical behavior of these nanomaterials and ordinary Portland cement (OPC) was studied. The nano-clay used in this investigation was nano-kaolin. The metakaolin was prepared by thermal activation of nano-kaolin clay at 750 °C for 2 h. The organic ammonium chloride was used to aid in the exfoliation of the clay platelets. The blended cement used in this investigation consists of ordinary Portland cement, carbon nanotubes and exfoliated nano metakaolin. The OPC was substituted by 6 wt.% of cement by nano metakaolin (NMK) and the carbon nanotube was added by ratios of 0.005, 0.02, 0.05 and 0.1 wt.% of cement. The blended cement: sand ratio used in this investigation was 1:2 wt.%. The blended cement mortar was prepared using water/binder ratio of 0.5 wt.% of cement. The fresh mortar pastes were first cured at 100% relative humidity for 24 h and then cured in water for 28 days. Compressive strength, phase composition and microstructure of blended cement were investigated. The results showed that, the replacement of OPC by 6 wt.% NMK increases the compressive strength of blended mortar by 18% compared to control mix and the combination of 6 wt.% NMK and 0.02 wt.% CNTs increased the compressive strength by 29% than control.     

Use of natural and thermally activated porphyrite in cement production

The aim of the present study was to investigate the use of porphyrite in the production of Portland cement. Natural and thermally activated porphyrites were used as a clay raw material and an activator, respectively, at 0, 10, 20, 30, 40 and 50 wt% in order to assess their effects on the cement properties. According to the test results, the compressive strength of the specimens decreased with increasing natural porphyrite content in various curing periods. However, the compressive strength of cement produced with 10 wt% porphyrite (activator) activated at 650 °C for 30 min showed a higher value (56 MPa in TPC-6) than cement without activator (51 MPa in RPC-2). Due to thermal activation, porphyrite activator containing a glass phase possesses an enhanced reactivity during clinker hydration that intensifies the synthesis of hydrosilicates and improves compressive strength accordingly. The X-ray diffraction analysis confirmed an intensive formation of Portland cement minerals such as C₃S, β-C₂S, C₃A and C₄AF. The addition of thermally activated porphyrite has also led to an improvement of the rheological behavior, stability to expansion, increase in setting time and decrease in specific surface area of cement. As prepared cement composites and concretes with improved properties meet the requirements of State Standards 310-86 and 10181-81 for Portland cement and concrete, respectively. The findings in this report indicate that porphyrite can be utilized both as a raw material and an activator in the production of cement.     

Physical and mechanical properties of durable sisal fiber–cement composites

Sisal fiber–cement composites reinforced with long unidirectional aligned fibers were developed and their physical–mechanical behavior was characterized in the present study. Flat and corrugated sheets were cast by a manual lay-out of the fibers in a self-compacted cement matrix and compressed with a pressure of 3 MPa. Direct tensile and bending tests were performed to determine the first crack, post-peak strength and toughness of the composites. Drying shrinkage, capillary water absorption and water tightness tests were performed to characterize the physical properties of the composites. To ensure the composite durability, the ordinary Portland cement matrix was modified by adding metakaolin and calcined waste crushed clay brick to consume the calcium hydroxide generated during Portland cement hydration. The durability of the newly developed composite was determined through accelerated aging conditions using the hot-water immersion test. The developed material presented a multiple cracking behavior under bending, even when subjected to 6 months of hot-water immersion under 60 °C. Scanning Electron Microscopy was used to investigate the micro-structure of the composites before and after aging.     

Effect of palm oil fuel ash fineness on the microstructure of blended cement paste

This paper presents the effect of palm oil fuel ash fineness on the microstructure of blended cement paste. Palm oil fuel ash (POFA) was ground to two different finenesses. Coarse and high fineness palm oil fuel ash, with median particle sizes of 15.6 and 2.1 μm, respectively, were used to replace ordinary Portland cement (OPC) at 0%, 20% and 40% by binder weight. A water to binder (W/B) ratio of 0.35 was used for all blended cement pastes. The amorphous ground palm oil fuel ash was characterized by the Rietveld method. The compressive strength, thermogravimetric analysis and pore size distribution of the blended cement pastes were investigated. The test results indicate that the ground palm oil fuel ash was an amorphous silica material. The compressive strengths of the blended cement pastes containing coarse POFA were as high as that of OPC cement paste. Blended cement paste with high fineness POFA had a higher compressive strength than that with coarse POFA. The blended cement pastes containing 20% of POFA with high fineness had the lowest total porosity. The Ca(OH)₂ contents of blended cement paste containing POFA decreased with increasing replacement of POFA and were lower than those of the OPC cement paste. In addition, the POFA fineness had an effect on the reduction rate of Ca(OH)₂. Furthermore, the critical pore size and average pore size of blended cement paste containing POFA were lower than those of the OPC cement paste. The incorporation of high fineness POFA decreased the critical pore size and the average pore size of blended cement paste as compared to that with coarse POFA.     

Effect of free water removal from early-age hydrated cement pastes on thermal analysis

In order to follow the progress of cement hydration by analytical techniques, the ongoing hydration reactions must be stopped by complete removal of free water. A comparison between solvent exchange, oven drying and vacuum drying, using thermal analysis, is presented for early-age hydrated cement pastes. Results show that oven drying at 105 °C accelerates hydration, causes dehydration of some hydrated cement phases and favours carbonation. Solvent exchange with ethanol, ether and methanol results in a strong absorption and an incomplete removal of solvents. FT-IR and XRD gave evidence of the formation of carbonate-like phases due to an interaction upon heating, i.e. during thermal analysis, between the strongly absorbed solvents and the cement compounds or hydrates. Vacuum drying reveals reliable results as no interaction products can be formed and the Ca(OH)₂ content, determined by thermal analysis, gives a good approximation of the real amount of Ca(OH)₂.     

Early hydration of calcium sulfoaluminate cement through electrical resistivity measurement and microstructure investigations

The electrical resistivities of the calcium sulfoaluminate (CSA) cement pastes at different water to cement (W/C) ratios, over a period of 1440 min (or 24 h) were measured by a non-contacting electrical resistivity apparatus. The electrical resistivity–time curve decreased slightly after mixing, and then increased sharply around the setting time, followed by two peaks, and finally developed at a very low rate up to 1440 min. The porosity plays a dominant role in the electrical resistivity development of the CSA cement paste in the earlier period of hydration (before the initial setting time), while the ion concentration plays a major role at the later period (after the second peak). The electrical resistivity–time curve demonstrates the process of ettringite formation and transformation to monosulfate in the CSA hydration system, and this is confirmed by the SEM and XRD observations. The high temperature and the insufficiency of gypsum in the CSA cement system during hydration result in the decomposition of ettringite at the first peak, while at the second peak the decomposition of ettringite is due to the lack of gypsum. The relationship between W/C and electrical resistivity at 1 h follows a negative trend but a positive trend is observed at 24 h. The relationship between the compressive strength and the electrical resistivity at 24 h for the different W/C ratios follows a negative relationship.     

Study of the reactivity of cement/metakaolin binders at early age for specific use in steam cured precast concrete

The paper presents the results of a hydration study performed in order to explain the significant increase in compressive strength at one day of age observed on steam cured mortars when 25% by mass of cement was replaced with a metakaolin. Two CEM I 52.5R cements, differing in reactivity, and a metakaolin (MK) were used. By means of XRD and thermal analysis carried out on cement pastes, blended or not with MK, the main results showed that the improvement in strength at one day of age could be explained by the occurrence of a pozzolanic reaction due to MK, thermo-activated by the high curing temperature (55 °C). The pozzolanic reaction was observed through the consumption of calcium hydroxide and an increase in the amount of C–S–H and C–S–A–H hydrated phases. This change in the hydration product nature and amount was more pronounced when MK was combined with the less reactive cement, in agreement with the mechanical results on mortars. These results are of great importance for the concrete industry where the current trend is to decrease the clinker content in cements (1 ton of clinker = 1 ton of CO₂ released). In particular, the interesting mechanical performance at early ages can be helpful for precast concrete manufacturing.     

Effect of red mud addition on the rheological behaviour and on hardened state characteristics of cement mortars

This paper reports on the use of red mud (RM) in mortars, applying design of experiments. Portland cement was replaced up to 50 wt.% RM, adjusting the relative amount of water (34–38 wt.%) in order to get mortars with suitable workability as defined by rheometry and flow table measurements. Temperature of hydration, compressive strength and water absorption were also determined. RM decreases the workability and increases the torque, but causes lower impact than water variation. The effect on initial yield stress depends on water content. Mortars with similar spread on table show different behaviour along the rheology test. Values of spread on table follow a quadratic model and RM exhibited an interactive effect with water. RM did not change the hydration process, but above 20% the maximum temperature decreases. The reduction of compressive strength is not constant and depends on the water added. Its variation also follows a quadratic model.     

Coarse blast furnace slag as a cementitious material,comparative study as a partial replacement of Portland cement and as an alkali activated cement

The cementitious performance of a coarse granulated blast furnace slag, 2900 cm²/g, was investigated in concretes of 230, 280 and 330 kg binder/m³. First, the slag partially replaced 30%, 50% and 70% of Portland cement, the strength reduced as the amount of slag increased; however, for high binder contents, similar strengths were attained for lower Portland cement contents. Second, the slag was alkali activated with sodium silicate (moduli 1.7 and 2) at 4%, 6% and 8% %Na₂O, the strength increased with the amount of slag in the concrete and developed faster as %Na₂O increased. The microstructures of both type of concretes were dense; however, the strengths of activated slag were superior at similar binder loads, indicating that the hydration products of activated slag are of higher intrinsic strength.     

Formation of hydrates of calcium aluminates in cement pastes with different dosages of SBR powder

The formation of hydrates of calcium aluminates in cement pastes in the presence of different dosages of styrene–butadiene rubber (SBR) powder was evaluated. The content of AFt, AFm, C₄AH₁₃ and CAH₁₀ in cement pastes was determined using XRD. The results showed that SBR powder facilitates the formation of AFt and enhances its stability. The enhanced formation of AFt results from a decrease in CH formation. AFm and C₄AH₁₃ appear after 7 and 28 days and both contents are decreased by SBR powder. Most C₃A reacts with CH after the gypsum is exhausted. CAH₁₀ is present in the cement pastes with SBR powder.     

Strength and toughness improvement of cement binders using expandable thermoplastic microspheres

The effect of Expandable Thermoplastic Microspheres (ETM) loading on the fracture resistance and indirect tensile strength of cement binders is studied. Portland white cement (PWC) was used as the matrix in the current study. Loadings of 0.1%, 0.35%, 0.5%, 0.75% and 1%, by weight, of ETM were added to the dry cement. Semi-circular bend specimens, 152 mm in diameter and 27 mm thickness with different notch depths were fabricated to study the crack resistance of the compounds, J_c. For the indirect tensile tests, circular specimens, 50 mm in diameter and 12.7 mm thickness were used. All specimens were left to cure under water for 7 days. A 2.5-fold increase in the indirect tensile strength was achieved at an ETM loading of 0.35% by weight. A nearly threefold increase in the fracture resistance occurred at the 0.1% ETM loading. The thermal resistivity of the compounds increased by 30% for a 1% Expancel loading. Fracture surface examination revealed that the ETM facilitated the permeation of water by creating pores. Thus, an optimum strength and fracture resistance was achieved between 0.1% and 0.4%.     

Effects of limestone replacement ratio on the sulfate resistance of Portland limestone cement mortars exposed to extraordinary high sulfate concentrations

A comparative study has been performed on the sulfate resistance of Portland limestone cement (PLC) mortars exposed to extraordinary high sulfate concentrations (200 g/l). PLCs have been prepared by using two types of clinkers having different C₃S/C₂S ratios and interstitial phase morphologies. Blended cements have been prepared by replacing 5%, 10%, 20% and 40% of clinker with limestone. Cubic (50 × 50 × 50 mm) and prismatic (25 × 25 × 285 mm) cement mortars were prepared. After two months initial water curing, these samples were exposed to three different sulfate solutions (Na₂SO₄ at 20 °C and 5 °C, MgSO₄ at 5 °C). Solutions were not refreshed and pH values of solutions were monitored during the testing stage. The compressive strength and length changes of samples have been monitored for a period of 1 year. Additional microstructural analyses have been conducted by XRD and SEM/EDS studies. Results indicated that in general, limestone replacement ratio and low temperature negatively affect the sulfate resistance of cement mortars. Additionally, clinkers of high C₃S/C₂S ratios with dendritic interstitial phase structure were found to be more prone to sulfate attack in the presence of high amounts of limestone.From the results, it is postulated that in the absence of solution change, extraordinary high sulfate content modified the mechanism of sulfate reactions and formation of related products. At high limestone replacement ratios, XRD and SEM/EDS studies revealed that while ettringite is the main deterioration product for the samples exposed to Na₂SO₄, gypsum and thaumasite formation were dominant products of deterioration in the case of MgSO₄ attack. It can be concluded that, the difference between reaction mechanisms of Na₂SO₄ and MgSO₄ attack to limestone cement mortars strongly depends on the pH change of sulfate solutions.     

Effect of concrete mixing parameters on propagation of ultrasonic waves

An experimental study was conducted to evaluate the effect of concrete aggregate gradation, water–cement ratio, and curing time on measured ultrasonic wave velocity (UPV). 30 × 30 × 10 cm Portland cement concrete slabs were cast for ultrasonic evaluation, while 10 cm diameter by 20 cm height cylinders were cast for compressive strength evaluation The slabs and cylinders were prepared using Portland cement and limestone aggregate. Two slabs were cast from each combination of coarse aggregate gradations and water cement ratio (0.40, 0.45, 0.50, and 0.55). Four ASTM gradations were considered, ASTM No: 8, 67, 56, and 4. These gradations have nominal maximum aggregate size 25, 4.75, 19.3, and 12.5 mm, respectively.The ultrasonic equipment used in this study was the portable ultrasonic non-destructive digital indicating tester (PUNDIT) with a generator having an amplitude of 500 V producing 54 kHz waves. The time needed to transfer the signal between the transducers was recorded and used to calculate the signal velocity, which was used as a parameter in the evaluation. Ultrasonic measurements were performed at 3, 7, 28, and 90 days after concrete casting.The results of the analysis indicated that water–cement ratio was found to have a significant effect on UPV. The UPV was found to decrease with the increase of water cement ratio. Aggregate gradation was also found to have significant effect on UPV. In general, the larger the aggregate size used in preparing Portland cement concrete, the higher the measured velocity of ultrasonic waves. Also, UPV was found to be increased as concrete curing time increased. Concrete compressive strength was found to be significantly affected by water–cement ratio and coarse aggregate gradation. Lower water–cement ratio produced higher concrete strength. Also, the concrete compressive strength increased as maximum aggregate size decreased.