Delayed ettringite formation (DEF) in mortars of white cement

In this study white cement CEM I-52.5 and white limestone cement CEM II-LL, A and B, with 15% and 25% limestone substitution, were studied. The way delayed ettringite forms, due to exposure to high temperatures (50 °C) and external sulphate attacks, was examined in the mortar samples.The mortars were immersed at 50 °C for 180 days in: (a) a saturated Ca(OH)₂ solution and (b) a 5% Na₂SO₄ solution. During the experiment’s duration, the mortar samples were being observed visually on a regular basis while their expansion was estimated on a weekly basis by measuring the change of length with a micrometer. At the end of the experiment, the mortar samples’ compressive strength was determined and the deterioration products were identified through means of X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM-EDAX), Thermogravimetry (TG) and Infra-Red Spectroscopy (FT-IR).Concluding it is evident that the amount of ettringite is proportional to the C₃A content of cement. Sulphates amount in cement is the controlling factor for heat induced ettringite formation since when they are consumed the reaction stops. On the other hand in the case of external sulphate attack another important controlling factor is the compressive strength of the cement; the higher compressive strength the lower the risk of expansion. Finally, in the case of external sulphate attack, limestone, when added to cement, was proved to enhance the durability against sulphates attack when compared to a cement of the same class.     

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.     

Thaumasite form of sulfate attack in limestone cement mortars: A study on long term efficiency of mineral admixtures

Concrete and mortar made from limestone cement may exhibit a lack of durability due to the formation of thaumasite. The addition of minerals that improve the concrete durability is expected to slow down the formation of thaumasite. In this work the effect of natural pozzolana, fly ash, ground granulated blastfurnace slag and metakaolin on the thaumasite formation in limestone cement mortar is examined. A limestone cement, containing 15% w/w limestone, was used. Mortar specimens were prepared by replacing a part of limestone cement with the above minerals. The specimens were immersed in a 1.8% MgSO₄ solution and cured at 5 and 25 °C. The status of the samples after a storage period of 5 years was reported based on visual inspection, compressive strength, mass measurements, ultrasonic pulse velocity measurements and analytical techniques. It is concluded that the use of specific minerals, as partial replacement of cement, inhibits thaumasite formation in limestone cement mortar.     

Microstructure changes in mortars attacked by sulphates at 5 °C

In this study mortars have been produced using ordinary Portland cement (CEM I – 32.5) and limestone cement with 15% limestone addition (CEM II/A-LL – 32.5). The mortars were immersed in a solution of 5% Na₂SO₄ at 5 °C for 6 months and the caused deterioration was observed visually at a regular basis. The deterioration product of the surface of both mortars has been identified as thaumasite by the means of XRD, FT-IR, DTA and SEM/EDAX analysis. The damage caused due to formation of thaumasite in both mortars was approximately the same and not influenced by the addition of limestone. Furthermore, expansion and compressive strength of the mortars were studied as a function of time and it was proved they were not influenced by thaumasite formation at the age of 6 months.     

Evaluation of sulfate resistance of Portland and high alumina cement mortars using hardness test

Portland cement and high alumina cement mortar specimens were exposed to cycles of drying at 40 °C, cooling at 20 °C and immersion in Na₂SO₄ and MgSO₄ solutions at 20 °C. The resistance of mortars was evaluated by visual inspection and by measuring the change in surface hardness and weight of the specimens. The decrease and increase in surface hardness were observed in both mortars by treating with Na₂SO₄ and MgSO₄ solutions, respectively. The combined effect of the chemical and physical attack by Na₂SO₄ was attributed to the complete failure of Portland cement mortar, whereas only marginal damage of high alumina cement mortar was believed owing to physical salt crystallization. No damage was observed in both mortars treated with MgSO₄ solution.     

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.     

Elastomeric influence of natural rubber latex on cement mortar at high temperatures using thermal degradation analysis

The importance of thermal endurance in relation to finishes prone to elevated temperatures cannot be over emphasized. Inclusion of elastomeric substances into mortar aimed at improving performance properties may therefore pose a serious threat. This paper presents experimental findings regarding elastomeric influence of natural rubber latex (NRL) – a typical elastomer – on cement mortar. Hardened cement paste, NRL-films, cement–latex blends, control and modified mortars containing 10% and 20% latex/water ratios were prepared and cured for 6 months. Microstructural units of samples were observed through SEM followed by subjection to TGA within a temperature range; 25–900 °C. The results indicate that NRL degrades to about 5% (by weight) at temperatures between 350 and 430 °C. Eventually, NRL-modified mortar was significantly affected by the softening of NRL-films present in the co-matrix. However, the overall resistance of the modified systems to thermal degradation was surprisingly improved by the inclusion of the elastomer.     

Concrete cover effect on reinforced concrete bars exposed to high temperatures

The mechanical properties of structural reinforcement steel have been investigated after the exposure to high temperatures. Plain steel, reinforcing steel bars embedded into mortar and plain mortar specimens were prepared and exposed to 20, 100, 200, 300, 500, 800 and 950 °C temperature for 3 h individually. The S420 deformed steel bars with diameters of ∅10, ∅16 and ∅20 were used. The mortar was prepared with CEM I 42.5 N cement and fly ash. The tension tests on reinforcements taken from cooled specimens were performed, and the variations in yield strength, ultimate strength and in resilience of three different dimensioned reinforcements were determined. A cover of 25 mm provides protection against high temperatures up to 400 °C. The high temperature exposed plain steel and the steel with 25-mm cover has the same characteristics when the reinforcing steel is exposed to a temperature 250 °C above the exposure temperature of plain steel.     

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.     

Effects of curing regimes and cement fineness on the compressive strength of ordinary Portland cement mortars

Curing techniques and curing duration have crucial effects on the strength and other mechanical properties of mortars. Proper curing can protect against moisture loss from fresh mixes. The objective of this experimental work is to examine the compressive strength of ordinary Portland cement mortars (OMs) under various curing regimes and cement fineness. Six different curing methods including water, air, water heated, oven heated, air–water, and water–air were applied to the specimens and also six groups of mortars were used. The results showed that the highest and lowest compressive strengths are attributed to the specimens of OPC mortar water cured using grounded OPC for duration of 6 h (OM–G6–wc) and OPC mortar air cured under room temperature with oven heated after demoulding of the specimens at 60 °C for duration of 20 h (OM–OH–ac), respectively. The maximum levels obtained of compressive strengths at 7, 28, and 90 days are 57.5, 70.3, and 76.0 MPa, respectively.     

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.     

Activation of Algerian slag in mortars

Blastfurnace slag has been widely used as a successful replacement material for Portland cement, and concrete of enhanced qualities can be achieved as a result. Due to the slag’s slow reactivity, however, the early-age mechanical properties may suffer. This paper reports the results of an investigation, carried out at Chlef University (Algeria), using Algerian slag, known to exhibit low reactivity due to its low CaO/SiO₂ ratio. The slag was activated mechanically by grinding the slag to 250, 360 and 420 m²/kg Blaine surface area, thermally by curing mortar specimens at 20°, 40° and 60 °C, and chemically by mixing the slag with two alkalis, NaOH and KOH at different concentrations. Samples were tested for compressive strength at the ages of 1, 3, 7, 28 and 90 days. All three methods enhanced the reactivity of the slag. The results indicated that the slag is very sensitive to temperature rise. Increase in fineness resulted in increased strength development and the fineness of the slag must be greater than that of the cement to achieve better performance. Alkali activation of slag results in increased strength development but the strength was lower than that of the control mortar.     

Mechanical properties of normal and high strength concretes subjected to high temperatures and using image analysis to detect bond deteriorations

The effect of high temperatures, up to 250 °C, on mechanical properties of normal and high strength concretes with and without silica fume was investigated, and image analysis was performed on split concrete surfaces to see the change in bond strength between aggregate and mortar. Specimens were heated up to elevated temperatures (50, 100, 150, 200, 250 °C) without loading and then the residual compressive and splitting tensile strength, as well as the static modulus of elasticity of the specimens were determined. For normal strength concrete residual mechanical properties started to decrease at 100 °C, while using silica fume reduced the losses at high temperatures. In terms of percent residual properties, high strength concrete specimens performed better than normal strength concrete specimens for all heating cycles. Image analysis studies on the split surfaces have been utilized to investigate the effect of high temperatures on the bond strength between aggregate and mortar. Image analysis results showed that reduced water–cement ratio and the use of silica fume improved the bond strength at room temperature, and created more stable bonding at elevated temperatures up to 250 °C.     

Effects of crushed glass cullet sizes,casting methods and pozzolanic materials on ASR of concrete blocks

The aim of this study is to investigate the influence of using different particle sizes of recycled glass, casting methods and pozzolanic materials in reducing the expansion due to alkali-silica reaction (ASR) of concrete blocks prepared with the use of crushed glass as fine aggregate. In this work, 25 × 25 × 285 mm mortar bar specimens were prepared using conventional wet-mixed and dry-mixed methods. Except for the control mortar bar, all the specimens were prepared by completely replacing river sand with different particle sizes of recycled glass. In addition, the influence of fly ash (PFA) and metakaolin (MK) content on the reduction of ASR expansion was also investigated. The flexural strength of the mortar bar specimens before and after they had been exposed to 1N NaOH solution was determined to complement the results of ASR expansion test. SEM was performed to examine the microstructure as well as nature of the cement binder-glass interfacial zone. The results reveal that ASR expansion reduced with reducing particle size of glass used. For the same given mix proportion, the dry-mixed method resulted in 44% less expansion when compared with the wet-mixed method. Both PFA and MK were demonstrated to be able to significantly reduce ASR expansion of the concrete glass blocks.     

Effects of calcination temperature of kaolinite clays on the properties of geopolymer cements

The aim of this work is to determine the most convenient calcination temperature of kaolinite clays in view of producing geopolymer cements. In this light, the clay fractions of three kaolin minerals were used. The clay fractions were characterized (chemical and thermal analyses and X-ray diffraction) and then calcined in the temperature range of 450 and 800 °C. The obtained amorphous materials were dissolved in a strongly alkaline solution in order to produce geopolymer cements whose pastes were characterized by determining their setting time, linear shrinkage and compressive strength. Hardened geopolymer cement paste samples were also submitted to X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy analyses. The setting time of geopolymer cement pastes produced from the clay fractions calcined at 450 °C was very long (test samples could be handled easily only after 21 days at the ambient atmosphere of the laboratory). For the clay fractions calcined between 500 and 700 °C, the setting time of geopolymer cement pastes reduced with increasing temperature and varied between 130 and 40 min. Above 700 °C, the setting time began to increase. The linear shrinkage of the hardened geopolymer cement paste samples aged between 21 and 28 days attained its lowest value around 700 °C. Above 700 °C, the linear shrinkage began to increase. The compressive strength of the hardened geopolymer cement paste samples was between 11.9 and 36.4 MPa: it increased with samples from the clay fractions calcined between 500 and 700 °C but dropped above 700 °C.It can be concluded that the most convenient temperature for the calcination of kaolinite clays in view of producing geopolymer cements is around 700 °C.     

Thermal expansion of self-consolidating normal and lightweight aggregate concrete at elevated temperature

In this study, the effects of aggregate type on the coefficient of thermal expansion of self-consolidating concrete produced with normal (SCC) and lightweight aggregate (SCLC) at elevated temperature were investigated. In experiments, two aggregate types, crushed limestone and pumice, were used. Different combinations of water/powder ratio and superplasticizer dosage levels were prepared for the SCC and SCLC mixtures. The total powder content (cement and mineral additives) was constant in the experiments. Thermal test was performed to accurately characterize the coefficient of thermal expansion (CTE) of SCC and SCLC aged 28 days using the dilatometer. The CTEs of SCC and SCLC were defined by measuring the linear change in length of concrete specimens subjected to a range of temperatures. Test temperatures were varied from 20 to 1000 °C at a heating rate of 5 °C/min. The results, in general, showed that SCC has higher CTE than normal weight concrete and that lightweight aggregate reduced the CTE of SCC due to their porous structure. The aggregate type has significant influence on the thermal expansion of SCC.     

Influence of calcined kaolin on mortar properties

The use of calcined clay, in the form of metakaolin (MK), as a pozzolanic material for mortar and concrete has received considerable attention in recent years. The present paper describes the results of a research project initiated to study the calcination of local kaolin at various temperatures (650–950 °C) and durations (2, 3 and 4 h) to produce MK with a high pozzolanic activity. The pozzolanic activity was assessed by 28-days compressive strength and hydration heat methods. The maximum identified activity was obtained at 850 °C for 3 h duration. An increase of both hydration heat and compressive strength was obtained when ordinary Portland cement was replaced by 10% MK. The use of ternary blended cement improves the early age and the long-term compressive strength. The durability was also enhanced as better acidic resistance was observed.     

Freeze–thaw resistance of blended cements containing calcined paper sludge

This work deals with the frost resistance of blended cements containing calcined paper sludge (source for metakaolin) as partial Portland cement replacements. Freeze–thaw tests were performed on blended cement mortars containing 0%, 10% and 20% waste paper sludge calcined at 650 °C for 2 h. Cement mortar specimens were exposed to freezing and thawing cycles until the relative dynamic modulus of elasticity fell below 60%. The performance of the cement mortars was assessed from measurements of weight, ultrasonic pulse velocity, compressive strength, mercury intrusion porosimetry and SEM. Failure of the control cement mortar occurred before 40 freeze/thaw cycles, while cement mortar containing 20% calcined paper sludge failed after 100 cycles. After 28 and 62 freezing and thawing cycles, cement blended with 10% and 20% calcined paper sludge exhibited a smaller reduction in compressive strength than the control cement.     

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.     

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.