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.     

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.     

How sulfates and increased temperature affect delayed ettringite formation (DEF) in white cement mortars

In this study,white cement CEM I and white limestone cement CEM II-LL A and Β with 15%, 25% and 35% limestone substitution were studied. The way delayed ettringite is forming due to exposure to increased temperature (50 °C) and external sulfate attack was examined in mortar samples which were immersed for 90 days in three different solutions: (a) saturated solution Ca(OH)₂ at 50 °C, (b) saturated solution of Ca(OH)₂ at 20 °C and (c) 5% w/w Na₂SO₄ solution at 50 °C. During this period mortar samples were visually observed regularly while their expansion was estimated on a weekly basis by measuring the change of length with a micrometer. At the end of the 90-days period the compressive strength of the mortars was determined and the deterioration products were identified through means of X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and thermogravimetric analysis (DTG). The results of this study show that DEF occurred in two forms. Samples cured at increased temperature contained DEF type I, which caused mediocre expansion and damage. Samples cured at increased temperature in the presence of sulfates produced DEF type II, which caused significant damage on the surface and exhibited high expansion.     

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.     

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.     

A study of the permeability and acid attack of corn cob ash blended cements

The durability of concrete made with corn cob ash (CCA) blended cement was investigated in this study. Permeability and chemical attack involving H₂SO₄ and HCl were the key parameters considered. Nine classes of CCA blended cements were employed with the CCA content ranging from 0% to 25%. The 0% CCA replacement involved the use of normal ordinary Portland cement and it served as the control. The water absorption of blended cement concrete was performed using 100 mm cube specimens of mix proportions 1:1½:3, 1:2:4 and 1:3:6 with 0.5, 0.6 and 0.7 water-to-binder ratios, respectively. The chemical attack test was carried out using 50 × 50 × 15 mm mortar specimens of mix proportions 1:1, 1:2 and 1:3 with water-to-binder ratio ranging between 0.26 and 0.29. The results indicated that the use of CCA blended cement reduces the water absorption of concrete specimens. Optimal reduction occurred at 10% CCA replacement for 1:1½:3 and 1:2:4 mix proportions and at 15% CCA replacement for 1:3:6 mix proportion. The resistance to chemical attack was improved as the addition of CCA up to 15% replacement level, caused a decrease in permeability and reduction in weight loss due to reaction of the specimens with HCl and H₂SO₄ acid water.     

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.     

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.     

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.     

Hydraulic binders of Fluorgypsum–Portland cement and blast furnace slag,stability and mechanical properties

The effect of various additives (Ca(OH)₂, K₂SO₄, Na₂SO₄, Al₂(SO₄)₃) was evaluated on the hydraulic character and stability of pastes of 50–75% Fluorgypsum, 15–30% Portland cement and 10–20% Blast furnace slag. Characterization included length changes, compressive strength, SEM, DTA and XRD. The combination of Na₂SO₄ and Al₂(SO₄)₃ favored early strength but caused detrimental expansion and strength losses after 90 days; whereas the use of only K₂SO₄ was favorable for strength and dimensional stability. The type of additive had a more important effect on stability and strength than the amounts of cement and slag. XRD indicated the presence of anhydrite, gypsum, ettringite, CaCO₃ and an unidentified phase, the interaction of these is proposed to explain the behavior of the cements investigated. SEM showed that cement and slag reacted forming C–S–H that enhanced the hydraulic character by engulfing the gypsum crystals.     

Properties of cement mortars containing clinoptilolite as a supplementary cementitious material

This paper presents a study of the properties and behavior of cement mortar with clinoptilolite which is one of the most common zeolite minerals found in nature. Six mortar mixtures were prepared by replacing the Portland cement with 0%, 5%, 10%, 15%, 20% and 30% clinoptilolite by weight. Test results showed that water demand, soundness and setting times of the cement pastes increased with the increase of clinoptilolite content. Compressive and flexural strength of the mortars containing clinoptilolite were higher than the control mixture. Dry unit weight of the mortars with clinoptilolite was lower than the control mortar. Clinoptilolite replacement decreased water absorption and porosity of the mortars. The control mortar showed less durability to carbonation compared to the mortars made with clinoptilolite at the end of carbonation tests. Freeze–thaw resistance of the mortars containing 5% clinoptilolite was higher than control mortar. The effect of clinoptilolite incorporation on high-temperature resistance seemed to be dependent on amount of clinoptilolite, temperature level, and the cooling method.     

Study on microstructure and mechanical property of interfacial transition zone between limestone aggregate and Sialite paste

The hydrating products as well as the microstructure at the interfacial transition zone (ITZ) between limestone aggregate and Sialite paste have been studied using XRD, SEM and EDAX methods. The inductive coupled plasma emission spectrometer (ICP) has been employed to investigate the solution phenomena of limestone by three kinds of different liquids. This paper reports the bonding strength of ITZ and the durability of Sialite concrete including water penetrability, chloride penetration, carbonated depth and freeze–thaw durability. It was found that Ca(OH)₂ crystals were not observed in the Sialite hydrating products at 28 days and at the ITZ. The interfacial bonding strength of Sialite was significantly stronger than that of ordinary Portland cement (OPC). The presence of rock-forming liquid comprising a variety of minerals was also effective in improving the interfacial bonding strengths between limestone aggregates and Sialite paste.     

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.     

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.     

Determination of chloride diffusivity through partially saturated Portland cement concrete by a simplified procedure

A recently proposed methodology for measuring the diffusion coefficient of chloride ions through concrete partially saturated with water, has been applied to five high-early-strength Portland cement mortars. The method is based on putting solid NaCl in contact with the concrete surface during the diffusion test. The results obtained indicate a strong dependence of the diffusion coefficient on the water saturation degree. The ionic diffusivities through these mortars are higher than those obtained with the same approach for five pozzolanic cement mortars of similar compositions. It has been also shown that the proposed test methodology and a different one based on the interaction of HCl(g) with the tested concrete surface, can yield comparable results. The D values obtained are of similar magnitude, and the same type of dependence on the water saturation degree is found through both methods, when applied to similar Portland cement concrete mixes with cement content of about 350 kg/m³ and water/cement ratios between 0.5 and 0.6.     

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.     

Effects of LOI of ground bagasse ash on the compressive strength and sulfate resistance of mortars

Raw bagasse ash collected from the Thai sugar industry has a high loss on ignition (LOI) of ～20%. When ground and ignited at 550 °C for 45 min, the LOI was reduced to ～5%. These high and low LOI of ground bagasse ashes were blended in the ratios of 1:2 and 2:1 by weight to give ground bagasse ashes with LOIs of 10% and 15%, respectively. Each of these ground bagasse ashes was used to replace Portland cement type I at 10%, 20%, 30%, and 40% by weight of binder to cast mortar.The results showed that the development of compressive strengths of mortars containing ground bagasse ash with high LOI was slower than that of mortar containing ground bagasse ash with low LOI. However, at the later age, both types of ground ash mortars displayed similar compressive strengths. Mortars containing high LOI (～20%) of ground bagasse ash at 20% and 30% by weight of binder could produce higher compressive strengths than a control mortar after 28 and 90 days, respectively. Mortar bars containing ground bagasse ash at 10% showed a greater potential sulfate resistance and displayed a reduce expansion compared to a control mortar. However, mortar bars containing high LOI (larger than 10%) of ground bagasse ashes showed greater deterioration from sulfate attack than the mortar bars containing low LOI (less than 10%) of ground bagasse ashes, especially at high replacement levels (30–40%).     

Durability of gamma irradiated polymer-impregnated blended cement pastes

This study is focusing on durability of the neat blended cement paste as well as those of the polymer-impregnated paste towards seawater and various concentrations of magnesium sulfate solutions up to 6 months of curing. The neat blended cement paste was prepared by a partial substitution of ordinary Portland cement with 5% of active rice husk ash (RHA). These samples were cured under tap water for 7 days. A similar paste was impregnated with unsaturated polyester resin (UPE) followed by gamma rays ranging from 10 to 50 kGy. The obtained data indicated that the polymer-impregnated specimens higher values of compressive strength than those of the neat blended cement paste. In addition, the polymer-impregnated blended cement specimens irradiated at a dose of 30 kGy and neat blended cement specimens were immersed in seawater and different concentrations of magnesium sulfate solutions namely, 1%, 3% and 5% up to 6 months. The results showed that the polymer-impregnated blended cement (OPC–RHA–UPE) paste irradiated at a dose of 30 kGy has a good resistance towards sulfate and seawater attack as compared to the neat blended cement (OPC–RHA) paste. These results were confirmed by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) studies.     

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.