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.     

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.     

Resistance of plain and blended cement mortars exposed to severe sulfate attacks

To emphasize the effect of physical sulfate attack on pozzolanic additions, the resistance of plain and blended cement mortars was investigated using 10% Na₂SO₄ and MgSO₄ solutions under four exposure regimes which included the standard conventional exposure and field-like exposures that created the physical sulfate attack. Although the performance of blended cement mortars was observed to be better under a conventional exposure regime, the damage in blended cement mortars was more severe under the exposure regimes that promoted the physical sulfate attack in a Na₂SO₄ environment. However, the physical attack by MgSO₄ was not apparent. Overall, MgSO₄ was found to be more damaging than Na₂SO₄ from the aspect of chemical attack; however Na₂SO₄ was more harmful than MgSO₄ as far as the physical attack is considered.     

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.     

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.     

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.     

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.     

Micro-analysis of the role of interfacial transition zone in “salt weathering” on concrete

When concrete elements are partially immersed in the sulfate environment, researchers always attribute “salt weathering”, “salt crystallization” or “physical attack” to the failure of concrete. However, there were few micro-analysis evidences to support this view. In this paper, an attempt was carried out to study whether salt weathering is really responsible for the concrete damage.As we know, the interfacial transition zone (ITZ) between paste and aggregate plays a determining role in the performance of concrete. In this paper, we focused on the role of ITZ in “salt weathering” on concrete. Concrete specimens, made with coarse aggregate and cement paste, were partially exposed to a 5% sodium sulfate solution and a 5% magnesium sulfate solution respectively, in a controlled environment (20 ± 2 °C, and 60 ± 5% RH). After 8 months of exposure, a micro-analysis is performed by means of XRD, ESEM and EDS. The experimental results showed that, in the upper part of concrete above the Na₂SO₄ solution, damage initiated in the ITZ between paste and aggregate due to the formation of ettringite and gypsum. Salt crystallization cannot occur on the paste surface in the ITZ, but it was found on the aggregate surface after damage initiation due to chemical sulfate attack. On the other hand, salt crystallization could occur in the carbonated concrete. There was no trace of salt crystallization in the concrete partially exposed to MgSO₄ solution.     

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 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%).     

Formulations of sulfobelite cement through design of experiments

This paper reports on the use of design of experiments for the formulation of sulfobelite clinkers. Samples made of 30–70 wt.%C₂S, 20–60 wt.%C₄A₃? and 10–25 wt.%C₄AF were formulated. The red mud generated from the Bayer process was used in some formulations as source of iron and aluminum. X-ray diffraction was used to define the high and low limits of phase compositions. Estimations of CO₂ emissions were also conducted. Derived cements (5 wt.% gypsum) were cured, and the temperature of hydration, compressive strength and density were determined. The Rietveld refinement showed that the percentages of phases in the clinkers are close to the expected ones. The presence of C₄A₃? is crucial to improve the mechanical strength at early ages. The use of red mud leads to the formation of C₃A and then the derived cements show faster hydration. In addition, C₂S is the major responsible for the generation of CO₂ emissions.     

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.     

Deterioration of dolostone by magnesium sulphate salt: An example of incompatible building materials at Bonaval Monastery,Spain

Since its abandonment 185 years ago, the XII century Santa Maria de Bonaval Monastery located in Guadalajara (Spain) has suffered significant deterioration: first the roof was lost, followed by partial collapse of the walls, moisture infiltration and extensive loss of stone surfaces due to salt weathering. This case study is a clear example of the incompatibility of some building materials: in this case, the combination of sulphate-bearing mortars and magnesium-rich stone and mortars leading to extensive weathering by magnesium sulphate crystallization. Samples of plaster, bedding and core mortars, stone fragments and flakes, salt crust and powders were collected, as well stone samples from the historic quarries located close to the Monastery. Characterization by XRD (X-ray diffraction), ESEM-EDS (environmental scanning electron microscopy with energy dispersive X-ray spectroscopy) shows that the most important stone-type used in the structure, dolostone, is mainly affected by magnesium sulphate salts (epsomite, MgSO₄ · 7H₂O), although other salts as kalicinite (KHCO₃) and mercallite (KHSO₄) were also detected. The connected porosity and pore size distribution determined by mercury intrusion porosimetry and capillarity behaviour suggest that the core mortar could easily be dissolved and the stone, plaster and bedding mortars are able to transport infiltrating solutions, giving rise to the precipitation of magnesium sulphate in the mortar joints and over the surface of the stone. Due to their chemical incompatibility, the combination of sulphate and magnesium-bearing mortars and stone with high magnesium content appears to be problematic and should be avoided in future restoration work.     

Mechanical and microstructural characterization of an alkali-activated slag/limestone fine aggregate concrete

Four limestone-based, alkali-activated slag fine aggregate concretes, two of which contained amorphous silica in the form of diatomaceous earth, were fabricated using different activating solutions (NaOH/waterglass or Na₂CO₃). Emphasis in this work was placed on using simple manufacturing methods and widely available materials, to ensure that these formulae are practical as construction materials in the developing world. Although cured only at room temperature, these fine aggregate concretes have good compressive strengths (～45 MPa) and their tensile strengths increased from ～2.6 MPa after 1 day of curing to ～4 MPa after 28 day for the NaOH-activated formulae. Samples activated with Na₂CO₃ had negligible tensile strengths after 1 day, increasing to ～2.5 MPa after 28 day. The main cementing phase was shown to be calcium–silicate–hydrates in all formulae; those activated with Na₂CO₃ also showed the presence of hydrotalcite. No evidence of geopolymeric phases was found, though incorporation of Na to form N–S–H that balance charges arising from Al substitution of Si in C–S–H is likely. Despite the short (～120 s) pot life of the strongest formula, NaCl was shown to be an effective retarding agent, which reduced the strengths of different formulae, at worst, by less than 25% after 28 day of curing.     

Use of palm oil fuel ash as a supplementary cementitious material for producing high-strength concrete

The objective of this study is to investigate the use of ground palm oil fuel ash with high fineness (GPA) as a pozzolanic material to produce high-strength concrete. Samples were made by replacing Type I Portland cement with various proportions of GPA. Properties such as the compressive strength, drying shrinkage, water permeability, and sulfate resistance, were then investigated. After aging for 28 days, the compressive strengths of these concrete samples were found to be in the range of 59.5–64.3 MPa. At 90-day the compressive strength of concrete containing GPA 20% was as high as 70 MPa. The drying shrinkage and water permeability were lower than those of high-strength concrete made from Type I Portland cement. When the concrete samples were immersed in a 10% MgSO₄ solution for 180 days, the sulfate resistance in terms of the expansion and loss of compressive strength was improved. The results indicated that GPA is a reactive pozzolanic material and can be used as a supplementary cementitious material for producing high-strength concrete.     

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.     

Use of mineral admixtures for enhanced resistance against sulfate attack

The effect of mineral admixtures such as nano silica, micro silica, fly ash and ground granulated blast furnace slag on the expansion of mortar bars caused by internal and external sulfate attack was investigated. According to data recorded through 12 months, all of the mineral additives, particularly slag, have significantly reduced the expansions caused by sulfate attack. It was shown that, the effectiveness of nano silica was also very significant. When used with sulfate contaminated sands, high sulfate resistant mixtures were produced with 4–6% nano silica replacement ratios. In the case of external sulfate attack, however, only 2% nano silica is most likely enough for keeping the expansions below 0.03% after 12 months.     

The assessment of porosity and pore size distribution of limestone Portland cement pastes

The cement production industry is one of the most energy and raw materials consuming one. Over the last years a great effort is performed in order to substitute clinker for less energy demanding materials. Nevertheless, construction industry needs durable materials with improved properties. Limestone is being used in blended cements widely. The most important parameter that affects all the properties of cement paste is its pore structure. In this study, four different limestone cements were produced and their pore structure was determined by means of mercury intrusion porosimetry (MIP) and nuclear magnetic resonance (NMR) cryoporometry. The conclusion of the study was that limestone addition affects the pore structure of the cement paste by increasing linearly the size of capillary pores from 20 nm to 40 nm when the maximum amount (35%) of limestone that is allowed by EN 197-1 is used. On the other hand the threshold diameter decreases exponentially and it is evident that limestone hardened cement pastes have many pores of the same size due to the filling effect that minerals additives have. Furthermore, limestone decreases the size of gel pores which is related to higher hydration rates. Hence, the use of limestone in cement produces a material that is structurally adequate to be used in construction.     

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.     

Study on early-age cracking of cement-based materials with superplasticizers

The addition of superplasticizers is an important approach to prepare high performance cement-based materials. The effect of polynaphthalene series superplasticizer (PNS) and polycarboxylate type superplasticizer (PC) on early-age cracking and volume stability of cement-based materials was investigated by means of multi-channel ellipse ring shrinkage cracking test, free shrinkage and strength test. The general effect of PNS and PC is to increase initial cracking time of mortars, and decrease cracking sensitivity of mortars. As for decreasing cracking sensitivity of mortars, PC > H-UNF (high-thickness-type PNS) > C-UNF (common-thickness-type PNS). To incorporate superplasticizers is apparently to increases free shrinkage of mortars when keeping the constant W/B ratio and the content of cement pastes. As for the effect of controlling volume stability of mortars, PC > C-UNF > H-UNF. Maximum crack width of mortars with PC is lower, but the development rate of maximum crack width of mortars with H-UNF is faster in comparison with control mortars. Flexural and compressive strength of mortars and concretes at 28 days increased with increasing superplasticizer dosages under drying conditions. C-UNF was approximate to H-UNF, but PC was superior to PNS in the aspect of increasing strength of cement-based materials.