Deterioration of mortar bars immersed in magnesium containing sulfate solutions

Mortars prepared with a CEM I and a CEM III/B binder were investigated in different magnesium sulfate solutions. The main deterioration mechanism for the CEM I was expansion, while surface erosion was dominant for CEM III/B. The presence of sodium, potassium and calcium in a magnesium sulfate solution led to less expansion and less surface deterioration for both, CEM I and CEM III/B, than which was observed in solutions containing only sodium or magnesium sulfate. The presence of a mixture of different cations seems to lower both the surface deterioration and the expansion and might explain why sulfate attack damages are not as frequent in the field as in laboratory tests. Sulfate binding before cracking/expansion is similar in the presence of all different solutions investigated, indicating that the speed of sulfate ingress and the amount of bound sulphate depends during the first months mainly on the binder.     

New procedure to investigate external sulphate attack on cementitious materials

In order to predict the durability of concrete structures in contact with sulphate rich environments, laboratory tests have to be sufficiently representative of the field conditions. This study is focused on the degradation of cement-based materials submitted to external sulphate attack. To be close to the natural degradation process, a new experimental procedure has been developed on the basis of two studies. The first was conducted on concrete and showed the deficiencies of common experimental programs only based on the measurement of the expansion. The second investigated the relation between the specimen size and the resistance to sulphate attack. As a result, the basic points of a representative procedure were identified. The mass and length of the specimens were monitored. The pH of the sodium sulphate solution was controlled, which allowed the assessment of leaching kinetics.This procedure was then validated by analysing, at macroscopic and microscopic scales, the influence of the water sulphate concentration on the behaviour of cementitious materials. The accelerating effect of high sulphate concentrations is due to the increase in leaching kinetics rather than the type of expansive products. Whatever the concentration, the same critical proportion of the cross section is required to generate global swelling.     

Resistance of different types of concretes to cyclic sulfuric acid and sodium sulfate attack

An improvement in accelerated testing as a way of predicting durability was proposed in this study. Accordingly, the behavior of different concrete mixtures was examined in relation to a cyclic exposure to sulfuric acid and sodium sulfate solutions, recording the expansion and mass loss of the test specimens for about 5 years. Three different cements – i.e. Portland limestone, blast furnace slag and pozzolanic cement – were used, the latter two both with and without silica fume (SF), to prepare the concretes for the study. Scanning Electron Microscopy (SEM) and energy-dispersive X-ray analysis (EDX) were used to correlate the samples’ microstructure and deformation.The lowest expansion was obtained by mixtures containing silica fume, although they were more susceptible to corrosion in acid. After a dormant period when no expansion occurred, the Portland limestone cement and blast furnace slag cement exhibited a large expansion that began suddenly and increased at an almost constant rate. This expansion correlated with the presence of cracks filled with calcium sulfate crystals in the core of the concrete samples.For comparison, the expansion of concretes specimens left in a sodium sulfate solution was also measured. The dormant period in the two-step expansion process seen in the Portland limestone and blast furnace concretes was shorter in the cyclic testing in sulfate and sulfuric acid, which can be considered as a model of accelerated deterioration, than in the latter.     

Concrete mixtures incorporating synthesized sulfonated acetophenone–formaldehyde resin as superplasticizer

Polymers in concrete have received considerable attention over the past 25 years. Water-soluble sulfonated acetophenone–formaldehyde (SAF) resin was produced in the laboratory from the reaction between acetophenone–formaldehyde, and sodium bisulfite. Its performance as a concrete admixture was evaluated through its effect on the (w/c) ratio, air content, setting time, compressive strength at different ages, water absorption and permeable pores. Also, the performance of the concrete when subjected to acidic environment using sulfuric acid (pH = 4), and sulphate attack using magnesium sulphate (pH = 6.5) were investigated. SAF resin could be classified as a high-range water reducer with retarding effect (Type F and G according to ASTM C494). It was found that concrete mixtures incorporating SAF resin-based admixture yielded higher compressive strength results compared with the control concrete mixtures, as well as they are more resistant to aggressive environments investigated due to the higher resistance to water movement.     

橡胶/混凝土盐冻循环后性能劣化及微观结构

制备普通混凝土(Normal concrete,NC)和橡胶/混凝土基体(Rubber/NC),研究盐冻循环60次内,表观现象、剥落量、抗压强度损失等性能指标劣化过程,采用超声波无损检测法评价混凝土盐冻循环破坏前后超声参数变化,建立相对波速、损伤度与抗压强度的关系,利用SEM观察盐冻循环损伤前后试件微结构变化。结果表明:随盐冻循环次数增加,混凝土试件表面剥蚀愈显著,剥落量增加,内部损伤、强度损失逐渐加剧,超声参数与抗压强度具有密切相关性;混凝土经历盐冻破坏后,内部结构呈疏松絮状,孔隙、裂纹愈加显现,密实度下降,造成宏观力学性能劣化。但弹性橡胶细集料掺入后有效缓解结冰压引起的内部开裂和孔隙扩大,各阶段橡胶/混凝土基体劣化程度均优于普通混凝土,以橡胶掺量 (与胶凝材料质量比) 10% (10%Rubber/NC)各性能指标最优,经历60次盐冻循环后,普通混凝土抗压强度损失率为58.5%,10%Rubber/NC抗压强度损失率为48.0%。      

Field performance of concrete exposed to sulphate and low pH conditions from natural and industrial sources

Laboratory investigations have been used to derive a high number of important details on sulphate attack on hardened concrete, but are not able to forecast the performance of this material under field conditions. In order to obtain reliable information on the long term behaviour of concrete, a survey is presented that looks at sulphate and sulphide containing environments in a particular region in mid-Europe. Twenty concrete structures have been sampled and analyzed.The classical idea of sulphate attack considering the migration of sulphate ions from ground or river water into concrete with subsequent phase transformation and damage has not been confirmed. This kind of exposure was found to be rare and no serious deterioration has been observed in connection with it. However, concrete is liable to be destroyed when in contact with sulphide bearing environments or if intimately mixed with gypsum. Disintegration and serious expansion requiring immediate repair has been observed. Information on all investigated structures are presented in this article and in more detail in a separate report.     

Sulfate attack and role of silica fume in resisting strength loss

This paper presents a detailed experimental study on the sulfate attack of Portland cement mortars, and the effectiveness of silica fume in controlling the damage arising from such attack. The test solutions used to supply the sulfate ions and cations were 5% sodium sulfate solution and 5% magnesium sulfate solution. Tap water was used as the reference solution. The main variables investigated in the study were the water/cementitious materials ratio, and the level of cement replacement. Compressive strength measured on 50 mm cubes was used to assess the changes in the mechanical properties of mortar specimens exposed to sulfate attack for 510 days. X-ray diffraction and differential scanning calorimetry were used to evaluate the microstructural nature of the sulfate attack. The test results showed that the presence of silica fume had a beneficial effect on the strength loss due to sodium sulfate attack. The best resistance to sodium sulfate attack was obtained with a SF replacement of 5–10%, but even then, a strength loss of 15–20% can be expected. On the other hand, mortars with silica fume were severely damaged in the magnesium sulfate environment. Further, the compressive strength loss actually increased with increasing SF content. The test results thus showed clearly that the use of SF in concrete exposed to magnesium sulfate solution is not recommended. The test results also showed that the w/cm ratio is the most critical parameter influencing the resistance of concrete to sulfate attack. All the tests reported in the study were carried out at 20 ± 1 °C.     

On the polymer modified cement mortars with different lightweight aggregates in marine environment

Having a light weight is a design objective for offshore and coastal structures and in order to reach this aim, use of lightweight concrete consisting lightweight aggregates and polymers is a feasible solution. In this study, materials obtained from locally available crushed volcanic scoria as main aggregate and some additional lightweight aggregates with ceramic, perlite and andesite as well polymers were used in the mixtures with different proportions (totally 14 mixtures) to propose feasible alternatives for coastal structure designers. To evaluate the design performances of the mixtures, their properties such as density, cylinder compressive strength, capillarity, loss of mass under the effects of the agents in marine environment such as sodium sulphate and sodium chloride were obtained experimentally. It was found that mixtures with scoria aggregates showed more favourable results and additional aggregates gave flexibility to the designer to lower densities up to 40 % ‐ 60 % of normal‐weight mixtures. Polymer modification also improved the performance of the related materials.     

Relative assessment of density and stability of foam produced with four synthetic surfactants

Selection of the surfactant has an impact on many of the foam properties as it affects the surface tension and gas–liquid interfacial properties. The objective is to produce stable aqueous foam of required density. These two characteristics are influenced by the type of surfactant, its concentration and foam generation pressure. This study compares the density and stability of foam produced using four synthetic surfactants namely sodium lauryl sulfate, sodium lauryl ether sulfate, sulfanol and cocodiethanolamide through a systematic experiment design based on response surface methodology. The relative performance has also been assessed in terms of their suitability for use in foamed concrete production based on ASTM test method. The effect of surfactant concentration has relatively lesser effect on foam density for sodium lauryl sulfate and sulfanol irrespective of foam generation pressure adopted. The drainage is proportional to the initial foam density for all the surfactant concentration for ionic surfactants at different foam generation pressures. For all the four surfactants under the optimum foam generation pressure, a stable foam with drainage less than 12% in 300 s (by considering economy as a factor) is achieved. From the foam stability test based on ASTM C 796-97, it is observed that all the four surfactants are suitable for use in foamed concrete production when optimized foam production parameters are adopted.     

Machining characteristics,wear and corrosion behavior of AZ91 magnesium alloy - fly ash composites produced by friction stir processing

In the present work, fine grained AZ91 magnesium alloy – fly ash composite has been successfully fabricated by friction stir processing. Microhardness measurements show marginally higher hardness with uniform distribution compared with the base material. No significant difference in the mean cutting force was observed during drilling of the base metal and the composite. However, lower cutting forces were recorded in the sub-surface region of the composites. Interestingly, decreased corrosion resistance was noticed for the composite compared with the base material. Lower mass loss has been observed for the composite during reciprocating wear experiments. The results strongly suggest that the surface composite of AZ91 magnesium alloy – fly ash exhibits better mechanical and wear properties. However, decreased corrosion resistance is a significant observation that warns the applicability of these composites in corroding environment.     

Resistance of self-consolidating concrete to ammonium sulphate attack

With the growing use of self-consolidating concrete (SCC) in various infrastructure applications, it has become necessary to establish reliable data on its long-term durability since its mixture design, rheology and consolidation are different than that of normal concrete. Several applications of SCC involve its exposure to chemical attack, particularly to sulphate-rich media such as wastewater treatment facilities, industrial and agricultural zones which also encompass ammonium cations. Thereby, this study aims at investigating the resistance of a wide scope of SCC mixture designs to ammonium sulphate attack. Specimens from 23 SCC mixtures were continuously immersed in a high concentration ammonium sulphate solution with controlled pH (6.0?C8.0) up to 54?weeks. The main test variables included the type of binder (single, binary, ternary and quaternary), air-entrainment, sand-to-total aggregates mass ratio, and the inclusion of fibre reinforcement (single and hybrid). The SCC specimens showed variable degrees of deterioration after 54?weeks, indicating different modes of degradation. The study highlights the role of ternary and quaternary binders in improving the resistance of SCC to ammonium sulphate attack. It is also emphasized that multiple performance indicators are needed to achieve a reliable assessment of cement-based materials under ammonium sulphate attack.     

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

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

Proportioning and characterization of lightweight concrete mixtures made with rigid polyurethane foam wastes

This paper presents the results of an experimental study concerning the incorporation of polyurethane (PUR) foam wastes into cementitious mixtures in order to produce lightweight concrete. A semi-empirical method is first proposed to predict the density of fresh PUR foam-based concrete mixtures. Seven concrete mixtures containing various PUR foam volume fractions (from 13.1% to 33.7%), and two reference concrete mixtures (without PUR foam) were prepared and characterized. In particular, their thermal and mechanical properties were determined. This permitted to quantify the influence of the PUR foam volume fraction on these parameters. Some specimens were maintained under water during 28 days, while the others were dried in air. The PUR-foam concrete thermal conductivity and compressive strength are, respectively, 2–7 times and 2–17 times lower than those of the reference mixture, depending on the volume fraction of PUR foam and on the curing conditions. Besides, the use of PUR foam in concrete implies a strong increase in the drying shrinkage and in the mass loss during the first seven days. These results can be related to the high porosity and the weak compressive strength of alveolar polyurethane.     

Thaumasite formation and deterioration in historic buildings

Thaumasite and ettringite can be found among the deterioration products of cementitious materials exposed to sulphate attack. This can occur in concrete structures, as well as in masonry walls of historic buildings erected before the advent of Portland cement. Masonry walls of historic buildings may contain gypsum or other sulphate salts for different reasons. When they need to be repaired, CSH and CAH, formed by the hydration of hydraulic binders used for restoration, can react with water and gypsum or sulphate salts and produce thaumasite and ettringite. Due to these reactions, degradations of the repaired historic buildings can occur causing expansion, cracking, spalling and strength loss. In order to assess preliminarily the chemical compatibility of a repairing cementitious material with the presence of gypsum or sulphate salts inside historic buildings, the Anstett test can be adopted. Alternatively, protection measurements, based on the hindrance of water to penetrate the walls, should be adopted since in the absence of water both thaumasite and ettringite cannot be produced, even in the presence of gypsum, or other sulphate salts inside the historic buildings.     

The Incompatibilities Between Chlorhexidine Diacetate and Some Tablet Excipients

The incompatibilities in a powder mixture for the manufacture of a chewable direct compression antiplague tablet preparate, were studied. The degree of loss of chlorhexi-dine diacetate in the presence of some common tablet exci-pients was studied under high relative humidity (80 % RH) and in water solution.

The most significant losses in water solution were observed with lubricants magnesium stearate and sodium lauryl sulphate. Under high relative humidity the greatest losses were, on the contrary, observed with the materials having a great water absorption capacity, such as sodium carboxy-methylcellulose and sodium starch glycolate.     

Density and strength characteristics of foamed gypsum

This paper deals with the effect of substances producing and assisting gas and foam formation on the properties of gypsum density and strength. The gas-based foaming method utilises aluminium sulphate, potassium alum or ammonium bicarbonate for the chemical production of gas bubbles in gypsum paste. The second method obtains foamed gypsum by means of air entrainment in wet gypsum paste. The foaming agents chosen were sodium lauryl sulphate and nonyl phenol ethoxylate that are widely employed in detergent production. Five different foaming techniques are obtained with these additives required for gas and foam production. Citric acid, as retarder, and carboxyl methyl cellulose (CMC), as viscosity increasing agent, were used for promoting foam and gas formation during the foaming experiments. Aluminium sulphate was discovered to be the one that achieves the most foaming in gypsum. The techniques utilising potassium alum, sodium lauryl sulphate and ammonium bicarbonate reduced the density of gypsum products by values varying between 30% and 35%. Foaming with nonyl phenol ethoxylate had an insignificant effect on density. The addition of citric acid and carboxyl methyl cellulose assisted with density reduction. Besides regulating the hardening time, citric acid supported density reduction by releasing gas through a reaction with marble powder. However, an addition of CMC above 0.1% or 0.2% causes a density increase by preventing bubbles to expand and merge with each other. The compressive strength of low-density gypsum products falls well below 10 Mpa, stipulated by TS 370 for building gypsum.     

Increased sulfate resistance of ggbs concrete in the presence of carbonate

Significant occurrence of the thaumasite form of sulfate attack (TSA) has only been identified in concrete containing carbonate aggregate, leading to a perception that the presence of calcium carbonate is undesirable in concrete exposed to sulfate attack. This paper reports on how the addition of a small percentage of calcium carbonate or calcium sulfate, affects the sulfate resistance of concrete containing ground granulated blastfurnace slag (ggbs). Three test methods were employed to assess sulfate resistance (all at 20 °C):

• Cubes were immersed in magnesium and sodium sulfate solutions and monitored for corner-loss and strength-loss, over six years.

• Mortar was sieved from fresh concrete and used to make prisms. These prisms were immersed in magnesium and sodium sulfate solutions and their expansions monitored for up to six years.

• In accordance with a draft of a European Standard for sulfate resistance, mortar prisms were prepared and monitored for expansion for one year.

• It was found that both calcium carbonate and calcium sulfate additions, had a consistent beneficial effect on the resistance of ggbs concrete to conventional sulfate attack, both in respect to expansion and in respect of disintegration. The paper discusses possible mechanisms for the improved resistance.

     

Influence of filler type on the properties of foam concrete

Most of the investigations on foam concrete in the past have been confined to neat cement paste, cement paste with partial replacement with admixtures and to cement–sand mixes. This paper reports the results of a systematic study to ascertain the influence of filler type (i.e., sand and fly ash) and the particle size of sand on the properties of moist cured foam concrete. This study shows that the consistency of mixture, for achieving pre-formed foam concrete of design density, mainly depends on the filler type. The flow behaviour of foam concrete is mainly influenced by the foam volume. A reduction in particle size of sand caused an improvement in strength of foam concrete. For a given density, replacement of sand with fly ash resulted in higher strength. Finer filler resulted in a higher ratio of strength to density.     

Thermal stability and fire behaviour of flame retardant high density rigid foams based on hydromagnesite-filled polypropylene composites

The present work deals with the development of new rigid polypropylene composite foams filled with high amounts of flame-retardant systems based on synthetic hydromagnesite, a basic magnesium carbonate obtained from an industrial by-product. A partially-interconnected cellular structure with a cell size around 100 μm was obtained for the hydromagnesite-filled PP foams. A 40% reduction of this cell size was observed when a small amount of a combination of montmorillonite and graphene layered nanoparticles was added to the hydromagnesite. The combination of hydromagnesite with an intumescent additive (ammonium polyphosphate) and layered nanoparticles led to improved thermal stability. In particular, the intumescent additive delayed the beginning of the thermal decomposition temperature and the layered nanoparticles split the second step of thermal decomposition in a third peak observed at higher temperatures. Improved flame retardancy, measured by means of cone calorimetry, was observed in the samples containing the intumescent additive. A novel normalized parameter, called foam efficiency ratio (FER), which takes into account the expansion ratio of the foam and the relation of its fire properties with that of the base solid, was also analyzed.     

The combined effect of chlorides and sulphates present in water and aggregates on properties of concrete

The combined effect of sulphates present in aggregates and high levels of chloride ions in the mixing water on some properties of concrete was studied. Different amounts of sulphates in sand (0.38, 0.50, 0.75 and 1.0% SO₃) with chloride contents in water (50, 750, 1000, 3000 and 6000 p.p.m. as Cl⁻) were used. The compressive strength was lower for concrete with higher sulphate contents at early ages. Small additions of sodium chloride to the mixing water decreased the compressive strength of concrete, particularly those with a low sulphate content.