Strength Development Of Mortars Research Articles

Influence of geometric shape, pore structure and surface modification of recycled fine aggregate on the rheology behaviour and strength development of mortar

The complex physical properties of recycled fine aggregate affect the rheological and mechanical properties of mortar, thereby limiting its application in cement-based materials. This paper evaluated the geometric shape and pore structure of natural fine aggregate (NFA), recycled fine aggregate (RFA), and recycled fine aggregate modified by water glass (MRFA), and studied its water absorption and bulk density. The influence of complex physical properties of RFA before and after modification on the rheological behavior, strength development, and microscopic properties of mortar was analyzed. The rheological tests were carried out on the fresh mortar with 7 mixes, and the compressive strength and flexural strength of hardened mortar after curing for 3, 7 and 28 days were tested. The results shown that the modification of water glass has improved the water absorption and geometric shape of RFA to some extent, resulting in a small increase in the apparent viscosity and yield stress of recycled mortar compared to NFA mortar. When the replacement level of RFA was less than 30 %, the compressive strength of recycled mortar was 13 % higher than that of NFA mortar. As the replacement level increased to 40 %, the incorporation of water glass also helped to improve the mortar strength. The irregular geometric shape and high water absorption of RFA were conducive to achieving dense mechanical bonding and reducing the Ca/Si ratio in the interface transition zone.

Effect of Embedded Depth of Copper-Nickel-Plated Sensor Probes on Compressive Strength Development of Mortar

Embedded sensors are widely employed for the structural health monitoring of structures constructed with concrete or mortar. Despite embedded sensors being actively used, there has been no study on whether or not the sensor probe placement within structures made of concrete or mortar influences their structural stability. The strength of small structures in particular could be affected by sensor probes embedded within them. To address the lack of research in this area, this study analyzed the effect of embedding positions of sensor probes on the compressive strength development of mortar. After the production of mortar specimens with the depth of the embedded sensor being controlled by the developed mold, compressive strength tests were conducted, and then test results were verified through finite element analysis. For testing, copper–nickel-plated sensor probes were embedded within the mortar because these sensor probes are popular commercial probes. The test results show that the compressive strength was 7.1 MPa when the sensor probe was embedded at a depth of 5 mm. In contrast, the compressive strength was 28.2 MPa at a depth of 30 mm. Since the compressive strength without the embedded sensor probe was 29.8 MPa, considering the results of this study, it is highly recommended that copper–nickel-plated sensor probes be embedded at least 30 mm from the surface of mortar structures.

Influence of main components of accelerators on mechanical property and hydration of Portland cement in a dry-hot geothermal environment

The behavior of accelerators in the shotcrete of tunnel structure is significantly affected by the high geothermal tunnel environment. The objective of this study is to investigate the influence of main components of alkali-free accelerators (aluminum sulfate, AS) and alkaline accelerators (sodium aluminate, NA) on mechanical property and hydration of Portland cement in a dry-hot geothermal environment of 80 ℃ (DH). Setting time test, compressive strength test, XRD, TGA, MIP and SEM were performed on cement systems with AS and NA. Results show that relative to standard curing environment, the cement hydration was promoted and the pore structure at early stage was optimized in DH, thereby significantly accelerating the setting time of cement pastes and improving the early compressive strength development of mortars. In contrast, the later cement hydration was retarded and pore structure was detrimental in DH, resulting in abnormally slow development of later strength and even reduction. Additionally, the stability of hydration products such as ettringite (AFt) and monosulfoaluminate hydrate (AFm) would decline in DH. DH could accelerate the dehydration and decomposition of AFt, and promote the transformation process from AFt to AFm at early stage. After 28 days, AFm would convert into hemicarboaluminate due to carbonation. The addition of AS elevated the hydration degree and optimized the pore structure, especially at later stage, thus exhibiting more stability in long-term strength. Nevertheless, the incorporation of NA would bring about serious deterioration phenomenon in DH, and the compressive strength almost stagnated after 1 day. This work provides a guidance for the real application of accelerators in the shotcrete of tunnel structure in a dry-hot geothermal environment.

Alkali-activated slag & fly ash as sustainable alternatives to OPC: Sorptivity and strength development characteristics of mortar

One approach to decreasing the environmental footprint of the construction industry is to replace ordinary Portland cement (OPC) with recycled slag and fly ash activated using alkaline materials. This article presents the outcomes of an experimental study that evaluated water absorption characteristics and strength development of mortar that uses blends of alkali-activated ASTM class F slag and fly ash binders. Mortar mixes were developed using three binder combinations: 100% slag and no fly ash (S1F0), 75% slag + 25% fly ash (S3F1), and 50% slag + 50% fly ash (S1F1). Slag and fly ash binders were activated using NaOH solution with molarity ranging from 10 mol/L to 16 mol/L mixed with sodium silicate solution. Two sets of samples were created, one set was immersed in an acidic medium after casting, which represented a closed environment, and the second was left in the lab exposed to air until the test day. For mortar cured in a closed system, the highest strength development during the first seven days after casting occurred in S1F0 mortar followed by S3F1, then S1F1. The observation indicates strength development is dominated by the fast reactivity of slag during the first 7 days. The pattern is similar for all NaOH activator concentrations evaluated in this study. Mortar samples with high slag content (S1F0 and S3F1) that were cured in air experienced a decrease in strength during the 28–90 day curing phase as the strength at the age of 90 days decreased relative to the strength at 28 days. However, S1F1 gained strength during the 28–90 day phase when NaOH molarity was the highest (16 mol/L). This is due to the dissolution and activation of fly ash at high solution alkalinity which contributed to the continued increase in strength. A strong linear correlation between the square root of time (t0.5) and water absorption existed in all activated mortars evaluated in the study, except for mortar samples prepared using NaOH with high concentration and the highest ratio of Na2SiO3/NaOH. Therefore, alkali-activated slag-based mortars share similar water capillary absorption characteristics with conventional cement-based mortar. It was found that increasing the NaOH concentration increases the global warming potential (gwp) and that the mix with the least environmental impact was the S1F1 developed using the lowest NaOH concentration of 10 mol/L and 50% fly ash.

Water absorption characteristics and rate of strength development of mortar with slag-based alkali-activated binder and 25% fly ash replacement

This article presents some of the results of an experimental study that evaluated water absorption characteristics and rate of strength development of geopolymer mortar where the binder consists of alkali-activated slag or slag with 25% fly ash replacement. Samples were cured in air at 22 ± 2 °C and 70% RH. From 1 to 7 days of curing, the increase in compressive strength was higher in mortar with 75%slag + 25% fly ash (S75F25) compared to samples with 100 slag binder (S100). After one day, fly ash begins to develop more N-A-S-H gel and contributes noticeably to strength development. Furthermore, the percentage change in strength development decreased continuously from 7 to 28 days of curing and became negative from 28 to 90 days. Ultimately, the 90-day strength decreased below the 28-day strength in all geopolymer samples. The decrease in strength may be due to loss of moisture causing precipitation of reactants rather than the formation of polymerization products. It was found that a strong linear relationship exists between water absorption of geopolymer mortar samples and the square root of immersion time (t0.5). Water absorption in S100 mortars decreased with an increase in the alkaline activator ratio of Na2SiO3/NaOH. This is because the free silicates from Na2SiO3 react with calcium dissolving from GBS and develop calcium-based hydrate C-(A)-S-H gel, which is typically dense and compact. When S100 mortar samples were cured for 90 days, the relationship between water absorption and t0.5 deviated from linearity, which indicates a coarser pore system and/or cracking, and is consistent with an observed decrease in compressive strength. Water absorption of S75F25 mortar exhibits a stronger correlation with t0.5 and a lower initial rate of water absorption, compared to S100, indicating a finer pore system and lower water absorption. Compressive strength at NaOH molarities of 10 mol/L was higher for S75F25 compared to S100. There is a strong correlation between the initial rate of water absorption and the compressive strength of geopolymer mortar.

Effects of the Curing Regime, Acid Exposure, Alkaline Activator Dosage, and Precursor Content on the Strength Development of Mortar with Alkali-Activated Slag and Fly Ash Binder: A Critical Review

Reductions of green gas emissions and the reuse/recycling of industrial byproducts are important for the mitigation of the environmental impact of the construction industry. The replacement of ordinary Portland cement (OPC) is a concrete binder with industrial byproducts that possess sufficient cementitious and pozzolanic properties, such as ground granulated blast furnace slag (GBS) and fly ash. This critical review analyzes the effect of some of the most critical parameters on the development of the compressive strength of concrete or mortar that consists of combinations of alkali-activated GBS and fly ash as binders. The review includes the effects of the curing environment, the proportions of GBS and fly ash in the binder, and the concentration of the alkaline activator on strength development. The article also reviews the effect of exposure as well as the age of samples at the time of exposure to acidic media on the development of concrete strength. The effect of acidic media on mechanical properties was found to depend not only on the type of acid but also on the alkaline activator solution, proportions of GBS and fly ash in the binder, and the age of the sample at the time of exposure, among other factors. As a focused review, the article pinpoints important findings such as the change in compressive strength over time when mortar/concrete is cured in an environment that permits the loss of moisture versus curing in a system that retains the alkaline solution and keeps reactants available for hydration and the development of geopolymerization products. The relative contents of slag and fly ash in blended activators have a significant impact on strength development. Research methods used include a critical review of the literature, a comparison of reported research findings, and identifying reasons for agreement or disagreement of findings.

Engineering properties and stability of high-performance mortar incorporating untreated and treated steel reducing slag aggregate

The purpose of this study was to investigate the effect of steel reducing slag aggregate (SRSA) on the mechanical and durability properties of high-performance mortar specimens. Two series of mortar specimens were produced for this study using treated and untreated SRSA. Each type of SRSA was used as a partial-to-complete replacement for natural fine aggregate at the following ratios: 25%, 50%, 75%, and 100%. The water-to-binder ratio (w/b) in all of the specimens was designed at 0.35. The mechanical properties and durability of the specimens were observed using slump flow, fresh unit weight, compressive strength, water absorption and porosity, ultrasonic pulse velocity (UPV), electrical surface resistivity (ESR), thermal conductivity (TC), sulfate attack resistance, and expansion rate. Scanning electron microscopy (SEM) was conducted for the microstructure analysis. The findings indicate that increasing the SRSA content is negatively correlated with mortar strength development and durability, with higher SRSA content reduced mortar compressive strength and durability performance comparing with reference mixture. However, most of the SRSA mortar mixtures exhibited good strength and excellent durability, with high UPV (≥3660 m/s) and ESR (≥20KΩ-cm) values. Notably, compared to the mortar specimens made using untreated SRSA, those made using treated SRSA exhibited better performance and better volume stability.

Utilization of Different Forms of Demolished Clay Brick and Granite Wastes for Better Performance in Cement Composites

Clay brick and granite waste are part of the waste generated by construction and demolition activities. The amount of these wastes generated is enormous, but on the one hand, they can be used as a raw material for cement mixtures; thus, it is important to find ways to utilize them efficiently. In this study, clay brick and granite waste were crushed and screened into two size fractions (0.15–2.36 mm for sand replacement and smaller than 0.15 mm for cement replacement), and a total of four different forms of recycled materials were obtained (recycled brick aggregate, recycled brick powder, recycled granite aggregate and recycled granite powder) and used in cement mortar. Various properties (workability, mechanical strength and drying shrinkage) of the mortars were assessed according to standardized test methods. The results showed that the various material forms had different effects on the various properties of cement mortar. At replacement ratios of 10% and 20%, recycled granite showed better workability when used as powder, whereas recycled brick used as aggregate had higher workability. In common, using recycled brick and recycled granite in the form of aggregate was advantageous for the strength development of mortar, while using them in the form of powder helped to mitigate drying shrinkage.

Evaluating the strength development of mortar using clinker fine aggregate with a combination of fly ash and its inhibitory effects on alkali–silica reaction and delayed ettringite formation

The aggregate composed of cement clinker, an intermediate cement product, improves strength development and mass transfer resistance of concrete. Fly ash (FA) is a supplementary cementitious material that can be substituted by cement. This study investigated the strength development of highly FA-substituted mortar mixed with cement clinker fine aggregate (CL) and tested its inhibitory effect against the alkali–silica reaction (ASR). In addition to these, this study provides the testing results of evaluating the effects of using cement clinker fine aggregate on delayed ettringite formation (DEF), which can be problematic for precast concrete products. The study results revealed that at 91 days of age, in the case of 80% replacement ratio of fly ash to cement, using CL exhibited similar strength development as mortar with limestone fine aggregate and no FA substitution. Furthermore, mortars with 70% and 80% FA substitution did not exhibit clear ASR-induced expansion even at 182 days of age. Lastly, mortar using clinker fine aggregate could suppress DEF-induced expansion at 182 days of age, which was similar to the effect obtained using FA. These results can promote the utilization of CL resources for concrete and using clinker as fine aggregate in precast concrete products.

INFLUENCE OF HIGH-TEMPERATURE CURING IN EARLY AGES ON COMPRESSIVE STRENGTH AND POZZOLANIC REACTION OF MORTAR MIXED WITH FINE MILLED FLY ASH

Compressive strength and pozzolanic reaction of specimens mixed with fine milled fly ash were examined. Mortar specimens mixed with three types of fine milled fly ash were cured under high-temperature condition in early ages. It was clarified that smaller particle size of the fly ash improved the early strength development of mortar. Moreover, both of finely milling fly ash particle and high-temperature curing in early ages remarkably accelerated the pozzolanic reaction of fly ash. The synergistic effect provided the acceleration of the strength development of mortar.

Synthesis of C-S-H seeds using the waste solution of acid treatment of recycled concrete aggregates and black liquor

• Finer and well-dispersed C-S-H seeds was prepared using the two waste solutions. • Lignin in black liquor kept C-S-H seeds globular rather than traditional foil-like. • C-S-H seeds from the two waste solutions had a more prominent accelerating effect. For preparing finer and well-dispersed calcium-silicate-hydrate (C-S-H) seeds at a lower cost, the waste solution of acid treatment of recycled concrete aggregates (RCAs) and black liquor from pulping were used for the synthesis of C-S-H seeds. The results show that C-S-H seeds can be successfully fabricated by chemical co-precipitation reaction using the two waste solutions. Interestingly, it was globular rather than traditional foil-like. Furthermore, C-S-H seeds synthesized using the two waste solutions had smaller particle size and better dispersibility. More importantly, it had a more prominent accelerating effect to the strength development of mortar, significantly enhancing the early and long-term compressive strength of mortar.

Blended Portland cement with high limestone loads modified with a waste glass based sodium silicate of different ratios SiO2/Na2O

This paper presents the results of one-part alternative low CO2 cements made of 60%limestone-30% portland cement ((LS-PC)m) and modified with 10% of a waste glass-based sodium silicate (SWG) synthesized by thermochemically processing blends of waste glass-NaOH of 3 different SiO2/Na2O ratios. The added alkali (from NaOH) contents in the (LS-PC)m was kept low, leading to under 2% Na2Oeq. The reaction products were characterized in pastes and the strength development in mortars. The addition of SWG and its SiO2/Na2O ratio had significative effects in the setting time, heat released, flowability, early and late mechanical strength, as well as in the reaction products formed. Lower alkali contents in the SWG did not drastically affect the flowability and setting time, also, the early strength was enhanced without impairing the late strength; in contrast, the opposite occurred for SWG with high alkali contents. The mortars reached acceptable 28-day strengths and similar 180-day strength as a reference Portland cement mortar. The results suggest that the SWG promoted the early formation of carboaluminates and C-S-H of different compositions, evidenced by the signals of six-fold coordinated Al and Q1, Q2(1Al) and Q2 sites, respectively, in the 27Al and 29Si RMN spectra. The studied binders could be a promising alternative, along with other low carbon binders, to contribute to reduce CO2 emissions.

Effects of accelerating and retarding agents on nucleation and crystal growth of calcium aluminate cement

Although calcium aluminate cement (CAC) has been widely used, one of the critical unclear issues in the field of cement chemistry focuses on certain mechanisms that manipulate the rate of cement hydration. A better understanding of hydration provides critical information such that issues, such as setting time, strength gain, and durability, can be better addressed. This study aimed to initially provide theoretical predictions using experimental information on the dissolution, nucleation, and precipitation mechanisms of ions, induction time (tind), interfacial free energy (γ), and the crystal growth mechanism of CAC containing accelerating and retarding agents in microstructural systems. We also aimed to investigate the early-age performance of CAC, including the setting time of pastes and strength development of mortars. Results found that adding accelerating and retarding agents affected the dissolution, nucleation, and precipitation mechanisms of the CAC hydration. This finally affected the phase formation of the final products.

Mechanical and durability properties of C–S–H-seeded cement mortar cured at fluctuating low temperatures with granulated blast furnace slag as fine aggregates

The conservation of natural resources, efficient use of industrial side-streams, and reduction of environmental impacts are the main targets of the construction sector worldwide. However, the low ambient temperatures and long harsh winter seasons in northern regions limit the use of industrial side-streams in construction activities under cold weather conditions due to their slow strength development rate. This study aimed to develop an eco-friendly construction material suitable for construction under cold weather conditions using blast furnace slag as a binder and fine aggregate admixed with a calcium silicate hydrate seed accelerator in mortar. Natural sand (NS) was volumetrically substituted with 25%, 50%, 75%, and 100% of granulated blast furnace slag (GBFS) fine aggregate in mortar cured at fluctuating low/freezing temperatures (+5 to −5 °C), representing the late fall and early spring seasons in northern regions. The mortar's compressive strength increased with the incorporation of GBFS aggregate over the first three days of curing and decreased thereafter. A denser interfacial transition zone was captured around GBFS aggregates than NS in the three days old mortars. The deceleration influences of fluctuating low/freezing temperatures on the compressive strength development of mortars were diminished with time. Capillary water absorption increased with higher GBFS aggregate contents. Mortar with 25–50 vol.% GBFS aggregates exhibited greater frost resistance than control mortar with NS only. The incorporation of GBFS aggregate enhanced the mortar's resistance against a sulfuric acid attack. This study demonstrates the potential of GBFS aggregate for use in construction under cold weather conditions.

The role of Pre-Heating and mineral additive modification on Long-Term strength development of calcium aluminate cement mortars

Strength development of Calcium Aluminate Cement (CAC) under various environmental conditions is an interesting subject of topic that pay attention of researchers for a long time. This special cement usually preferred in applications where quick setting ability, high early strength and resistance to chemical attack is required. However, uncertainty in the hydration process and long-term strength problems, due to conversion reactions in combination with high initial material cost, limit the application of this cement.In order to propose a solution to the above-mentioned problems, blended mortar mixtures were prepared by replacing CAC with variable amounts of microsilica, anhydrite CaSO4 and Portland cement within the scope of this study. The compressive strength development of mortar specimens up to 400 days were investigated under semi-dry curing conditions at 20 ± 5 °C and 60 ± 20% RH. During the initial stages of hydration, a distinct pre-heating based curing procedure was applied to the conjugate specimens. Preliminary stabilization of the unstable hydration products is targeted by this pre-heating method. Accordingly, the crystal structure of the hydration products of specimens was analyzed with the support of microstructural studies (XRD, DTA-TGA, SEM-EDS). The pore size distribution and sub-capillary porosity of specimens were also determined by mercury intrusion porosimetry. The effectiveness of the mineral modification and pre-heating based curing method (exposure to 50 ± 1 °C heating for 21 days) on the long-term strength development of CAC is discussed with the aid of microstructural analysis results. The results showed that it is possible to stabilize the hydration process of CAC and its strength becomes predictable in the long term, if appropriate mineral additives in proper replacement ratios are employed in the mixture design.

Contribution of fly ash to mortar strength development under steam and internal curing

The purpose of this study is to quantitatively evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar by using cementing efficiency factor (k-value) that represents strength development performance as a binder of fly ash. In addition, the pozzolanic reaction of fly ash was evaluated from the viewpoint of calcium hydroxide consumption by using thermogravimetry and differential thermal analysis as well as the degree of fly ash reaction by using selective dissolution method. The result indicated that steam curing improved early compressive strength and internal curing improved compressive strength and k-value at all ages. Also, a linearrelationship between the degree of fly ash reaction and the k-value was shown regardless of the age and the replacement ratio of fly ash.

Clinkering design of sulfobelite cements using clay overburden residue from bauxite mining

Sulfobelite (belite-rich calcium sulfoaluminate (BCSA)) cements are promising alternatives to ordinary Portland cement (OPC) due to their lower carbon dioxide generation and lower energy consumption in the clinkering process. Additionally, they are cost effective because of their less-restrictive raw meal composition compared with that of calcium sulfoaluminate (CSA), in which expensive bauxites are used as the alumina source. Diverse mining residues have been proved to be reliable alumina sources for BCSA production, including the clay residues that cover bauxites (Belterra Clay (BTC)) in the Brazilian Amazon region. The clinkering settings of BCSA using a different (less alumina) BTC were examined in this work. The mineralogy of the BCSA clinkers was optimised by a design of experiments and studied using X-ray powder diffraction, scanning electron microscopy and thermal analysis. The raw meal composition, boron doping and firing temperature were investigated to obtain the optimal clinker composition. Hydration of the cement pastes was investigated by calorimetry and the strength development of mortars was tested and compared with that of a commercial OPC. The BCSA mortars produced with gypsum additions developed compressive strength comparable to that that OPC within 28 days. The main hydrated phase was ettringite, with minor amounts of kuzelite, straetlingite and gibbsite.

Pozzolanic reactivity of aluminum-rich sewage sludge ash: Influence of calcination process and effect of calcination products on cement hydration

The application of aluminum-based flocculant in wastewater treatment results in a large amount of aluminum-rich sewage sludge. This work investigated the influence of calcination process on physicochemical characteristics and pozzolanic activity of aluminum (Al)-rich sludge ash and studied the effect of sludge ash on cement hydration. The results showed that higher calcination temperature from 600 ℃ to 900 ℃ increased the amorphous content in sludge ash. The pozzolanic activity of sludge ash calcined at 800 ℃ and 900 ℃ was confirmed by Frattini test. In view of strength activity index of blended mortar and energy conservation, the optimal calcination condition of sewage sludge ash was calcined at 800 ℃ with air-cooling. The addition of sludge ash promoted the transformation of ettringite to monosulfate phase in cement paste. However, the high Al concentration dissolved from S6 and S7 ash inhibited significantly the cement hydration and resulted in low compressive strength values of the blended mortars. The pozzolanic reaction of S8 and S9 ash produced more hydration heat and additional Al-bearing products such as katoite and monosulfate which contributed to the strength development of mortars. Furthermore, the heavy metals in sewage sludge can be immobilized in ash structure during calcination process and the structure of hydration products, which ensures the environmental security of sludge ash utilization in construction materials.

Acceleration mechanism of nonisothermal microwave heating on strength development of mortar

Based on the finding of little understanding of acceleration mechanism of microwave curing, this work studies the mechanism in terms of thermal and nonthermal effects of microwaves on mortars. Mortar is cured by regimes designated with varied curing intensity in order to distinguish thermal and nonthermal effects, which are normally difficult to be told apart. They are distinguished by the inconsistency of temperature and compressive strength evolution kinetics. Microstructural characteristics are studied to explore an insight to the effects. It is found that thermal effects constantly exist and nonthermal effects are initiated by threshold energy. Although they are both responsible for accelerating mortar hardening, thermal effects are more effective. Nonthermal effects increase the branching and cross-linking of aluminosilicate chains. The resultant three-dimensional C-S-H morphology is detrimental for packing densely at nanoscale, which may explain the less acceleration effectiveness of nonthermal effects on strength development.

Mechanical and durability assessment of cement-based and\xa0alkali-activated coating mortars in an aggressive marine environment

Coatings are used as practical solutions against the intrusion of corrosive ions into concrete structures, particularly, in the harsh marine environment. In the present study, the effectiveness of using cement-based and geopolymer-based coatings produced using by-product materials has been evaluated. Silica fume and GGBFS at their optimum dosages were incorporated into mortar mixtures as a cement replacement, and mixtures of NaOH or KOH and sodium silicate solutions were used in the alkali-activated mortars. Shrinkage test, RCMT, and capillary absorption test as common experiments for durability analysis, as well as tests related to the mechanical and bonding properties including compressive strength test, pull-off test, and shear bonding strength test were carried out on the specimens. According to the results, both geopolymer and cement-based mortars improved the compressive and bonding strengths, and chloride diffusion resistance of coatings compared to the OPC mortar. Silica fume was found to be more effective in the strength development of mortars at young ages, while GGBFS was more responsible for acting as a filler and producing further gel in the older ages. The major drawback with geopolymer mortars is the high rate of water absorption and shrinkage coefficient in the early hours, which shows the importance of curing of these mortars at young ages. Overall, the mix design produced with 30% GGBFS and 7.5% silica fume showed the highest durability and mechanical properties and proved to be more compatible with the harsh environment of the Persian Gulf.