---

Abstract: A geotechnical engineer may sometimes encounter fine-grained clay soils. Improvement of the engineering properties of those soils is necessary in order to achieve suitable construction sites. One of the methods for soil modification is to use additives, which are economically justifiable as well as being abundantly produced and accessible. Lime is one of the additives that results in reinforcement of the soil through Pozzolanic reactions. But in case that the soil consists of sulfate ions or when a stabilized soil becomes prone to sulfate water, then the presence of lime not only doesnot decrease the swelling of the stabilized layer, but also it acts to the contrary and causes the increase in swelling and reduces the strength. Addition of fly ashes reduces the destructive effects of the sulfates (such as bulk increase and cracking) and increases the abatement of carbonation in a soil stabilized by lime or cement while rising the PH level of the aggregate and calcium ion, which results in expedient pozzolanic reaction. This article discusses the impact of fly ash addition on the geotechnical properties of soil and lime aggregate. The ML soil was used in this study with various percentages of fly ash and a given percentage of lime. Then the effect of fly ash on lime and soil aggregate was studied through shear strength and Atterberg limits tests. These tests were performed for 7-day and 28-day saturated specimens of different percentages so that a set of diverse fly ash and lime aggregates would be tested. The results represent the extraordinary impact of fly ash on the shear strength as the 28-day samples showed a 138% increase while the 7-day samples had a 90% rise. Also the cohesion parameter was increased up to 700% in the 28 –day samples while it showed a 600% rise in the 7-day samples. The internal friction angle also showed a significant increase, especially since the low price of fly ash makes it economically advantageous. This article would at least be useful for rehabilitation of problematic soils and for application of novel results and technologies in the geotechnical engineering.

---

.Setting time is considered as an identification boundary of fresh and hardened of concrete. Initial set refers to a change from a fluid to a rigid state, accompanied by a rapid temperature rise in the body of concrete structures. The early hydration of C3A and C3S in presence of gypsum plays an important role in concrete rheology, and setting points of concrete. The final set, on the other hand, express the start of strength development. Knowledge of the initial setting time will help not only in determining the time available for the transportation, placement, and consolidation of concrete; but also in identifying the effectiveness of various admixtures and pozzolanic materials. The use of different types of pozzolanic materials has allowed to reduce the carbon dioxide emission per ton of produced cement. In the present work, the effects of silica fume, fly ash and slag on the setting times and strength development of different concrete mixtures are examined. The setting times were conducted accordance to ASTM C403. The Concrete specimens were prepared with three water cement ratios of 0.35, 0.45, and 0.55. Silica fume at 7%, fly ash at 15% and 25% and slag at 25% and 35%, by weight of cement, were used as replacement for cement. The results showed that the setting times of the concrete increased as water-cement ratio increased and the lowest setting times corresponded to the specimens with 0.35 water-cement ratio. The setting time of the specimens with water cement ratios of 0.45 and 0.55 increased by 10% and 18%, respectively, compared to that for 0.35. Using silica fume, fly ash and slag, as replacement for cement, retarded the setting times of the mixtures. The more replacement contents led to increase of setting times. The setting time of the fly ash specimens at 15% and 25% increased by 6% and 12%, whereas addition of slag at 25% and 35% retarded the setting time by 11% and 18%, respectively, compared to those for reference specimens with 0.45 water-cement ratio. Silica fume caused a slight increase of setting time by 5.5% compared to those without pozzolan materials. These may attributed to the low cement content in the pozzolanic specimens which resulted to the low friction surface between cement particles and on the other hand enhancement of effectiveness of the super-plasticizer in the mixtures which led to the increase of setting times. The results also indicated that silica fume, as a very fine supplementary cementitious material, enhanced both the early and later age strength. Fly ash and slag, as replacement for cement, increased the later stage strength of the specimens, but had a negative impact on the early- age strength. This may attributed to the low pozzolanic activity of these cementing materials in the early age, however in the later ages which calcium hydroxide content increased in the mixtures the secondary induced gel enhanced the compressive strength. A power relationship was proposed for the early age strength and the final setting time relationships

---

Today, concrete is used as the most widely used and common building materials in the development of civil and economic infrastructure.
 Concrete is made from ordinary Portland cement, and due to the high consumption of concrete and the growing need for cement production, research shows that in the future the demand for cement concrete will increase, and on the other hand cement production requires the consumption of natural resources such as electricity. And fossil fuels, as well as the release of about 7% of CO2 gas into the environment, and its production process consumes the most energy after steel and aluminum, so the provision of alternative products to move towards sustainable development is essential.

In the medium and long term strategies to reduce global warming due to the cement industry, the development of energy-efficient technologies to reduce CO2 emissions and their development in the market will be very key.

The capabilities of geopolymer cement in the field of energy storage and reduction of CO2 emissions are very significant compared to Portland cement, so that this technology, while providing comparable functions with commercial cementitious materials, can reduce CO2 emissions from the cement industry. Reduce by as much as 80%.

---

A significant number of engineering structures around the world are exposed to fire on a daily basis. The most important effect of fire on the structure is elevated temperatures, which may reach more than 1000 degrees Celsius and cause not only thermal stresses and deformations but also diminished mechanical properties of materials comprising the structure. Fire-related collapses have been observed in numerous structural fires. However, many reinforced concrete structures exposed to fire do not demonstrate notable apparent damage and survive despite having experienced elevated temperatures before the fire is put out. Estimating the residual strength of such structures is of critical importance when deciding whether such structures can be safely used after fire. Moreover, in many industrial applications, there is a need to concrete that can withstand repeated long-term cycles of elevated temperatures without diminished mechanical properties. The objective of this paper is to investigate the effects of silica fume and fly ash as two widely used supplementary cementitious materials on the residual strength of concrete exposed to elevated temperatures and evaluate while such materials can be of benefit in improving the strength retention in case of heat exposure. Using 19 mix designs, a series of 570 concrete cylinders was fabricated using different water to cement ratios (0.35, 0.5, and 0.65), silica fume replacement ratios (0, 10, and 15 percent), and fly ash replacement ratios (0, 10, 20, and 30 percent). The specimens were cured in water for 56 days, after which they were placed in a rate-controlled large-scale electrical furnace, and their residual compressive and tensile strengths were measured before heat, and after heat exposure for 2-, 12-, and 24-hour heating cycles with temperatures reaching 200, 400, and 600 degrees Celsius. To eliminate the risk of explosive spalling, all specimens were preheated at a temperature of 100 degrees for 24 hours before the main heating cycle. Results showed that the compressive and tensile strengths did not reduce noticeably after exposure to 200 degrees but demonstrated a significant drop after exposure to 400- and 600-degree cycles. In many cases, the residual compressive and tensile strengths of specimens were found to be smaller than those predicted in previous studies. The square root equation widely used in the literature was found to provide a reasonable lower-bound estimate of the residual splitting tensile strength of concrete from the residual compressive strength; however, a linear trend was identified to provide a more accurate estimate for the results of this study. Moreover, due to less scatter, the splitting tensile strength was found to be a better indicator of heat damage in the structure than the compressive strength. The use of silica fume did not result in a meaningful trend in the residual compressive strength but reduced the residual tensile strength of specimens. Fly ash, on the other hand, could increase the residual compressive strength of the specimens but reduces the residual tensile strength. The results suggest that generally, and with few exceptions, these two supplementary cementitious materials are not recommendable choices for improving the strength retention of concrete in case of heat exposure.

---

The durability of a concrete structure is highly dependent on the strength and permeability of the surface layer, as it is the surface layer that must prevent the entry of materials that can initiate or enhance the harmful effects on concrete. Sulfates are one of the most common destructive factors in concrete in most parts of Iran, especially in the southern regions of the country where concrete is exposed to seawater (which contains sulfate compounds). Also, the lack of compaction of the surface layer, due to the difficulty of vibration in limited spaces between molds and rebars and other accessories, is one of the main reasons for the poor durability of reinforced concrete structures exposed to environmental factors. Naturally, incorrect vibration results (worming, detachment, dehydration) have stronger negative effects on permeability and therefore durability. Self-compacting concrete with suitable properties is free from these defects and as a result, materials with less inconsistency and uniform permeability have less weaknesses for environmental harmful factors and, therefore, have better durability. Therefore, considering that the "torsion" test shows good sensitivity to surface changes of concrete, so in this study, using the "torsion" test, the effect of magnesium sulfate on the surface strength of self-compacting concretes has been investigated. In the "twist" test, a 5 cm diameter metal cylinder is glued to the surface of the test site using epoxy resin adhesive. Then, using a conventional hand-held tachometer, a torsion anchor is inserted into the metal cylinder to break the test object. The equipment used in the "twist" test is very cheap, simple and accessible compared to other corresponding tests. The damage from the "torsion" test is very superficial and minor, and by causing failure in the test object itself, it directly determines its strength. Self-compacting concrete mixing designs were studied by replacing 25, 35 and 45% cement with fly ash filler, and a conventional concrete mixing design as a control to study the effect of magnesium sulfate on the strength of self-compacting concrete. Self-compacting concrete tests including slump flow test, slump flow time of 50 cm, V-shaped funnel, V-shaped funnel increase time (5 minutes) and L-shaped mold were performed on self-compacting concrete acceptance criteria on self-compacting concrete mixing designs. . The results indicate that sulfated water not only does not have a negative effect on the surface strength of self-compacting concrete containing fly ash, but also provides better curing conditions for these samples. The use of fly ash also makes the magnesium sulfate solution a more suitable medium than ordinary water for the surface strength of self-compacting concretes. The process of obtaining surface strength in almost all self-compacting samples treated in magnesium sulfate solution is more than that in ordinary water. However, in the case of ordinary concrete, the process of obtaining the surface strength of all samples placed in magnesium sulfate solution is less. For self-compacting samples treated in magnesium sulfate solution, with increasing the percentage of fly ash, the surface resistance of 3 and 7 days decreases. But the 28-day surface resistance increases.