Alkali Leaching Research Articles

Alkali circulating leaching of arsenic from copper smelter dust based on arsenic-alkali efficient separation

Leaching arsenic from solid waste selectively and removing arsenic from alkaline leachate efficiently are two key points in alkali treatment of copper smelter dust, and the latter is challenging. In this study, composite salt precipitation of magnesium ammonium arsenate (NH4MgAsO4·6H2O), similar to magnesium ammonium phosphate (NH4MgPO4·6H2O), was proposed to solve the difficult problem of separation arsenic from alkali. Based on the thermodynamic analysis, the selective leaching of arsenic from copper smelting dust was carried out in the NaOH-Na2S system. In the alkali leaching system, more than 80% arsenic can be leached out from the dust with the diffusion-controlled type in the Avrami model, while the leaching rates of valuable metals are less than 0.5%. For the strong alkaline leachate containing arsenic obtained by alkali leaching, the selective removal of arsenic was achieved by adding magnesium salt and ammonium salt. With the change of the amount of magnesium salt and ammonium salt, the sedimentation performance and composition of the arsenic slag changed accordingly. At the mole ratio of NH4+: As = 8:1 and Mg2+: As = 1.5:1, 96.38% of arsenic was removed, and the content of arsenic in the arsenic slag composed of MgNH4AsO4·6H2O reached 28.96%. On this basis, the circulating alkali leaching of copper smelter dust based on arsenic-alkali separation was successfully carried out. The whole scheme is not only economical and safe, but also achieves the reuse of wastewater without secondary pollution, which provides an alternative solution for the treatment of arsenic containing solid waste.

Liberation and Enrichment of Metallic Iron from Reductively Roasted Copper Slag

Iron recovery from copper slag generated during copper production by the pyrometallurgical method has been widely investigated to achieve resource utilization. Liberation and enrichment of metallic iron from reductively roasted copper slag were explored in this work. Results show that metallic iron, quartz solid solution, and cristobalite solid solution are the main phases in reductively roasted copper slag, and most metallic iron particles are wrapped by silica. A concentrate with 74.39% Fe content and 83.14% Fe recovery is obtained through grinding–magnetic separation. For comparison, the metallic iron particles are liberated through alkali leaching of silica. The Fe content in leaching residue reaches 78.17% by removing 88.96% SiO2 and further increases to 90.45% by magnetic separation with Fe recovery of 85.20%. Therefore, alkali leaching–magnetic separation is suitable for treating reductively roasted copper slag because of the high Fe content in concentrate and comprehensive recovery of silica.

Extraction of vanadium and enrichment of titanium from modified Ti-bearing blast furnace slag

An alkali leaching-acid leaching method was proposed to extract vanadium and enrich titanium from modified Ti-bearing blast furnace slag (MTBBFS). The optimal alkali leaching conditions were NaOH concentration 2.0 mol/L, leaching temperature 95 °C, and leaching time 90 min. The leaching ratio of V was 91.30%. After 4 times cyclic alkaline leaching, the vanadium concentration of leaching solution increased to 14.70 g/L and the solution could be used to recover vanadium by the precipitation of ammonium salt. The optimum acid leaching conditions were HCl concentration 2.0 mol/L, leaching temperature 65 °C, and leaching time 60 min. The TiO2 content of synthetic rutile and the total recovery ratio of Ti were 99.15% and 97.77%, respectively. Due to the TiO2 content of the rutile >99.00%, the rutile was a high-quality feedstock for titanium dioxide with the chloride process. Moreover, the mechanism of vanadium extraction and the mechanism of alkali leaching-acid leaching reaction process were discussed

Mechanochemical activation on selective leaching of arsenic from copper smelting flue dusts

A novel application, including mechanochemical pre-treating and alkali leaching, for arsenic selective leaching from copper smelter flue dusts (CSFUs) was developed to overcome the disadvantages of hydrometallurgical methods. Compared with raw CSFU powders, the mechanical-activated ones showed higher maximum arsenic leaching efficiency (increased by ~20%), and lower apparent activation energy (decreased by ~7 kJ·mol−1). Furthermore, this novel process only consumed half of alkali and sulfides and needed one-third of the leaching time to compare with the ones used in the traditional alkali leaching process. The promoting effect of mechanical force on arsenic leaching firstly relied on the physical property changes of CSFU powders, including a decrease of particle sizes and an increase of the specific surface. Secondly, mechanochemical force converted As5+ species into reduced phases (e.g. As2O3, NaAsO2), and thio-arsenates (e.g. AsO2S23−, AsO3S3−), which could spur its leaching due to their stronger mobilities in the alkali solution within sulfides. Finally, mechanochemical activation could be facilitated to separate discrete soluble arsenic species or incorporated ones from sulfate minerals in the CSFUs. This work may have important implications for the development of new eco-friendly technologies for purifying arsenic-bearing materials.

Role of waste-based geopolymer spheres addition for pH control and efficiency enhancement of anaerobic digestion process.

In anaerobic digestion processes, pH has a vital role due to the direct impacts on the microbial community. An eco-friendly approach has been applied to control pH in anaerobic bioreactors, using waste-containing fly ash geopolymer spheres (GS) instead of powdered chemical compounds, to promote continuous alkalis leaching. The influence of GS porosity and concentration on the behavior of anaerobic sequential batch reactor treating cheese whey was evaluated. Results showed that the use of GS with the highest concentration and porosity promoted an increase in methane yield up to 30%, compared to the assay with powdered chemical compounds addition. In addition, GS boosted butyric acid production to the detriment of propionic acid, which favored methane production by a factor up to 1.2. This innovative approach indicates that GS addition can regulate pH in anaerobic digesters treating challenging wastewaters and, simultaneously, improve not only its efficiency but also the sustainability of the entire process.

Assessment of Potential Impacts of Peach Kernels and Cardoon as Co-Firing Fuels with Lignite Through Experiments on Reactivity and Ash Behavior

Objective : The reactivity and combustion performance of mixtures of lignite with peach kernel and cardoon were investigated, through thermal analysis experiments, for assessing the potential impacts of these wastes as co-firing fuels. Methods: Additionally, a control method was applied to reduce problematic elements from biomass ashes in order to mitigate deposition phenomena in co-firing units. The results showed that combustion performance of raw fuels followed the order cardoon>peach kernel>lignite, however in case of blends, this order was reversed lignite/peach kernel> lignite/cardoon, revealing synergistic effects between component fuels. Results: The combustion performance of lignite was improved by blending it with biomass fuels. Deposition problems should be expected in boilers firing peach kernel and cardoon fuels above 900°C. Conclusion: After alkalis leaching, the reactivity and efficiency of woody materials and mixtures with lignite were enhanced, while the slagging/fouling propensity was reduced.

Environment-Friendly Preparation of Adsorptive TiO <sub>2</sub>/SiO <sub>2</sub> Core-Shell Nanospheres and V2o5 Microrods from Spent SCR Catalysts by Ultrasonic-Coupled Alkali Leaching Process

Spent selective catalytic reduction (SCR) catalysts comprising of valuable V2O5 and TiO2 are usually disposed as hazardous waste because of the toxic V2O5 ingredients. Therefore, recovery of V2O5 and TiO2 from spent SCR catalysts has highly environmental benefits as well as economic value. In this study, an ultrasonic-coupled alkali leaching process was developed for environment-friendly preparation of adsorptive TiO2/SiO2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts. The results show that the synergy of ultrasonic and alkali leaching significantly enhance the separation of V2O5 from spent SCR catalysts, and the addition of NaOH decreases by 70% compared with the traditional roasting-leaching process. The as-prepared adsorptive TiO2/SiO2 core-shell nanospheres exhibit superior adsorptive and photocatalytic performance for 2,4-dinitrophenol removal, and 99.9% of 2,4-dinitrophenol was oxidized within 120 min. In addition, high-purity V2O5 microrods with high H2 S removal rate were efficiently prepared from low-grade filtrate using a hydrothermal precipitation method, which was superior to the traditional NH4VO3 precipitation and solvent extraction process. This study provides a potential method for controlling the environmental pollution of V2O5 and recycling of valuable adsorptive TiO 2 /SiO 2 core-shell nanospheres and V2O5 microrods from spent SCR catalysts.

Complete Extraction of Amorphous Aluminosilicate from Coal Fly Ash by Alkali Leaching under Atmospheric Pressure

One of the potential sources of alumina and mesoporous silica is the coal-fired thermal plants waste known as the coal fly ash (CFA). The studies of the alumina extraction from CFA are often focused on the preliminary desilication, but the efficiency of the alkali desilication is low due to formation of the desilication product—Na6[Al6Si6O24]·Na2X (DSP). This research is focused on the possibility of CFA desilication without formation of DSP using a leaching process with higher liquid to solid ratios (L/S) and alkali concentrations. The experimental data were analyzed using an artificial neural network (ANN) machine learning method and a shrinking core model (SCM). The investigation of the CFA morphology, chemical and phase composition before and after leaching were carried out by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), inductively coupled plasma optical emission spectrometry (ICP-OES) and X-ray diffraction (XRD). The present work shows that it is possible to avoid formation of DSP if using the L/S ratio >20 and concentration of Na2O—400 g/L during CFA leaching. The kinetics analysis by SCM showed that the process is limited by the surface chemical reaction at T <100 °C, and by diffusion through the product layer at T >100 °C, respectively. The SEM images of the solid residue after NaOH leaching under conditions that prevent the DSP formation show mullite particles with an acicular structure.

The pH of Cement-based Materials: A Review

Cement-based materials (CBMs), such as paste, mortar and concrete, are highly alkaline with an initial high pH of approximately 12.0 to 13.8. CBMs have a high pH due to the existing oxide mineral portlandite and alkali metal contents in Portland cement. The high pH of concrete provides excellent protection and reinforces the steel bars against corrosion. The pH of concrete does not remain constant due to ageing and other defect-causing factors, such as chloride ingress, alkali leaching, carbonation, corrosion, acid attack, moisture and biodegradation process. Reducing the concrete pH has negative impact on the strength, durability and service life of concrete buildings. However, the high pH of concrete may also cause concrete structure deterioration, such as alkali silica reaction, porosity and moisture related damages in concrete structures. The pH of CBMs can be influenced by high temperatures. For instance, the extremely high volume (85%–100%) of slag-blended cement pastes shows considerable pH reduction from 12.80 to 11.34 at 800 °C. As many concrete structure deterioration are related to concrete pH, using an accurate and reliable method to measure pH and analyse the durability of reinforced concrete structure based on pH values is extremely important. This study is a comprehensive review of the pH of CBM in terms of measurement, limitations and varying values for different CBM types.

Fundamental study on the acceleration of the neutralization of alkaline construction sludge using a CO2 incubator

The recycling ratio of construction sludge in Japan has gradually increased over the past 20 years. However, the increase in this ratio has become marginal. One of the main reasons for the stagnation is the alkalinity of the recycled sludge after treatment, as alkaline construction sludge can impact the environment owing to factors such as alkali leaching. Therefore, this study explored whether the neutralization of alkaline construction sludge can be accelerated with a controlled CO2 concentration during the production of recycled soils at recycling plants. A CO2 incubator was introduced in the laboratory to accelerate the neutralization of the alkaline sludge to investigate the effects of the initial water content and the layer thickness of the sludge on the neutralization process. The effects of the degree of CO2 concentration on the process were also examined, as were the contributions of other factors to the neutralization rate. First, the mixture conditions were explored to simulate alkaline construction sludge having a wide range of pH values (hydrogen-ion exponent) to investigate the effects of the initial pH on the neutralization rate. The results revealed that the neutralization rate of simulated alkaline sludge subjected to atmospheric curing was affected by the initial pH. Sludge with high alkalinity, such as a pH of 11 or 12, cannot fall below the upper limit of the pH regulated by the water pollution prevention act even after a dozen days. Consequently, sample surveys were conducted to capture the variation in the pH of actual construction sludge at several intermediate treatment facilities. The sample surveys showed that the pH of the sludge samples, which were in an either solid or semi-solid state, fell in the range of 6.9–13.0, with the samples having a pH of around 11 being dominant. Therefore, CO2 curing using a CO2 incubator was implemented in the laboratory on alkaline sludge that simulated the characteristics observed in the sample surveys. The laboratory test results revealed that, at a constant sludge height, a higher CO2 concentration increased the neutralization rate of the alkaline sludge. However, the contribution of the CO2 concentration to the acceleration of the neutralization decreased with a reduction in the initial water content. The test results also suggested that the contribution of the initial water content to the acceleration of the neutralization was more dominant than that of the sludge height for the present experimental conditions.

Comparison of mechanical behaviour of geopolymer and OPC-based well cement cured in saline water

Well cement plays a vital role in maintaining wellbore integrity in a carbon capture and storage project. To date, ordinary Portland cement (OPC)-based well cement has been used and there are contrasting research findings on its suitability as well cement. This research aims to use geopolymer as well cement, and major aim of this work is to analyse the mechanical behaviour of both geopolymer and OPC subjected to various down-hole conditions including different salinity (0–30% NaCl) and confinements (0–15 MPa). This research comprises experimental and numerical works. Under the experimental work, series of unconfined compressive strength (UCS) test were conducted for geopolymer and OPC samples cured in saline water with 0–30% NaCl salinity for a period of 45 days. COMSOL multiphysics software was used for the numerical study, and first, laboratory scale uniaxial stress–strain behaviour of both geopolymer and OPC was validated numerically. The model was then extended to study the triaxial mechanical behaviour under different salinity (0–30% NaCl) and confining pressures (0–15 MPa) conditions. Based on the results, it was noticed that UCS of geopolymer increases with salinity due to the high resistance against alkali leaching in saline water compared to fresh water. The UCS of OPC increases with salinity up to 10% NaCl, and then it reduces with any further increase in salinity. The UCS of geopolymer is 6% to 66% higher than that of OPC depending on the salinity conditions and geopolymer possesses higher UCS than OPC for any given salinity condition. COMSOL multiphysics numerical software predicted the stress–strain behaviour of geopolymer and OPC accurately up to yield point. Failure strength of geopolymer in saline water is 3% to 61% higher than that of OPC for a given confining pressure. Findings of this research indicate that geopolymer can be a good alternative for OPC based well cement under deep down-hole conditions.

An effective separation process of arsenic, lead, and zinc from high arsenic-containing copper smelting ashes by alkali leaching followed by sulfide precipitation.

Separation of arsenic and valuable metals (Pb, Zn, Cu, Bi, Sn, In, Ag, Sb, etc.) is a core problem for effective utilization of high arsenic-containing copper smelting ashes (HACSA). This study developed an effective separation process of arsenic, lead, and zinc from HACSA via alkali leaching followed by sulfide precipitation. The separation behaviors and optimum conditions for alkali leaching of arsenic and sulfide precipitation of lead and zinc were established respectively as follows: NaOH concentration 3.81 M; temperature 80°C; time 90 minutes; liquid-to-solid ratio 4:1; agitation speed 450 revolutions/minute (r/min) and 2.0 times of theoretical quantity of sodium sulfide (Na2S); temperature 70°C; and time 60 minutes. The results indicated that the leaching rates of As, Pb, and Zn were 92.4%, 36.9% and 13.4%, respectively. More than 99% of lead and zinc were precipitated from the alkali leachate. The scanning electron microscopy/energy dispersive X-ray spectroscopy study confirmed that arsenic was dissolved from HACSA into the alkali leachate. Furthermore, lead and zinc were precipitated as sulfides from the alkali leachate. The proposed process was a good technique for separation of arsenic and enrichment of valuable metals for further centralized treatment separately. It provided high separation efficiency of arsenic and valuable metals, as well as low environmental pollution.

Inhibition of Efflorescence in Na-Based Geopolymer Inorganic Coating.

Coating is one of the most important high-value-added application cases in geopolymer materials. However, efflorescence can easily cause discoloration and reduce the esthetic impression of the coating surface, thus limiting its application; hence, inhibition of efflorescence is one of the most important techniques in the application of geopolymer coatings. Efflorescence is a spontaneous behavior in a Na-based geopolymer, involving the migration of soluble alkalis. Alkalis are dissolved by water and diffuse to the material surface through nocuous pores, and then react with CO2 to produce white carbonate products. To inhibit efflorescence in geopolymer coating, this article reports a structure modification method using polydimethy siloxane (PS) and mica. To explore the inhibition mechanism, the effects of PS and mica on the pore structure, water absorption, alkali leaching, and efflorescence product were investigated. The experimental results showed that a harmful pore structure and instinctive water absorption of the geopolymer strongly contributed to efflorescence. PS and mica could reduce the pore size distribution and porosity and are helpful to establish a waterproof structure, leading to water absorption and the alkali leaching rate being significantly suppressed. Both high water glass and water content play a critical role in the increase of efflorescence, but even under a high content of water glass and water used in geopolymer coating, this method shows an 80–90% efflorescence reduction, which is much higher than that of other studies. In practical engineering, when the geopolymer coating is applied after modification, even if it is exposed to the field environment for a long time, there is no efflorescence deposit on the coating surface. It is feasible to limit water ingression in a geopolymer, which effectively blocks the efflorescence reaction process. This method is simple and practical and can be applied in practical engineering applications of geopolymer coatings conveniently.

Concentration of Rare Earth Elements (Sc, Y, La, Ce, Nd, Sm) in Bauxite Residue (Red Mud) Obtained by Water and Alkali Leaching of Bauxite Sintering Dust

One of the potential sources of rare-earth elements (REE) is the industrial waste known as red mud (bauxite residue), in which the majority of REE from the initial bauxite are concentrated via the Bayer process. Therefore, the studies of the subject, both in Russia and outside, focus almost exclusively on red mud processing. This article looks into the possibility of REE concentration into red mud by leaching an intermediate product of the bauxite sintering process at Russian alumina refineries, namely electrostatic precipitator (ESP) dust. The experimental works were performed by X-ray diffraction (XRD)and electron probe microanalysis (EPMA) of the sinter and sinter dust. The determination of major and rare-earth elements in the sinter from the rotary kilns and in the ESP dust before and after leaching was carried out by X-ray fluorescence (XRF) and plasma mass spectrometry (ICP-MS). The study showed that it is possible to obtain red mud that contains three times more REE than traditional waste red mud after two-stage leaching ESP dust in the water at 95 °C followed by leaching in an alkaline-aluminate liquor at 240 °C. The shrinking core model was used to study the kinetics of leaching of the original ESP dust and water-treated dust in alkaline-aluminate liquor. The study showed the change in the limiting stage of the alkaline leaching process after water treatment, with the activation energy growing from 24.98 to 33.19 kJ/mol.

Suspension calcination and alkali leaching of low-grade high-sulfur bauxite: Desulfurization, mineralogical evolution and desilication

To enable the utilization of low-grade and high-sulfur bauxite, the suspension calcination was used to remove the sulfur and the activate silica minerals, and the calcinated bauxite was subjected to a desilication process in NaOH solution under atmospheric pressure. The desulfurization and desilication properties and mineralogical evolution were studied by X-ray diffraction, thermogravimetry-differential thermal analysis, scanning electron microscopy, and FactSage methods. The results demonstrate that the suspension calcination method is efficient for sulfur removal: 84.21% of S was removed after calcination at 1000°C for 2 min. During the calcination process, diaspore and pyrite were transferred to α-Al2O3, magnetite, and hematite. The phase transformation of pyrite follows the order FeS2 → Fe3O4 → Fe2O3, and the iron oxides and silica were converted into iron silicate. In the alkali-soluble desilication process, the optimum condition was an alkali solution concentration of 110 g/L, a reaction time of 20 min, and a reaction temperature of 95°C. The corresponding desilication ratio and alumina loss ratio were 44.9% and 2.4%, respectively, and the alumina-to-silica mass ratio of the concentrate was 7.9. The Al2O3·2SiO2, SiO2, and Al2O3 formed during the calcination process could react with NaOH solution, and their activity decreased in the order of Al2O3·2SiO2, SiO2, and Al2O3.

The Development of Accelerated Test Method for Internal Sulfate Attack by Delayed Ettringite Formation

Delayed Ettringite Formation (DEF) is an internal sulfate attack caused by heat-induced decomposition and/or prevention of normal ettringite formed during the initial hydration of cement at elevated temperature (above about 70°C) and its re-crystallization in the hardened matrix. This reaction is a physico-chemical phenomenon inducing an expansion of the cement paste that could lead to cracking of cementitious matrix. These cracks result in a decrease in the mechanical performances and durability parameters of the material. However, the internal sulfate attack is characterized by a very slow reaction kinetics and therefore it is difficult to study it in laboratories. This research developed an accelerated method focused on mortar specimens; it is based on electrochemical techniques in order to speed the leaching of alkalis that could be accelerated the DEF.

Treatment of Secondary Dust Produced in Rotary Hearth Furnace through Alkali Leaching and Evaporation–Crystallization Processes

This paper presents the results of an experimental study on the extraction of KCl and the improvement of the zinc grade of secondary dust obtained from rotary-hearth-furnace secondary dust (RHF secondary dust) using alkali leaching (Na2CO3 solution) and evaporation–crystallization processes. The effects of the liquid–solid ratio and Na2CO3 content on the element leaching ratio in the alkali leaching process, as well as the effects of the volume–evaporation ratio and cooling temperature on KCl extraction in the evaporation–crystallization process, were investigated. The results showed that the optimum liquid–solid ratio was 6:1, and the optimum quantity of Na2CO3 was 1.5 times the basic quantity. The recovery ratio of zinc reached 95.23%, and the leaching ratio of K reached 79.01%. The experimental results of the evaporation–crystallization process demonstrated that the evaporation temperature was 80 °C, the volume evaporation ratio was 50%, the cooling temperature was 25 °C, and the mass fraction of K2O in the obtained crystals was 58.99%.

Synthesis and characterization of rice husk-based magnetic porous carbon by pyrolysis of pretreated rice husk with FeCl3 and ZnCl2

Rice husk-based magnetic porous carbon with high porosity and magnetization was synthesized by pyrolysis of pretreated rice husk with FeCl3 and ZnCl2. Thermogravimetric analyses (TGA) of the precursors were used to characterize the mechanism of the pyrolysis progress. The effects of ZnCl2 impregnation ratio and pyrolysis temperature on the properties of magnetic porous carbon were identified by ultimate analyses, scanning electron microscopy (SEM), N2 adsorption-desorption isotherms, X-ray diffractometer (XRD), Raman spectrometer and vibrating sample magnetometer (VSM). Based on the results, the optimum ZnCl2 impregnation ratio of 1.0 and pyrolysis temperature of 600 °C were obtained when considering both of the porous characteristics and magnetic properties. Finally, the synergistic effect of combined pretreatment of hydrothermal carbonization (HTC) and alkali leaching and simultaneous activation and magnetization was confirmed as the possible reaction mechanism of synthesis of magnetic porous carbon from rice husk with the surface area of 1344.67 m2/g and saturation magnetization of 3.64 emu/g.

A Comparative Study in Vanadium and Tungsten Leaching from Various Sources of SCR Catalysts with Local Difference

Direct leaching with NaOH can be an economically acceptable method for vanadium (V) and tungsten (W) recovery from spent selective catalytic reduction (SCR) catalysts. However, different chemical-physical characteristics of catalysts would affect the V and W leaching. In this paper, the V and W leaching behavior of various sources of SCR catalysts with a local difference (yellow and gray color) were systematically investigated with alkali leaching solution under ambient pressure. Different leaching efficiencies from yellow and gray color areas were correlated with oxidation states and species of V and W on catalyst surfaces, as characterized by X-ray photoelectron spectroscopy (XPS), Raman, Fourier transform infrared spectroscopy (FTIR), and other analytic methods. For the V leaching efficiency, the samples from a gray area of catalysts (40.0–51.0%) were lower than that from the yellow area (66.8–69.8%). The higher molar ratio of V3+ and a lower molar ratio of V5+, and the lower total V content on the surface of the samples from the gray area could be the main reasons for the lower V leaching efficiency. As for the W leaching efficiency, the samples from the gray area (44.6–57.3%) were slightly higher than that from the yellow area (38.0–52.6%) of catalysts. The less total W content of surface species and stronger interaction among V–W–Ti of yellow area samples resulted in the lower leaching efficiency. These differential leaching efficiencies needed to be taken into consideration for recovering V and W from spent SCR catalysts.

The effect of cement mortar composition on the pH value

Cement-based materials (CBMs) initially start their life at a high pH of about 12.5-13.5, which is further reduced due to several factors. These include ageing, penetration of moisture, carbonation, chloride ingress, alkali leaching, corrosion, and other biodegradation processes. A less reported factor enhancing the above reduction is incorporation of supplementary cementitious materials (SCMs) as partial replacement of cement due to the consumption of Ca(OH)2 in their pozzolanic reaction. In this study, pH and effect of Ca(OH)2 contents of cement mortars having 50% of fly ash and ground granulated ballast furnace slag were studied up to the age of 150 days. The results obtained strongly indicate that pH is not only dependent on the Ca(OH)2 content in cement mortars as specified in the most previous studies.