Resistance of alkali-activated slag concrete to acid attack

This paper presents an investigation into the durability of alkali-activated slag (AAS) concrete exposed to acid attack. To study resistance of AAS concrete in acid environments, AAS concrete was immersed in an acetic acid solution of pH=4. The main parameters studied were the evolution of compressive strength, products of degradation, and microstructural changes. It was found that AAS concrete of Grade 40 had a high resistance in acid environment, superior to the durability of OPC concrete of similar grade.     

Durability of geopolymer materials in sodium and magnesium sulfate solutions

This paper presents an investigation into the durability of geopolymer materials manufactured using class F fly ash and alkaline activators when exposed to a sulfate environment. Three tests were used to determine resistance of geopolymer materials. The tests involved immersions for a period of 5 months into 5% solutions of sodium sulfate and magnesium sulfate, and a solution of 5% sodium sulfate+5% magnesium sulfate. The evolution of weight, compressive strength, products of degradation and microstructural changes were studied.In the sodium sulfate solution, significant fluctuations of strength occurred with strength reduction 18% in the 8FASS material prepared with sodium silicate and 65% in the 8FAK material prepared with a mixture of sodium hydroxide and potassium hydroxide as activators, while 4% strength increase was measured in the 8FA specimens activated by sodium hydroxide. In the magnesium sulfate solution, 12% and 35% strength increase was measured in the 8FA and 8FAK specimens, respectively; and 24% strength decline was measured in the 8FASS samples. The most significant deterioration was observed in the sodium sulfate solution and it appeared to be connected to migration of alkalies into solution. In the magnesium sulfate solution, migration of alkalies into the solution and diffusion of magnesium and calcium to the subsurface areas was observed in the specimens prepared using sodium silicate and a mixture of sodium and potassium hydroxides as activators. The least strength changes were found in the solution of 5% sodium sulfate+5% magnesium sulfate. The material prepared using sodium hydroxide had the best performance, which was attributed to its stable cross-linked aluminosilicate polymer structure.     

Sulfate attack on alkali-activated slag concrete

This paper presents an investigation into durability of alkali-activated slag (AAS) concrete in sulfate environment. Two tests were used to determine resistance of AAS concrete to sulfate attack. These tests involved immersion in 5% magnesium sulfate and 5% sodium sulfate solutions. The main parameters studied were evolution of compressive strength, products of degradation, and microstructural changes. After 12 months of exposure to the sodium sulfate solution, the strength decrease was up to 17% for AAS concrete and up to 25% for ordinary Portland cement (OPC) concrete. After the same period of exposure to the magnesium sulfate solution, the compressive strength decrease was more substantial, up to 37% for OPC and 23% for AAS. The main products of degradation were ettringite and gypsum in the case of Portland cement and gypsum in AAS. OPC samples had significant expansion, cracking, and loss of concrete, while AAS samples were not expanded but cracked in the test. During experiments with the sodium sulfate solution, some increase in strength of AAS concrete was recorded, likely due to continuing hydration.     

Modelling acid attack on concrete: Part I. The essential mechanisms

At present, a computer model is being developed to predict the corrosion of concrete construction components subjected to acidic solutions with pH values ranging between 4.0 and 6.5, i.e. exposure classes XA3 down to XA1 according to DIN EN 206-1. The concrete may contain Portland cement based binders with dissolvable or acid resistant aggregate. Calcium aluminate cement is also considered. The concrete degradation is characterised by a corroded layer of high porosity whose thickness is determined by the combination of dissolution, precipitation and transport processes which depend on cement chemical composition, binder reactivity, aggregate reactivity, grading curve as well as concrete composition. It is also intended to include the effect of abrasion. The model components will be described in detail in forthcoming publications.     

Behavior of low-calcium fly and bottom ash-based geopolymer concrete cured at ambient temperature

This paper presents the results of an experimental study on the behavior of fly ash-, bottom ash- and blended fly and bottom ash-based geopolymer concrete (GPC) cured at ambient temperature. A total of 10 bathes of GPC and a single batch of ordinary Portland cement concrete (OPC) were manufactured. The tests of compressive strength, elastic modulus, flexural strength, workability, drying shrinkage and absorption capacity were carried out to determine the properties of fresh concrete and mechanical and durability-related properties of hardened concrete. Test parameters included the mass ratio of fly ash-to-bottom ash, liquid alkaline-to-coal ash binder ratio, coal ash content and concrete type. The results indicate that the selected parameters significantly affect the microstructure and the behavior of GPCs. It is seen that bottom ash-based GPCs exhibited significantly lower geopolymerization than that of the fly ash-based GPCs, resulting in the inferior behavior of the former compared to the latter.     

Magnesium sulfate attack on hardened blended cement pastes under different circumstances

This paper describes the sulfate resistance of some hardened blended Portland cement pastes. The blending materials used were silica fume (SF), slag, and calcium carbonate (CaCO₃, CC?). The blended cement pastes were prepared by using W/S ratio of 0.3. The effects of immersion in 10% MgSO₄ solution under different conditions (room temperature, 60 °C, and drying-immersion cycles at 60 °C) on the compressive strength of the various hardened blended cement pastes were studied. Slag and CC? improve the sulfate resistance of ordinary Portland cement (OPC) paste. Mass change of the different mixes immersed in sulfate solution at 60 °C with drying-immersion cycles was determined. The drying-immersion cyclic process at 60 °C accelerates sulfate attacks. This process can be considered an accelerated method to evaluate sulfate resistance of hardened cement pastes, mortars, and concretes.     

Geopolymeric materials prepared using Class F fly ash and elevated temperature curing

This paper reports the results of the study of the influence of elevated temperature curing on phase composition, microstructure and strength development in geopolymer materials prepared using Class F fly ash and sodium silicate and sodium hydroxide solutions. In particular, the effect of storage at room temperature before the application of heat on strength development and phase composition was studied. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and SEM were utilised in this study.Long precuring at room temperature before application of heat was beneficial for strength development in all studied materials, as strength comparable to 1 month of curing at elevated temperature can develop in this case only after 24 h of heat curing. The main product of reaction in the geopolymeric materials was amorphous alkali aluminosilicate gel. However, in the case of sodium hydroxide activator in addition to it, traces of chabazite, Linde Type A, Na-P1 (gismondine) zeolites and hydroxysodalite were also present. The type of zeolite present and composition of aluminosilicate gel were dependent on the curing history.     

元宝山电厂粉煤灰几种利用方法

元宝山电厂采用静电除尘器收集粉煤灰，质量较好，部分已达Ⅰ级灰标准。本文就其用于配制砌筑水泥，作混凝土掺和料和混凝土减水剂进行介绍。     

活性粉煤灰作水泥混合材的应用研究

研究了一种适合于水泥生产的粉煤灰活化方法。活化后的粉煤灰用作水泥混合材，可以提高粉煤灰水泥１～２个标号，且达到早强水泥标准，从而完全克服了粉煤灰水泥因早期强度低不能用于建筑工程的缺陷。活化后的粉煤灰的形状便于运输、贮存和应用。     

The effect of matrix composition and calcium content on the sulfate durability of metakaolin and metakaolin/slag alkali-activated mortars

The work in hand presents the results of an experimental research on the effect of different precursors (binders) used in alkali-activated materials (AMM) and its composition (i.e. SiO₂/Al₂O₃ molar ratio) on their sulfate durability. A reference matrix is formed from the activation of metakaolin (MK); this matrix was modified by the partial replacement of MK with either 20 wt% silica fume (SF) or 20 and 40 wt% blast furnace slag (BFS), so that the SiO₂/Al₂O₃ molar ratio of calcium-free and calcium-rich AAM changed from 3.0 to 3.9. The properties assessed prior to the durability testing were: density (pycnometry), compressive strength, capillary sorption and oxygen permeability. The sulfate durability was investigated by exposing the matrices to a magnesium sulfate solution for 30, 90 and 180 days of attack, after which the residual compressive strength was determined. The reductions in strength after each period of testing were correlated with variations in the pH of the sulfate solutions and with geometry changes (expansion) measured in cylinders exposed to the durability tests. X-Ray diffraction was used to determine the minerals formed onto the surface of the samples after magnesium sulfate attack. The results show that the MK-based AAM present a higher resistance to magnesium sulfate attack. Furthermore, the partial replacement of MK with BFS is responsible for reductions in the mechanical properties after attack to sulfate. This is associated with the formation of ettringite and gypsum in the presence of calcium from BFS, besides the loss of alkalinity from the migration of alkali (Na⁺) to the solution.     

The effect of ionic contaminants on the early-age properties of alkali-activated fly ash-based cements

Alkali-activated fly ash-based cements are concrete binders that utilise fly ash as their major solid raw material. The solid particles are activated using concentrated silicate and hydroxide solution to produce high-strength products. Due to the highly alkaline nature of the solution, precipitation of the reactive species, both from the solids and from the solution, proceeds at a very fast rate. This renders short setting times, which can be advantageous or disadvantageous depending on the practical situation. The present work examines the effects of inorganic salt addition towards the setting and rheological characteristics of the early pastes. Compressive strength, Fourier transform infrared spectroscopy (FTIR) and X-ray diffractograms were collected to examine the hardened products. It was found that calcium (Ca) and magnesium (Mg) salts shortened the setting time by providing heterogeneous nucleation centers in the initial paste solution. Potassium salts retarded setting only to the cements, which used less sodium silicate in the initial solution for activation. Managed ionic contamination can be used to increase the product early strength. However, its long-term effects still need to be identified.     

Alkali-activated cement based on natural SiO2-containing material: Part I. Strength, hydration, microstructure and durability

An alkali-activated cement (AAC) based on natural SiO₂-containing materials—grounded porcellanite (Pr) and highly dispersed pure quartz sand—was examined. Sodium hydroxide was used as an alkali activator. The pressed specimens were prepared and were cured in an autoclave at a pressure of 1.6 MPa and a temperature of 205 °C. It was shown that the strength of cement as well as compound and the microstructure of its hydration products depend on the cement composition. It was distinguished that autoclave-cured cementing matter comprises secondary quartz and the mass of sodium hydrated silicates along with the initial Pr crystal phases. After a 2-year storage under water, 15% Na₂SO₄, and Dead Sea water, the strength of specimens decreased by 17.5-20%. Control specimens, prepared with Portland cement and immersed in a 15% Na₂SO₄ solution for 2 weeks, were broken up completely. Positive results of long-term durable tests suggest that an AAC based on natural raw material would be stable in other salt solutions.     

Limitations of Köch-Steinegger test to evaluate the durability of cement pastes in acid medium

The durability of ordinary Portland cement and ground granulated blast furnace slag pastes in buffered acetic/acetate medium (pH 4.5) was studied by means of the Köch-Steinegger test. Results show that flexural strength measurement is not a good parameter to evaluate the degradation degree of cement paste in acid medium because two effects take place with opposite consequences on flexural strength as a result of acid attack: a densification of the cement paste in the specimen core and a degradation of the outer surface with loss of resistance.     

Effect of partial replacement of geopolymer binder materials on the fresh and mechanical properties: A review

The growth of demand for concrete raises concerns about the consumption of natural resources and ordinary Portland cement. Geopolymer composites show promise as a sustainable alternative for conventional cement concrete. Considering the wide range of potential geopolymer composites applications (including suitability for transportation infrastructure, underwater applications, repair and rehabilitation of structures as well as recent developments in 3D printing), the desired fresh and mechanical properties of the geopolymer composite may vary between applications: for example, rapid setting can be a merit for certain applications and a demerit for others. Therefore, the desired fresh and mechanical properties (e.g., workability, setting time, compressive strength, etc.) can be controlled for a given geopolymer source material through its partial substitution by natural or by-product materials. Recognizing the critical role of various replacement materials in enhancing the potential applications of geopolymer composites, the present review was undertaken to quantify and understand the effect of partial replacement by fly ash, metakaolin, kaolin, red mud, slag, ordinary Portland cement, and silica fume on the setting time, workability, compressive strength and flexural strength of various source materials addressed in the literature. The review also provides insights into research gaps in the field to promote future research.     

Resistance of sodium polyphosphate-modified fly ash/calcium aluminate blend cements to hot H2SO4 solution

Sodium polyphosphate-modified Class F fly ash/calcium aluminate blend (SFCB) cements were prepared at room temperature and their resistance to hot acid erosion was evaluated by submerging them in H₂SO₄ solution (pH 1.6) at 90°C. Sodium polyphosphate preferentially reacted with calcium aluminate cement (CAC) to form amorphous Ca(HPO₄).xH₂O and Al₂O₃.xH₂O gel, rather than fly ash. These amorphous reaction products, which bound the partially reacted and unreacted CAC and fly ash particles into a coherent mass, were responsible for strengthening and densifying the SFCB specimens at room temperature, playing an essential role in mitigating their acid erosion. In these cements, the extent of acid erosion depended primarily on the ratio of fly ash/CAC; namely, those with a higher ratio underwent a severe erosion. This effect was due to the formation of a porous structure, which allowed acid to permeate the cement easily, diminishing the protective activity of Ca(HPO₄).xH₂O and Al₂O₃.xH₂O gel against H₂SO₄.     

Durability of traditional plasters with respect to blast furnace slag-based plaster

Blast furnace slag is a residue of steel production. It is a latent hydraulic binder and is normally used to improve the durability of concrete and mortars. Slag could be also used as rendering mortar for masonry and old buildings. Today, cement and hydraulic lime are the most popular hydraulic binders used to make plasters. They are characterised by a low durability when exposed to the action of chemical and physical agents.The aim of this study was to provide a comparison between the physical-mechanical properties of some renders made with ordinary Portland cement, hydraulic lime, or slag. Furthermore, an investigation was carried out to analyse mortar resistance to several aggressive conditions like acid attack, freezing and thawing cycles, abrasion, sulphate aggression, cycles in ultraviolet screening device, and salt diffusion. The specimens, after chemical attack, have been characterised from the chemical-physical [specific surface according to the BET (Brunauer-Emmet-Teller) method], crystal-chemical (X-ray diffraction, XRD), and morphological (scanning electron microscopy, SEM) points of view.     

The microstructure and durability of fly ash-based geopolymer concrete: A review

Fly ash-based geopolymer concrete (FABGC) is a type of environment-friendly building material that displays remarkable mechanical properties and durability. It has the potential for extensive application in the field of civil engineering. This paper considers the related research on the microstructure and durability of FABGC to systematically summarize the results on its alkali-activated reaction, pore structure, and interface characteristics. The degradation mechanisms of FABGC in various corrosive environments are analyzed, and the factors that affect its microstructure and durability are discussed. It is observed that aluminosilicate gel produced by the alkali-activated process of FABGC has an optimizing effect on the pore structure and interfacial transition zone. An effective development of the microstructure can improve the durability of FABGC to a certain extent. At present, there is no consensus on the research conclusions on the microstructure and durability of FABGC. Therefore, further research is required.     

Resistance of self-consolidating concrete to sulfuric acid attack with consecutive pH reduction

Self-consolidating concrete (SCC) is increasingly being used in numerous concrete applications some of which are vulnerable to sulfuric acid attack. The mixture design of SCC is different than that of normal concrete, and thus its long-term durability characteristics are still uncertain. This study aims at investigating the resistance of a variable range of SCC mixture designs to sulfuric acid attack. The main test variables include the cementitious materials type (single, binary, ternary and quaternary binders), the sand-to-total aggregates mass ratio, and the inclusion of fibre reinforcement (single and hybrid). The investigation comprised two consecutive 6-week phases of immersion of test specimens in sulfuric acid solutions with a maximum pH threshold of 2.5 and 1.0, respectively. In total 24 SCC mixtures were tested. The study reveals that the rate of attack, as expressed by mass loss versus time, is controlled by different factors at each exposure phase. The advantages of blended binders and hybrid (steel + polypropylene) fibres in improving the resistance of SCC to sulfuric acid attack are highlighted. Microanalysis conducted upon test termination elucidates the damage mechanisms, and it is shown that there is no direct correlation between the rate of attack expressed by mass loss and the compressive strength loss after exposure to sulfuric acid.     

Using limestone aggregates and different cements for enhancing resistance of concrete to sulphuric acid attack

A research program was undertaken to improve concrete's resistance against sulphuric acid attack. Six concretes were investigated, four using calcareous limestone aggregates and two using silicious aggregates. Cements used in these concretes included a portland cement, a binary cement containing ground granulated blast furnace slag, and two ternary cements containing slag and silica fume or fly ash and silica fume. All the concretes had the same water/cement ratio of 0.4, with compressive strengths in the range of 45 MPa and 58 MPa at the age of 28 days. In the experiment, concrete cylinders were immersed in 1% sulphuric acid solution and they were periodically examined for appearance, measured for mass change and tested in compression up to 168 days. The concrete using limestone aggregates and the ternary cement containing silica fume and fly ash performed the best.     

Mechanical properties and microstructure evaluation of fly ash - Slag geopolymer foaming materials

The existing fly ash-slag foaming geopolymer materials generally have the shortcomings of low fly ash content and low porosity. It is urgent to develop geopolymer foaming materials with high fly ash content and high porosity. Using fly ash and slag as the main raw materials, geopolymer foaming materials were prepared by alkali activation. The effects of activator content and sodium silicate modulus on the macroscopic mechanical properties, pore structures and microstructures of geopolymer foaming materials were studied. The experimental results showed that when the activator content was 21% (wt.) and the modulus of sodium silicate was 1, the specimen exhibited the best performance. The compressive strength of the specimen reached 2.18 MPa at 28 d, the porosity was 63.07%, and the average pore sizes of macroscopic pores were 920 μm. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS) analysis showed that when the content of activator was 21% and the modulus of sodium silicate was 1, the reaction grade of the system was the highest, reached 55.12%, meanwhile the main product Sodium silicate hydrate (N-A-S-H) gel produced the largest amount. The fractal dimension calculations showed that the spatial complexity of a specimen with large pores was greater than that of a specimen with small pores. This study can provide a basis for the design of geopolymer foaming materials with high proportion of fly ash and high porosity.