Alkali Activated Cement Concrete Research Articles

Lifecycle assessment of alkali activated cement concrete

Climate change is one of the most important environmental problems that our planet Earth is facing. This is due to the increased emission of greenhouse gases such as carbon dioxide. Concrete, the most consumed material in the construction industry is reported to be responsible for about 8% of worldwide carbon dioxide emissions. The manufacturing of ordinary Portland cement is both resource and energy-intensive and is accountable for 1.35 billion tons of carbon dioxide emitted annually. Hence potential alternative to Portland cement widely recognized is the adoption of alkali-activated cement. Alkali-activated cement commonly utilizes industrial by-products such as fly ash, GGBS, etc. along with alkali activators such as sodium silicate and sodium hydroxide. The literature review indicates that the environmental impact due to the usage of Portland cement can be reduced by the adoption of alkali-activated cement. However, the manufacture of alkali activators is likely to contribute to the emission to the environment. In addition, the heat curing commonly adopted during the production of concrete to activate the alkalis might also have a bearing. Hence a comparative study using the lifecycle assessment (LCA) method is carried out to assess the impact due to the production of alkali-activated cement concrete using supplementary cementitious materials (SCM) fly ash and GGBS with varying proportions of alkali activators (sodium silicate and sodium hydroxide). Data is extracted from the published literature corresponding to two different compressive strength ranges of OPC concrete and alkali-activated cement concretes that have utilized four varying proportions of alkali activator ratios. It is then analyzed by the ‘cradle to gate’ approach using LCA software SimaPro. The impact assessment is done using the ReCiPe 2016 method. A comparison of results and their interpretation is done based on its compressive strength ranges, the alkali activator ratios, and the effect due to change in the SCMs utilized.

Calorimetric Studies of Alkali-Activated Blast-Furnace Slag Cements at Early Hydration Processes in the Temperature Range of 20-80 °C.

In the hydration process of inorganic cements, the analysis of calorimetric measurements is one of the possible ways to better understand hydration processes and to keep these processes under control. This study contains data from the study of thermokinetic processes in alkali-activated blast-furnace slag cements compared to ordinary Portland cement (OPC). The obtained results show that, in contrast to OPC, the heat release values cannot be considered as a characteristic of the activity of alkali-activated blast-furnace slag cements. In addition, it is concluded that in the case of OPC cements, cumulative heat release is a criterion for the selection of effective curing parameters, while in the case of alkali-activated blast-furnace slag cements, a higher heat rate (which increases sharply with increasing temperature from 20 to 40 °C) is a criterion. From the point of views of thermokinetics, the rate of heat release at temperatures up to 40 °C can be a qualitative criterion that allows to choose the parameters of heat curing of alkali-activated cement concretes. By introducing a crystallo-chemical hardening accelerator, such as Portland cement clinker, into the composition of alkali-activated blast-furnace slag cements, it is possible to accelerate the processes not only in the condensation-crystallization structure formation stage, but also in the dispersion-coagulation structure formation stage. Portland cement clinker increased the efficiency of thermal curing at relatively non-high temperatures.

Mechanical Behavior and Frost-Resistance of Alkali-Activated Cement Concrete with Blended Binder at Ambient Curing Condition

Concrete is the most commonly used construction material because of its various advantages, such as versatility, familiarity, strength, and durability, and it will continue to be in demand far into the future. However, with today’s sensitivity to environmental protection, this material faces unprecedented challenges because of its high greenhouse gas emissions, mainly during cement production. This paper investigates one of the promising cement replacement materials, alkali-activated cement (AAC) concrete. Being produced mainly from byproduct materials and having a comparable structural performance to conventional concrete, AAC concrete can transform the construction industry. Mechanical properties such as compressive and flexural strength and the relationship between them are studied. Different source materials such as fly ash (FA), ground granulated blast furnace slag (GGBS), silica fume (SF), and Metakaolin (MK) are used. The effect of the source materials and the activator solutions on the concrete performance is studied. Furthermore, the freeze-thaw resistance of the concrete is studied. The study results showed that the behavior of AAC depends highly on the source material combinations and type used. The effect of the alkaline solution is also dependent on the source material used. Mixes with higher GGBS content showed the highest strength, while mixes with MK showed the highest flexural strength. The freeze-thaw test results showed that proper design of AAC concrete with lower water content is critical to achieving a good resistance.

Alkali-activated slag concrete with paper industry waste.

Pulp and paper manufacturing and recycling industries are a resource-intensive sector, generating 25–40% of the annual municipal solid waste worldwide. Waste includes abundant volumes of paper sludge, as well as the product of its incineration, namely paper sludge ash. These two waste materials are both predominantly landfilled. There is thus a drive for additional valorisation routes for these materials. This short communication focuses on the potential use of paper sludge ash in alkali-activated cement concrete; this type of concrete was estimated to potentially reduce CO2 emissions by up to 5–6 times, while it can also incorporate waste materials or industrial by-products in its composition. The paper presents a laboratory study assessing the feasibility of structural alkali-activated cement concrete with ground granulated blastfurnace slag (a by-product of steel production) and paper sludge ash. Paper sludge ash is used mainly as a source of Ca(OH)2 in the alkaline activator solution, and secondly as an additional source of aluminosilicates. A number of factors potentially affecting the activation process and the resulting concrete quality were investigated, including different dosage of activators, curing conditions and curing time. Mixes with paper sludge ash in the activator system developed high early concrete strengths at ambient temperatures and maintained adequate strengths for structural concrete. Further mix optimisation and mechanical and durability testing, accompanied by material characterisation, are required to establish the advantages of using this waste material in structural alkali-activated cement concrete.

Parametric study on bond of GFRP bars in alkali-activated cement concrete

Bond behaviour plays an important role in the design and performance of reinforced-concrete structures. In this study, finite-element modelling is used to perform a parametric study. The bond between the glass fibre-reinforced polymer (GFRP) bar and alkali-activated cement concrete is modelled by surface-based cohesive behaviour. The accuracy of the model is validated by comparing model predictions with experimental results. The effect of concrete cover, bar diameter, compressive strength, lead length, embedment length and GFRP elastic modulus on bond behaviour is investigated. Each of these parameters are varied based on a range of applicable values to study their influence on bond behaviour. The parametric study showed that bond behaviour is mainly affected by concrete cover, bar diameter, embedment length and the compressive strength of the concrete. The effect of the elastic modulus of the GFRP bar is not as pronounced as that of the other parameters, while the influence of lead length can be avoided by providing enough unbonded length at the loaded end. The parametric study is further used to calibrate a well-known bond equation and develop a new regression equation for predicting the maximum bond stress. The predicted results from these equations showed a good agreement with the experimental results as well as those of the finite-element model.

Bond properties of steel and sand-coated GFRP bars in Alkali activated cement concrete

The bond performance of glass fibre reinforced polymer (GFRP) bars and that of steel bars embedded in Alkali Activated Cement (AAC) concrete are analysed and compared using pull-out specimens. The bond failure modes, the average bond strength and the free end bond stress-slip curves are used for comparison. Tepfer’s concrete ring model is used to further analyse the splitting failure in ribbed steel bar and GFRP bar specimens. The angle the bond forces make with the bar axis was calculated and used for comparing bond behaviour of ribbed steel bar and GFRP bars in AAC concrete. The results showed that bond failure mode plays a significant role in the comparison of the average bond stress of the specimens at failure. In case of pull-out failure mode, specimens with ribbed steel bars showed a higher bond strength while specimens with GFRP bars showed a higher bond stress in case of splitting failure mode. Comparison of the bond stress-slip curves of ribbed steel bars and GFRP bars depicted that the constant bond stress region at the peak is much smaller in case of GFRP bars than ribbed steel bars indicating a basic bond mechanism difference in GFRP and ribbed steel bars.

Alkali-aggregate reaction in alkali-activated cement concretes

The paper discloses the results of a study on structure formation processes in the interfacial transition zone “cement stone − aggregate” of the alkali-activated cement concretes. The results indicate the importance of the content of Al2O3 in the cement and aggregate. Together with components that are able to interact actively with alkalis in the presence of reactive SiO2, the processes taking place during the alkali-aggregate reaction can have weather destructive or constructive effect. The constructive effect is caused by incorporating the corrosion products into alkaline aluminosilicate compounds, thus retarding the internal corrosion of the concrete and providing the higher durability even in case of alkali-activated cements and alkali-susceptible aggregates with relatively high alkali content.

Efficiency of the Alkali-Activated Cement Concretes for Sea Construction

There are shown studies on development of the alkali-activated slag cements and concretes using Chinese raw materials. Such cements and concretes are able to provide high performance characteristics of materials and constructions under exposure of sea climate. The tests had been done for compliance with the requirements set in the national standards of P.R.China and were determined characteristics of strength, wet conditions shrinkage, freeze-thaw resistance, corrosion and weather resistance, biodegradability, water permeability, bond strength between concrete and reinforcement. The obtained positive results of tests allowed to recommend them for a pilot-scale production of the developed materials in sea construction for protection of the sea coastal line in the P.R.China.

Plastic shrinkage cracking and bleeding of concrete prepared with alkali activated cement

Restrained shrinkage cracking that appears on concrete surfaces during construction, or while concrete in the curing process adversely affect the long-term durability and other performance attributes of concrete-based-infrastructures. This study investigates the effects of early-age surface cracking on restrained shrinkage concrete made of alkali activated cement. Concrete produced using this class of cement differs from ordinary Portland cement concrete in terms of surface qualities and some other properties. Bleeding which could be a main cause of plastic shrinkage cracking was measured and compared with that of Portland cement. Rheological tests were performed to gain further insight into factors that may influence the early-age performance of the cement paste. Portland cement was used as control for comparative assessment of the alkali activated cement performance. Experimental test showed that alkali activated cement improved the characteristics of the newly produced concrete. The concrete prepared using alkali activated cement has low tendency to bleeding and higher resistance to plastic shrinkage, also, the viscosity and yield stress of the alkali activated cement paste were relatively higher when compared to those of ordinary Portland cement paste. Test data collected from rheological and bleeding tests on the alkali activated cement concrete were used to explain its desired resistance to plastic shrinkage cracking.

Properties of ambient cured blended alkali activated cement concrete

This paper presents results of the development and strength properties of ambient-cured alkali activated geopolymer concrete (GPC). The study looks at the strength properties, such as compressive strength, splitting tensile strength, and elastic modulus of such concretes and its dependency on various parameters. The parameters studied in this work are the type and proportions of pre-cursor materials, type of activator and their respective ratios and the curing time. Two types of pre-cursor material; low calcium fly ash (FA) and ground granulated blast furnace slag (GGBFS) were activated using different proportions of sodium silicate and sodium hydroxide solutions. The results indicate that ambient cured geopolymer concrete can be manufactured to match strength properties of ordinary Portland cement concrete (OPC). The strength properties of GPC are dependent on the type and ratio of activator and the proportion of GGBFS used. Increasing the percentage of GGBFS increased the compressive and tensile strengths, while reducing the setting time of the mix. The effect of GGBFS on strength was more pronounced in mixes that contained sodium silicate as activator solution. Unlike OPC, ambient-cured GPC containing sodium silicate gain most of their strength in the first 7 days and there is no change in strength thereafter. However, GPC mixes not containing sodium silicate only achieve a fraction of their strength at 7 days and extended curing is required for such concretes to gain full strength. The results also indicate that the elastic modulus values of GPC mixes without sodium silicate are comparable to OPC while mixes with sodium silicate have elastic modulus values much lower than ordinary concrete.

Bond behaviour of GFRP reinforcement in alkali activated cement concrete

Bond plays a key role in the performance of reinforced concrete structures. Glass fiber reinforced polymer (GFRP) reinforcing bar and alkali activated cement (AAC) concrete are promising alternative construction materials for steel bars and Ordinary Portland Cement (OPC) concretes respectively. In this study, the bond behaviour between GFRP bars, and AAC and OPC concretes is investigated by using beam-end test specimens. Sand-coated GFRP bars with 12.7mm and 15.9mm diameters and embedment lengths of six and nine times the bar diameter were used. The free end and the loaded end bond slip relationships, the failure mode and the average bond stress were used to analyse each of the specimens. Additionally, the distribution of tensile and bond stress along the embedment length was investigated by installing strain gauges along the embedment length. The results of the study indicate that the tensile and bond stress distribution along the embedment length are nonlinear, and the nonlinearity changes with the load. Finite element analysis was also performed to further study the bond stress distribution along the embedment length of the bar. From the finite element analysis, it was found that the bond stress distribution depends on the embedment length of the specimens; approaching uniform distribution as the embedment length decreases.

Bond of spliced GFRP reinforcement bars in alkali activated cement concrete

Abstract Glass Fibre Reinforced Polymer (GFRP) bars and Alkali Activated Cement (AAC) concrete are promising materials for replacing steel bars and ordinary Portland cement concrete. These materials can provide a solution to the associated corrosion and environmental issues. However, no design recommendations are available on splicing of GFRP bars in AAC concrete. This may be mainly due to insufficient data that would guide such recommendations. In this paper, the results of testing beams with spliced bars are presented mainly from consideration of the bond property between the spliced GFRP bars and the AAC concrete. The effects of splice length, compressive strength of concrete, and stirrup confinement on the bond behaviour are investigated. Ultimate moment capacity, failure modes, cracking pattern, and strain values in the bars have been determined and used in the analysis. A three-dimensional finite element model (FEM) was then developed. The FEM results show that the model accurately reproduces the experimental behaviour of splice tests with regard to load-deflection curves and failure modes. The reported results and numerical analysis are intended for understanding distribution of tensile and bond stresses in GFRP reinforced AAC concrete. These distributions were observed to be dependent on the applied load.

The Development of Alkali-activated Cement Mixtures for Fast Rehabilitation and Strengthening of Concrete Structures

This study was dedicated at design of alkali-activated (slag) cements with high-modulus soluble silicates and concretes based on them which would possess workability retention, quick and steady strength gain, high strength characteristics, etc. Specific features of mode of deformation (in tension under bending stresses) of fine aggregate alkali-activated cement concretes were studied. The results of this study showed that the modified alkali-activated cements and concretes exhibited high tensile strength in bending and high ratios of this strength to compressive strength. The results of analysis of deformation of the alkali-activated cement concretes under action of bending stresses showed that high values of crack resistance, impact resistance and increased deformability could be expected. Basic strength and deformation characteristics of the developed repair mortars and concretes after additional optimization of their constituent composition are provided and discussed.

Reuse of Waste from the South West of England in Alkali-Activated Cement Concrete

The extraction of china clay in the South West of the UK generates waste in a mass ratio of 1:9 for china clay: waste. Currently, part of the coarser waste, “stent” and sand named “china clay sand” (CCS) in this study, is used as building stone or secondary aggregate in concrete and asphalt but the finest waste fraction, called “mica” waste, is used only for the restoration of old quarries. Looking for innovative solutions for the needs of a new Eco-town in the UK, and with regard to uses commercially applicable to construction and of low environmental impact, the china clay waste is being studied as an aggregate in alkali-activated cements (AAC). Aiming to replace primary aggregates with wastes in low risk construction materials, a series of AAC concrete based on a 50% GGBS and 50% fly ash (FA) blend and an equivalent Portland concrete series were produced. In the mixes the primary aggregate was steadily replaced by forms of the waste and tests in compression showed a decreasing trend in strength accordingly. The two series of concrete follow approximately the same ratios of decrease although in absolute values the AAC series reached higher range of strengths on the 28 day compared to the Portland series. While the use of CCS did not have any negative impact, the addition of mica decreased the strength up to 25% more.

Mechanism of preventing the alkali–aggregate reaction in alkali activated cement concretes

Processes of structure formation taking place in the interfacial transition zone “cement paste – aggregate” have been studied on a variety of cement model systems. The results of this study suggest that, depending upon the contents of components capable of actively interacting with alkalis in the presence of reactive SiO2 in the cement and aggregate, the processes taking place during an alkali-aggregate reaction could be constructive or destructive in character. So-called “constructive processes” are attributed to binding the corrosion reaction products with the formation of the alkaline aluminosilicate hydrates. The results of this study have been taken as a base in developing a mechanism of preventing the alkali–aggregate reaction in the alkali activated cement concretes through the introduction of additional quantities of Al2O3-containing substances.

도로포장 보수재 활용 가능성 평가를 위한 알칼리 활성 시멘트 모르타르 기초연구

PURPOSES : This study is to evaluate the feasibility of using the alkali activated cement concrete for application of partial-depth repair in pavement. METHODS : This study analyzes the compressive strength of alkali activated cement mortar based on the changes in the amount/type/composition of binder(portland cement, fly ash, slag) and activator(NaOH, <TEX>$Na_2SiO_3$</TEX>, <TEX>$Na_2CO_3$</TEX>, <TEX>$Na_2SO_4$</TEX>). The mixture design is divided in case I of adding one kind-activator and case II of adding two kind-activators. RESULTS : The results of case I show that <TEX>$Na_2SO_4$</TEX> based mixture has superior the long-term strength when compared to other mixtures, and that <TEX>$Na_2CO_3$</TEX> based mixture has superior the early strength when compared to other mixtures. But the mixtures of case I is difficult to apply in the material for early-opening-to-traffic, because the strength of all mixtures isn't meet the criterion of traffic-opening. The results of case II show that NaOH-<TEX>$Na_2SiO_3$</TEX> based mixtures has superior the early/long-term strength when compared to NaOH-<TEX>$Na_2SiO_3$</TEX> based mixtures. In particular, the NaOH-<TEX>$Na_2SiO_3$</TEX> based some mixtures turned out to pass the reference strength(1-day) of 21MPa as required for traffic-opening. CONCLUSIONS : With these results, it could be concluded that NaOH-<TEX>$Na_2SiO_3$</TEX> based mixtures can be used as the material of pavement repair.