New model for natural carbonation prediction in reinforced concrete – concept and validation on chemical admixtures

Abstract: A new model for prediction of natural carbonation in reinforced concrete structures has recently been developed and presented in (Ekolu, 2018). For brevity, the model is referred to as ESS model. It employs concrete strength as the primary property depicting core carbonation behaviour of any given concrete while other factors induce secondary influence. Chemical admixtures in concrete significantly influence concrete behaviour. Using experimental data from the literature (Dan-Herrera et al., 2015), the model behaviour is verified under use of various types of chemical admixtures of concrete including internal curing, shrinkage reducing, viscosity modifying, and high range water reducing admixtures. Class F fly ash was also used in the concrete mixtures as a supplementary cementitious material. Verification results show the model predictions to be meaningful and consistent with robustness

Mathematical modeling of the pessimum action of chlorides on the Extent of delayed ettringite formation. Part 1: formulation

Abstract: Internal sulfate ions are the main cause of delayed ettringite formation, which leads to expansive damage of cement concretes. On the other hand, the exposure of cementitious systems to chloride ions causes corrosion of reinforcement bars that finally seriously shortens the service life of reinforced concrete structures. It has been found that, when concomitantly present in cementitious systems, chlorides interact with sulfate ions. Particularly, for a given concentration of sulfate ions present, low to moderate concentrations of chloride ions bring about serious deterioration of concretes due to high amount of ettringite formed, while higher contents of chlorides tend to reduce and even completely eliminate ettringite formation. The objective of the present project is to mathematically formulate the interaction between chloride and sulfate ions in cementitious materials. Such knowledge is useful for accurate consideration of the action of chlorides on concretes used in sea environments, thus predicting more precisely their service life (this being a far important step in design). Kinetic analysis with pure reactants involved in individual steps implicated in the whole mechanism that pertain to the formation of, or destruction of ettringite in saline solution can be made. Monitoring each of these chemical reactions may help in establishing reaction rate equations, from which rate constants may be obtained. In the whole mechanism, it then possible to express reaction rates of individual compounds involved, chlorides ions and ettringite included. This finally gives a system of partial differential equations of compound concentrations as a function of time, one solution of which can be put in a form of an equation of concentration of ettringite formed as a function of concentration of chlorides. This article is the first part of a three-part study: In this part is formulated the approach (from reaction kinetics theory) followed for establishment of the mathematical equation; in the second part will be presented and discussed lab kinetics results and their use down to the mathematical model; the third and last part deals with applicability and limitations of the developed model, assessed in terms of expansion observed on steam-cured mortars made with cements used in the South African building industry

Strength and durability effect of modified zeolite additive on concrete properties

Abstract:This paper presents an investigation into the effect of a modified zeolite additive (PWC) on strength and durability behaviour of concrete. The additive, a blend of selected alkaloids and zeolite, is commercially available and effectively used in soil stabilization for road construction. However, its influence on behaviour and performance for use in concrete has not been explored. In this research, concrete samples were prepared by incorporating PWC and/or 30% fly ash in the mixes. PWC was used in proportions of 0.6 %, 0.8%, and 1.0 % of the weight of cement. Concretes or mortars of 0.5 water/cementitious ratio were subjected to split tensile strength, compressive strength, oxygen permeability, sorptivity, and porosity tests. Hydration behaviour of the cementitious systems and the pozzolanic effect of PWC additive were studied using differential thermal analysis and thermogravimetric analysis. Results show that PWC is effective in improving concrete strength when used at optimum proportions found to be between 0.6 % and 1.0 %. Sorptivity property of concrete was improved with use of PWC regardless of its proportion in the mix, while permeability of concrete improved only in the presence of 30 % fly ash

Modification in clay concrete properties during fluid flow permeability measurement

Abstract: In this paper, two methods consisting of triaxial water permeability and water penetration were used to evaluate changes occurring in the pores of clay concretes, during the tests. Triaxial permeability is generally used for concrete with higher permeability while concrete with very low permeability are suited for the penetration method. Clay concrete specimens of 0 to 40% clay content were used in the study. The concrete mixes had water-to-cementitious ratios (w/cc) of 0.70, 0.75, 0.80, 0.85, and the cementitious content 380 and 450 Kg/m3. Results show that concrete gains moisture during wetting at a much faster rate than it loses during subsequent drying. This could be explained by the contribution of suction pressure created upon drying. When water penetration pressure is applied, more water is driven into pore space than could be responsible for changing the network of the voids. Pore structure during drying may certainly be different in size and shape than its form during wetting, leading to a consequent effect on the permeability of the clay concretes. The modification could be one reason that the moisture gain percentage in clay concretes was higher than in normal concretes

Effect of mix parameters on strength of geopolymer mortars - experimental study

Abstract: In this article, an investigation is reported on development of strength in South African fly ash (FA) – based geopolymer mixtures. Locally available Class F, FA from one of the coal power stations was used in the investigation. The alkali-activator used consisted of sodium silicate (SS) and sodium hydroxide (SH) mixed in varied ratios of 1.0, 1.5, 2.0, 2.5 and 3.0 SS to SH. The SS of silicate modulus = 2.5 was used but the SH concentration in the activator was varied to 10, 12, 14M NaOH. Mortars of 2.25 aggregate/binder ratio were used to prepare 50 mm cubes. In preparing mortar mixtures, the liquid to solids (L/S) ratios were varied to L/S = 0.3, 0.4, 0.5 and 0.6. Mortar cubes were cast and cured at 80oC for 7 days then tested for compressive strength. It was found that all three parameters consisting of SS/SH ratio of the activator, concentration of NaOH used in the activator and the L/S ratio, showed significant influence upon compressive strength development. The optimum strength of the geopolymer mortar mixtures was obtained at SS/SH = 2.0, 12M NaOH concentration ad L/S = 0.5

Enhancing the reactivity of aluminosilicate materials towards geopolymer synthesis

Abstract: Geopolymers are alternative materials to portland cement, obtained by alkaline activation of aluminosilicates. They exhibit excellent properties and a wide range of potential applications in the field of civil engineering. Several natural aluminosilicates and industrial by-products can be used for geopolymer synthesis, but a lot of starting materials have the disadvantage of poor reactivity and low strength development. This paper presents a comprehensive review of the main methods used to alter the reactivity of aluminosilicate materials for geopolymer synthesis, as reported recently in the literature. The methods consist of mechanical, thermal, physical separation and chemical activation, of which mechanical activation is the most commonly employed technique. The reactivity of the activated aluminosilicate materials is mainly related to the activation method and the treatment parameters. Chemical activation by alkaline fusion is a promising method allowing preparation of one-part geopolymer materials, an alternative class of geopolymeric binders. However, the resulting alkaline-fused geopolymer products are vulnerable to attack by excessive alkalis

A concrete reactive barrier for acid mine drainage treatment

Abstract: Pervious concrete was investigated for potential use as a permeable reactive barrier (PRB) for treatment of acid mine drainage (AMD). Pervious concrete mixtures of varied water-cement ratios = 0.50, 0.40, 0.35, 0.30, 0.27 and cement contents = 300, 360, 380, 400 kg/m3 were prepared. Dolomite and granite aggregates types of 9.5 mm size were employed. Tests done were density, compressive strength, porosity. Water treatment was determined by analysis of the influent and effluent AMD after passage through the pervious concrete. It was found that a filter thickness of at least 500 mm was required to increase the pH of acidic mine water from 2.8 pH to 5 - 7 pH value, and corresponding reduction in electrical conductivity. When used in the filter mix design, the granite aggregate gave better treatment performance compared to the dolomite aggregate. The concrete PRB treatment led to effective removal of major metals from the AMD. The treatment reduced the metals in the AMD by 30% SO4, 99% Fe, 50-83% Mn, 85% Ca, 30% TDS. There was, however, a noticeable increase in magnesium concentration in the water effluent by 49-66% Mg. These results are short-term tests and further work is underway on the system’s life expectancy

Quality of water recovered by treating acid mine drainage using pervious concrete adsorbent

Abstract : In this paper, a batch experiment was conducted to evaluate the water quality obtained from using pervious concrete (PERVC) technology to treat acid mine drainage (AMD). The study proposes an innovative application of PERVC as a permeable reactive barrier liner in evaporation ponds. The effectiveness of PERVC adsorbent in removing heavy metals was compared with that of zero-valent iron (ZVI) of particles sizes 1.0 to 1.8 mm. The AMD used in the study was obtained from abandoned gold and coal mines. PERVC mixtures consisted of granite aggregate and ordinary portland cement CEM I 52.5R (CEM I) or CEM I containing Class F 30% fly ash (30%FA) as a cement replacement material. ZVI was prepared from a mixture of silica sand and iron grit of specific sizes. PERVC and ZVI media were used to conduct batch reactor tests with AMD, for a period of 43 days at a ratio of one litre of reactive material to three litres of AMD. The quality of treated AMD was compared against effluent discharge standards. The contaminants Al, Fe and Zn were effectively removed by both PERVC and ZVI. Also, both adsorbents reduced Ni, Co and Cu to levels below those measured in raw AMD. However, PERVC was more effective in removing Mn and Mg while ZVI was ineffective. Although PERVC removed more heavy metals and with greater efficiency than ZVI, the PERVC – treated water showed high pH levels and exhibited elevated Cr6+ concentrations, owing to leaching from the cement and fly ash materials used in PERVC mixtures

Disintegration of concrete construction induced by acid mine drainage attack

Abstract: In this paper, microanalytical investigation was conducted on disintegrated field concrete that had been used to construct a weir within a coal mine in South Africa. The concrete was in contact with polluted mine water, commonly referred to as acid mine drainage (AMD). Accordingly, the concrete weir had been exposed to dynamic conditions associated with flowing AMD. Investigations were conducted by optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometry (EDX), and X-ray diffraction (XRD). The field concrete samples examined consisted of soft broken concrete chunks and whitish powdery substance that had crystallized and formed a surface coating on the widely cracked locations of the deteriorated concrete. No evidence of sulphate attack or pyrite oxidation was found in the investigation; acid attack mechanism was diagnosed

Evaluation and microanalytical study of ZVI /zeolite substrate mixtures for treating acid mine drainage using reactive barriers - removal mechanisms

Abstract: Batch and column experiments were performed to evaluate contaminant removal from acid mine drainage (AMD) using volcanic ash zeolite (VA) with or without zero-valent iron (ZVI) media. Two types of AMD were used i.e. WZ with pH of 2.43 taken from goldfields and TDB with pH of 2.93 collected from coalfields. It was found that VA substrate performed similarly or better than ZVI reactive media whose pH reached 7.0 to 8.5. VA was effective in removing heavy metals despite attaining a relatively lower maximum pH of 5.5. Metals Al, Fe, Zn were generally completely removed from both types of AMD by both the VA and ZVI substrates and their 50:50 mixtures. Removal of Mg and Mn was influenced by the type of AMD. Generally, the substrate comprising 50:50 ZVI/VA mixture was found to treat both AMD types fairly consistently, achieving complete removal of major elements in TDB and a majority of other elements in WZ..