Performance characteristics of lightweight aggregate concrete containing natural pozzolan

Pozzolans are materials of natural or artificial origin that are not cementitious by themselves, but form a hydraulic cement when mixed with lime hydrate due to their aluminosiliceous composition. The reaction of pozzolan with lime already existing in cement or liberated during the hydration process modifies some properties of the cements and the resulting concrete.     

Influence of spent catalyst used for catalytic cracking in a fluidized bed on sulphate corrosion of cement mortars: I. Na2SO4 medium

The aim of the work was to determine the effect of spent catalytic cracking in a fluidized bed (FBCC), catalyst used for as a partial substitute for cement or sand, and of sodium sulphate solutions of concentration 16,000 or 52,000 mg SO₄²⁻/dm³ on the mechanism of sulphate corrosion and on expansion of mortars compacted according to prENV 196-10, as well as on mechanical strength of mortars compacted according to the Polish Standard PN-EN 196-1. The observations indicate that the sulphate corrosion of the mortars proceeds via at least two different mechanisms depending on the concentration of sodium sulphate solution and on the concentration and form of pozzolanic additive used. The extent of corrosion damage depends mainly on the concentration of the aggressive solution and on the degree of compaction of the mortar. No unequivocal correlation was found between the expansion of mortars compacted according to prENV 196-10 and the mechanical strength of mortars compacted according to PN-EN 196-1. The greatest expansion was observed in mortars, which did not exhibit the lowest compressive strength. Therefore, it is questionable to use the measure of elongation, particularly in poorly compacted mortar samples, as the sole standard method for determining resistance to sulphate corrosion in sodium sulphate media.     

Lime-pozzolana mortars in Roman catacombs: composition, structures and restoration

Analyses of microsamples collected from Roman catacombs and samples of lime-pozzolana mortars hardened in the laboratory display higher contents in carbonated binder than other subaerial Roman monuments. The measured environmental data inside the Saint Callistus and Domitilla catacombs show a constant temperature of 15-17 °C, a high CO₂ content (1700 to 3500 ppm) and a relative humidity close to 100%. These conditions and particularly the high CO₂ concentration speed-up the lime calcitization roughly by 500% and reduce the cationic diffusion to form hydrous calcium aluminosilicates. The structure of Roman catacomb mortars shows (i) coarser aggregates and thicker beds on the inside, (ii) thin, smoothed, light and fine-grained external surfaces with low content of aggregates and (iii) paintings and frescoes on the outside. The observed high porosity of the mortars can be attributed to cracking after drying linked with the high binder content. Hardened lime lumps inside the binder denote low water/mortar ratios for slaking. The aggregate tephra pyroclasts rich in aluminosilicate phases with accessorial amounts of Ba, Sr, Rb, Cu and Pb were analysed through X-ray diffraction (XRD), electron microprobe analysis (EMPA) and also by environmental scanning electron microscopy (ESEM) to identify the size and distribution of porosity. Results support procedures using local materials, special mortars and classic techniques for restoration purposes in hypogeal backgrounds.     

Use of ground clay brick as a pozzolanic material to reduce the alkali-silica reaction

The objective of this experimental study was to use ground clay brick (GCB) as a pozzolanic material to minimize the alkali-silica reaction expansion. Two different types of clay bricks were finely ground and their activity indices were determined. ASTM accelerated mortar bar tests were performed to investigate the effect of GCB when used to replace cement mass. The microstructure of the mortar was investigated using scanning electron microscopy (SEM). The results showed that the GCBs meet the strength activity requirements of ASTM. In addition, the GCBs were found to be effective in suppressing the alkali-silica reaction expansion. The expansion decreased as the amount of GCBs in the mortar increased.     

Supplementary cementing materials in concrete: Part I: efficiency and design

Many solid industrial by-products such as siliceous and aluminous materials (fly ash, silica fume, slags, etc.) as well as some natural pozzolanic materials (volcanic tuffs, diatomaceous earth, etc.) may be characterized as supplementary cementing materials (SCM) as they exhibit cementitious and/or pozzolanic properties. Due to plenty of these materials and their large variations on physical and chemical composition, the development of a general design for their use in concrete is required. In this work, the concept of an efficiency factor is applied as a measure of the relative performance of SCM compared with Portland cement. Artificial materials of various compositions and some natural pozzolans were studied. Compressive strength and accelerated chloride penetration tests were performed. With regard to these characteristics, efficiency factors for these materials were calculated. A mix design strategy to fulfil any requirements for concrete strength and service lifetime was developed and it enables concrete performance to be accurately predicted.     

The effect of zinc oxide additions on the performance of calcined sodium montmorillonite and illite shale supplementary cementitious materials

The objectives of this study were to use activation treatments on sodium montmorillonite and illite shale, to alter early hydration or later pozzolanic reactivity when used as supplementary cementitious materials (SCMs). For comparison purposes, treatment methods were also applied to the highly reactive pozzolan, metakaolin, and the inert filler, quartz. Activation treatment strategies included the addition of 0.15 wt% zinc oxide and the use of thermal treatments to the SCMs at temperatures of 650 °C, 830 °C and 930 °C. The use of zinc oxide additions increased the early hydration rate of SCM-containing pastes, yet introduced a chemical retardation and negatively impacted early compressive strengths. Moreover, the results suggest that retardation was inversely correlated with the pozzolanic reactivity of the SCM used. Thermal treatment methods were effective at influencing the SCM pozzolanic reactivity, with montmorillonite calcined at 830 °C and illite calcined at 930 °C behaving as late-reacting pozzolans. SCMs calcined at these temperatures resulted in higher 90 day compressive strengths compared to mortars containing the quartz filler. Overall, this study provides insight into different strategies that maybe used to enhance the reactivity of impure calcined clays in order to facilitate their acceptance into the concrete industry.     

Use of ultrafine rice husk ash with high-carbon content as pozzolan in high performance concrete

Rice husk ash (RHA) has been generated in large quantities in rice producing countries. This by-product can contain non-crystalline silica and thus has a high potential to be used as cement replacement in mortar and concrete. However, as the RHA produced by uncontrolled burning conditions usually contains high-carbon content in its composition, the pozzolanic activity of the ash and the rheology of mortar or concrete can be adversely affected. In this paper the influence of different grinding times in a vibratory mill, operating in dry open-circuit, on the particle size distribution, BET specific surface area and pozzolanic activity of the RHA is studied, in order to improve RHA’s performance. In addition, four high-performance concretes were produced with 0%, 10%, 15%, and 20% of the cement (by mass) replaced by ultrafine RHA. For these mixtures, rheological, mechanical and durability tests were performed. For all levels of cement replacement, especially for the 20%, the ultra-fine RHA concretes achieved superior performance in the mechanical and durability tests compared with the reference mixture. The workability of the concrete, however, was reduced with the increase of cement replacement by RHA.     

Effect of large amounts of natural pozzolan addition on properties of blended cements

In this study, the effects of 35, 45, and 55 wt.% natural pozzolan addition on the properties of blended cement pastes and mortars were investigated. Blended cements with 450 m²/kg Blaine fineness were produced from a Turkish volcanic tuff in a laboratory mill by intergrinding portland cement clinker, natural pozzolan, and gypsum. The cements were tested for particle size distribution, setting time, heat of hydration, compressive strength, alkali-silica activity, and sulfate resistance. Cement pastes were tested by TGA for Ca(OH)₂ content and by XRD for the crystalline hydration products. The compressive strength of the mortars made with blended cements containing large amounts of natural pozzolan was lower than that of the portland cement at all tested ages up to 91 days. Blended cements containing large amounts of pozzolan exhibited much less expansion with respect to portland cement in accelerated alkali-silica test and in a 36-week sulfate immersion test.     

The origin of the pozzolanic activity of calcined clay minerals: A comparison between kaolinite, illite and montmorillonite

This paper investigates the decomposition of three clayey structures (kaolinite, illite and montmorillonite) when thermally treated at 600 °C and 800 °C and the effect of this treatment on their pozzolanic activity in cementitious materials. Raw and calcined clay minerals were characterized by the XRF, XRD, ²⁷Al NMR, DTG and BET techniques. Cement pastes and mortars were produced with a 30% substitution by calcined clay minerals. The pozzolanic activity and the degree of hydration of the clinker component were monitored on pastes using DTG and BSE-IA, respectively. Compressive strength and sorptivity properties were assessed on standard mortars. It was shown that kaolinite, due to the amount and location of OH groups in its structure, has a different decomposition process than illite or montmorillonite, which results in an important loss of crystallinity. This explains its enhanced pozzolanic activity compared to other calcined clay–cement blends.     

Strength and durability aspects of calcined kaolin-blended Portland cement mortar and concrete

Economic and sustainability arguments require carefully assessing the potentialities of indigenous resources for the production of mortar and concrete for the construction industry. In Vietnam, significant efforts should be bestowed on urban development, coastal protection and harbour construction works. In a joint Vietnamese-Dutch co-operation program, the practical use for this purpose of relevant resources in Northern Vietnam is assessed experimentally. This paper concentrates on kaolin, which is widely available in this region. The key issues this paper is dealing with are the effects of partial replacement of Portland cement by calcined kaolin in mortar and concrete on compressive strength as well as on durability characteristics of mortar and concrete mixes pertinent to the coastal environment. Workability measures are also mentioned. Data are therefore presented on compressive strength development over a maximum curing period of 180 days of mixes in which the water to binder ratio was varied between 0.40 and 0.53. Moreover, partial replacement was considered in the range from 0% to 30% by weight. The results of this study render possible the assessment of optimum replacement percentages of Portland cement by calcined kaolin, and the associated strength gain. Additionally, this paper reports on the performance aspects of similarly blended mortar and concrete specimens stored for a period of one year in a low concentration of a sodium sulfate solution. It could be concluded that a strength gain due to blending will be accompanied by improved durability in this environment.