Effect of heat treatment on reactivity-strength of alkali-activated natural pozzolans

Natural pozzolans can be activated and condensed with sodium silicate in an alkaline environment to synthesize high performance cementitious construction materials with low environmental impact. The nature of the starting materials including mineral composition, chemical composition and crystal structure groups affects the formation of the geopolymer gel phase. In this paper, the pozzolanic activities of five natural pozzolans are studied. From XRD and XRF results, most of the raw materials contain zeolite clay minerals and have a high loss on ignition. Therefore, before use, samples were calcined at 700, 800 and 900 °C, respectively. The improvement in pozzolanic properties was studied following heat treatment including calcinations and/or elevated curing temperature by using alkali solubility and compressive strength tests. The results show that pozzolan containing sodium zeolite clinoptilolite can be used to prepare a moderate to high strength binder by heat treatment and calcinations can impart disorder hornblende as a constituent of pozzolan with no amorphous phase to prepare a moderate strength binder.     

火山灰质粉对砂浆性能与微结构的影响

为加强我国区域火山灰资源利用，为工程建设提供优质建筑原材料，选取泸定桥、泸霍两地典型火山灰弃岩资源磨制成火山灰材料，研究不同种类和粒径的火山灰质材料基本特性。采用火山灰等质量取代水泥制备火山灰-水泥砂浆，分析火山灰对水泥砂浆力学性能与微观结构的影响及机理。结果表明，随着火山灰粒径（d50）的降低，砂浆各个龄期的抗压强度和火山灰活性指数升高，但浆体的流动性降低。掺入火山灰会导致浆体的孔隙率增大和孔结构粗化。当火山灰粒径相似时，泸定桥组砂浆的28d活性指数小于泸霍组。此外，蒸汽养护可以促进各组砂浆的抗压强度和活性增长。     

Development of corn cob ash blended cement

In an attempt to convert waste product into useful material for the construction industry, this research considered the use of corn cob ash (CCA) as a pozzolan in cement production. The study investigated the chemical composition of CCA. Factory production of the CCA – blended cement was carried out by replacing 0%, 2%, 4%, 6%, 8%, 10%, 15%, 20% and 25% by weight of Ordinary Portland Cement clinker with CCA. The 0% replacement serves as the control. The results showed that CCA is a suitable material for use as a pozzolan as it satisfied the minimum requirement of combined SiO₂ and Al₂O₃ of more than 70%, which a good pozzolan for manufacture of blended cement should meet. The blended cements produced also satisfied both NIS 439:2000 and ASTM C 150 requirements especially at lower levels (<15%) of CCA percentage replacement. Based on the test results, it was concluded that CCA could be suitably used in blended cement production.     

Improving hydraulic properties of lime–rice husk ash (RHA) binders with metakaolin (MK)

To improve long-term hydraulic properties of binders from RHA and lime, 25–75% MK was added to RHA. Binders were formulated and properties were compared to that containing RHA or MK as only pozzolans. The lime–pozzolan ratio was 1:3. The properties tested after 7, 28 and 56 days were: absolute density and fineness of the binders, initial setting time, chemical and mineralogical composition of hydrated binders, flexural and compressive strengths and water absorption of mortars. The micrographs of the hardened binder pastes at 56 days permitted to evaluate the densification of different matrixes and the development of pores. From the results obtained, it was concluded that, MK increased the density of mixtures and decreased their grindability. The presence of MK decreased the SiO₂ content of binders and increases their Al₂O₃ and Fe₂O₃ contents. Calcium-silicate hydrates (CSH) gel and gehlenite (C₂ASH₈) were the main phases formed during the pozzolanic reaction in the presence of MK. No reduction in flexural and compressive strengths was observed after 28 days for binders containing MK. The mixture of 25% MK and 75% RHA which is recommended gave flexural and compressive strengths higher than binder with RHA or MK as the only pozzolan. Water absorption of mortars was less than 20%.     

Compressive strength and shrinkage of mortar containing various amounts of mineral additions

Three mineral additions largely used in cementitious materials were tested in order to follow the shrinkage behaviour for 1 year of observation when they substitute a part of cement. The tests were carried out on standardized mortars specimen where cement was replaced by 5%, 15% and 25% of limestone, 10%, 20%, 30% of natural pozzolan and 10%, 30% and 50% of slag. The substitution of cement by 10%, 20% and 30% of limestone powder, natural pozzolan and slag respectively involves an optimal improvement of compressive strength of mortar. The separate quantification of the autogeneous and drying shrinkage development shows the effective contribution of each addition on microstructure modification and of the additional hydrates production. The microstructure was improved in the presence of limestone and of a moderate rate of slag, whereas it remains normal with natural pozzolan. The replacement rate of an active addition lower than 10% led to an additional hydrates production. This overproduction which accompanies the autogeneous shrinkage is more pronounced when cement is largely replaced by limestone. The evolutions of strength and shrinkage of mortars follow the same tendency from where it is easier to find a linear relationship giving the shrinkage deformation according to the compressive strength.     

Deicer salt-scaling resistance of non-air-entrained roller-compacted concrete pavements

This paper investigates the effects of various pozzolans on the fresh and hardened performance of non-air-entraining, roller-compacted concrete pavement. Deicer salt-scaling resistance of the specimens was evaluated for freeze–thaw cycle conditions with either a long or a short freezing period. Water–cement ratios were 0.36 and 0.43. Natural pozzolan, fly ash and silica fume were used as partial replacements for the cement, by 8%, 20%, and 20%, respectively. The results showed that the pozzolans generally improved the consistency of the mixtures compared with reference samples. Introducing silica fume improved compressive strength and, particularly, salt-scaling resistance of the specimens. Salt scaling decreased as the duration of the freeze period in the freeze–thaw cycle decreased.     

苏丹谢里克水电站火山灰替换粉煤灰可行性的分析与探讨

在苏丹谢里克水电站前期筹备过程中，对苏丹当地的四种天然火山灰进行了专门的室内检测试验．包括：火山灰掺量对活性指数的影响试验，火山灰抑制骨料碱活性效果试验，火山灰对胶凝材料水化热和混凝土强度的影响实验，对火山灰替换粉煤灰的可行性进行了专门的分析研究。     

Effects of industrial by-product based geopolymers on the strength development of a soft soil

Portland cement is traditionally used as a binder in ground improvement projects on soft soil foundations. The use of cement in ground improvement projects, however, is fraught with both, financial and environmental concerns due to its relatively high cost, the use of natural resources and the high carbon footprint from cement production. Attempts are being made to find alternative environmentally friendly binders with a low carbon footprint using industrial by-products such as fly ash (FA) and slag (S). Using waste by-products such as FA and S to produce geopolymer binders, as novel green cementitious materials, may provide an environmentally friendly and effective ground improvement option. In this study, the effect of adding geopolymers to a soft soil was investigated for usage in deep soil mixing (DSM) applications. The soil was a soft marine clay known as Coode Island Silt (CIS). Different combinations of FA and S with six combinations of sodium and potassium based liquid alkaline activators (L) were added to the soil to study the effects on its engineering and chemical properties. These changes were evaluated via an unconfined compression strength (UCS) test, scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDS) tests. The tests were conducted after 3, 7, 14 and 28?days of curing. Based on the results, the important role of L in strength development was studied, and the combination of 30% NaOH with 70% Na₂SiO₃ was found to achieve the highest strengths. Furthermore, increasing the S content was found to result in significant improvements in strength. The excellent correlation between strength and stiffness shown in the results are expected to help in the development of relationships for strength prediction of these green binders in geotechnical applications. This study shows that FA and S based geopolymers can be used as sustainable binders in DSM projects, with significant environmental benefits.     

Use of natural and thermally activated porphyrite in cement production

The aim of the present study was to investigate the use of porphyrite in the production of Portland cement. Natural and thermally activated porphyrites were used as a clay raw material and an activator, respectively, at 0, 10, 20, 30, 40 and 50 wt% in order to assess their effects on the cement properties. According to the test results, the compressive strength of the specimens decreased with increasing natural porphyrite content in various curing periods. However, the compressive strength of cement produced with 10 wt% porphyrite (activator) activated at 650 °C for 30 min showed a higher value (56 MPa in TPC-6) than cement without activator (51 MPa in RPC-2). Due to thermal activation, porphyrite activator containing a glass phase possesses an enhanced reactivity during clinker hydration that intensifies the synthesis of hydrosilicates and improves compressive strength accordingly. The X-ray diffraction analysis confirmed an intensive formation of Portland cement minerals such as C₃S, β-C₂S, C₃A and C₄AF. The addition of thermally activated porphyrite has also led to an improvement of the rheological behavior, stability to expansion, increase in setting time and decrease in specific surface area of cement. As prepared cement composites and concretes with improved properties meet the requirements of State Standards 310-86 and 10181-81 for Portland cement and concrete, respectively. The findings in this report indicate that porphyrite can be utilized both as a raw material and an activator in the production of cement.     

工业和生活废弃物的利用与混凝土的使用安全

随着天然砂石自然资源的日渐枯竭,工业废弃物、建筑垃圾和污泥等加工后作为人造粗、细集料用于混凝土,掺入水泥中作混合材,以及作为混凝土的掺合料和矿物外加剂,促进了资源再生利用和节能减排工作.加强废弃物的成分研究,规定其有害物质限量,防止利用不当,造成对环境的二次污染,影响混凝土的耐久性与使用寿命,应是当前研究的重点.     

Corrosion of steel rebar embedded in natural pozzolan based mortars exposed to chlorides

Corrosion of steel is considered the most important durability problem of reinforced concrete. The application of supplementary cementitious materials has been proposed in order to mitigate this durability problem, reduce the production costs and control the emission of greenhouse gases. Mexico is rich in volcanic areas from which natural pozzolanic materials can be obtained. This paper examines the use of such natural pozzolans as a partial substitute of normal portland cement in reinforced mortar specimens. Compositions with substitution levels of 0%, 10% and 20% by mass of normal Portland cement of natural pozzolanas were investigated. The specimens were exposed to penetration of chlorides. Compressive strength, corrosion potential, polarization resistance, electrical resistivity, and chloride content of the mortars were determined in order to characterize the physical, mechanical, electrical, and electrochemical behavior of the mortar as well as the embedded steel. It was found that the use of pozzolan has resulted in a significant increase in mortar resistivity and corrosion initiation time for the same cover depth, and as a result, decreases the rate of corrosion of rebars once corrosion was initiated.     

Traditional organic additives improve lime mortars: New old materials for restoration and building natural stone fabrics

Portland cement has low chemical and physical affinity for traditional building materials. This hinders the restoration of historical buildings and modern rustic architecture where natural stone is used. Lime mortars used in construction since c.12,000 B.C. were recovered, and attempts were made to enhance their properties. Various additives were selected on the basis of their properties and historical use. These include polysaccharides (opuntia, also known as nopal used either as a powder or as mucilage), proteins (animal glue and casein) and fatty acids (olive oil). Six types of lime mortar were formulated and characterized. Compressive strength, water-resistance, carbonation speed, porosity, texture and mineral composition were measured. We propose new lime mortars with added value, i.e. increased mechanical properties and water-resistance, different carbonation speeds, and different textures. They are all compatible with traditional building materials, so they can be used in the restoration of architectural heritage and modern architecture where natural stone is used.     

Effect of chemical additives on the consolidation behaviours of mini-PVD unit cells –from macro to micro

The mechanism of adding NaCl, CaCl₂ and small amounts of cement/lime (1–4% by dry weight) into clay slurry for mitigating the effect of apparent clogging around PVDs during vacuum consolidation was investigated by mini-PVD unit cell consolidation tests as well as microstructure observations via scanning electron microscopy (SEM) imaging. The consolidation test results indicated that for the specimens with CaCl₂, cement and lime additives the rate of consolidation increased considerably, while the effect of the NaCl additive was limited. The SEM images show that for the specimen without additive, there were obvious localized deformation-induced microstructure anisotropies. For specimens with CaCl₂ and cement/lime additives, the microstructures of the soils tended to be isotropic. The additives tested reduced the thickness of the diffusive double layer around the clay particles and promoted the formation of a more stable flocculated microstructure, therefore mitigating the effect of apparent clogging.     

矿物组成对低热硅酸盐水泥抗海水侵蚀性能的影响

为研究不同矿物组成对低热硅酸盐水泥抗海水侵蚀能力的影响,阐明低热硅酸盐水泥抗海水侵蚀的机理,利用分析纯化学试剂和水泥原材料分别制备硅酸二钙单矿和水泥熟料,并将具有不同矿物组成的低热硅酸盐水泥净浆试件在人工模拟海水中浸泡28d。通过强度发展规律、物相分析和综合热分析,发现硅酸二钙和铁铝酸四钙可以稳定低热硅酸盐水泥在海水中的强度发展,并阐明了海水中复杂盐离子与水泥水化产物反应的机理,建议适当提高铝酸三钙含量以增强低热硅酸盐水泥的抗海水侵蚀能力。     

Study on the pozzolanic properties of a natural Cuban zeolitic rock by conductometric method: Kinetic parameters

Natural zeolite is a type of mineralogical material containing large quantities of reactive SiO₂ and Al₂O₃. These materials have been used in the cement industry as a supplementary cementing material. Like other pozzolanic materials the zeolite contributes to improve the mortar and concrete performances mainly through the pozzolanic reaction with calcium hydroxide (CH). The pozzolanic reactivity of this type of material in comparison with other pozzolans is of great interest.The current paper presents a study about the pozzolanic activity of a natural Cuban zeolitic rock. The pozzolanic activity in a zeolitic material/CH solution is investigated by means of electrical conductivity measurements and latter on the kinetic parameters are quantified by applying a kinetic–diffusive model to process. The kinetic parameters that characterize the process (in particular the reaction rate constant) were determined in the fitting process of the model. The index of pozzolanic activity evaluated according to the obtained values of the reaction rate constant permits to characterize the pozzolanic activity of these materials in a rigorous way.The results show that this Cuban zeolitic rock is a natural material with high pozzolanic activity. Also, these results allow corroborating the direct influence of the fineness of zeolitic material on the pozzolanic and mechanical properties.     

Sodium sulfate attack on plain and blended cements

This paper presents an investigation into the sodium sulfate resistance of three portland cements of various compound compositions and 24 blended cements produced by replacing 10%, 20%, 30% and 40% of these cements with a fly ash and a natural pozzolan. Sulfate expansion of mortar specimens was evaluated using ASTM C1012 test procedure. No acid titration was made to change pH of the sulfate solution. In addition to 5% sodium sulfate concentration prescribed in the standard, 18 selected cements were also exposed to sodium sulfate of 3% concentration. A mathematical expression was developed to estimate the sulfate susceptibility of cements regarding their C₃A, C₃S/C₂S ratio, mineral additive content as well as concentration of sulfate solution.     

影响道路水泥干缩率的几个因素

试验研究了道路水泥中各矿物组成以及掺加不同的混合材(如矿渣、粉煤灰、煤矸石、石膏等)对其干缩率的影响.结果表明,干缩率并不只是随C3A含量的增大而增大,它还与其他矿物含量有关,是各矿物共同作用的结果;在本文实验条件下,掺不同混合材的水泥干缩率不同,掺粉煤灰优于掺矿渣,而掺矿渣则又优于掺煤矸石.掺适量石膏可以补偿水泥的体积收缩,但石膏掺量过多又会影响水泥的安定性.     

高温作用下无机矿物聚合物混凝土的力学性能研究

无机矿物聚合物混凝土在高温下的力学性能和普通混凝土有很大的区别.利用对比试验分析了无机矿物聚合物胶凝材料中各种原材料对无机矿物聚合物混凝土在高温下的抗压强度的影响,并和普通混凝土在高温下的抗压强度变化规律进行了对比,以期为无机矿物聚合物混凝土的耐火设计及理论分析时参考.     

Stiffness and strength development of the soft clay stabilized by the one-part geopolymer under one-dimensional compressive loading

In this paper, the one-part alkali-activated geopolymer was adopted as the soil binder to stabilize the soft clay under the one-dimensional compressive loading. Factors influencing the stiffness and strength of the one-part geopolymer stabilized soil such as the proportion of silicon-aluminum raw materials, the mass ratio of solid NaOH (NH) to raw materials and the water-binder ratio were taken into account. The stiffness development of the one-part geopolymer stabilized soft clay under one-dimensional compressive loading was investigated by an improved transducer system equipped with a pair of bender extender elements and the tactile pressure sensor, from which the time history of earth pressure coefficient (i.e., K₀) and elastic wave velocity (i.e., the compression wave velocity V_P and the shear wave velocity V_S) of the stabilized soil sample could be measured, respectively. Furthermore, the unconfined compressive strength (UCS) of the stabilized soil sample cured under varied one-dimensional compressive loadings was tested to reveal the strength development of the geopolymer stabilized soil sample. Among different mixing proportions, the alkali-activated binary precursor [90% ground granulated blast furnace slag (GGBFS) and 10% fly ash (FA)] exhibits the highest performance in terms of the V_S, V_P and UCS of the geopolymer stabilized soil sample when the activator/precursor and water/solid ratio are 0.15 and 0.7 (i.e., the 4.67 mol/L of NH solution), respectively. The prediction of UCS for the one-part geopolymer stabilized soft clay was proposed and established based on the elastic wave velocity (i.e., V_S and V_P). The outcome of the current study sheds light on the practical use of the one-part alkali-activated geopolymer as a soil binder in ground improvement.     

Sulfate resistance of plain and blended cements exposed to wetting–drying and heating–cooling environments

Exposure conditions significantly affect the resistance of cements to sulfate attack. This article investigates the sulfate resistance of ordinary portland cement (OPC), sulfate resistant portland cement (SRPC), and blended cements with different proportions of natural pozzolan and Class F fly ash when subjected to different exposure regimes. Plain and blended cement mortar specimens were stored under three different conditions: (i) continuous curing in lime-saturated water, (ii) continuous exposure to 5% Na₂SO₄ solution at room temperature, and (iii) cyclic exposure to 5% Na₂SO₄ solution at room temperature in which the cycles consisted of wetting–drying and heating–cooling. The sulfate resistance of cements was evaluated by measuring the reduction in compressive strength and length change of mortar specimens up to one year of exposure. This study revealed that the performance of blended cements under sodium sulfate solution at room temperature was better than that of SRPC with a 3.6% C₃A content when the length change was considered. However, for the structures exposed to sulfate attack and cycles of wetting–drying and heating–cooling, SRPC was found to perform better than blended cements when the compressive strength losses were considered.