New insights into the mechanisms of carbon dioxide mineralization by portlandite

Portlandite (Ca(OH)₂; also known as calcium hydroxide or hydrated lime), an archetypal alkaline solid, interacts with carbon dioxide (CO₂) via a classic acid–base “carbonation” reaction to produce a salt (calcium carbonate: CaCO₃) that functions as a low-carbon cementation agent, and water. Herein, we revisit the effects of reaction temperature, relative humidity (RH), and CO₂ concentration on the carbonation of portlandite in the form of finely divided particulates and compacted monoliths. Special focus is paid to uncover the influences of the moisture state (i.e., the presence of adsorbed and/or liquid water), moisture content and the surface area-to-volume ratio (s_a/v, mm⁻¹) of reactants on the extent of carbonation. In general, increasing RH more significantly impacts the rate and thermodynamics of carbonation reactions, leading to high(er) conversion regardless of prior exposure history. This mitigated the effects (if any) of allegedly denser, less porous carbonate surface layers formed at lower RH. In monolithic compacts, microstructural (i.e., mass-transfer) constraints particularly hindered the progress of carbonation due to pore blocking by liquid water in compacts with limited surface area to volume ratios. These mechanistic insights into portlandite's carbonation inform processing routes for the production of cementation agents that seek to utilize CO₂ borne in dilute (≤30 mol%) post-combustion flue gas streams.     

INVESTIGATIONS ON THE INTERFACIAL ZONE BETWEEN AGGREGATE AND PASTE OF PORTLAND CEMENT

The composition and structure of theinterfacial zone between aggregate and paste ofportland cement as well as the orientationcoefficient of portlandite (Icn) werepreliminarily studied by XRD and SEMmethods.The main products in the zone are C-S-H gel,portlandite,AFt and pores,micro-cracks as well as unhydrated portlandcement clinker particles.Potlandite not onlygrows well but exists in orientating forms,andnear the interface it exists in a state parallel tothe surface of aggregate by its(001)latticeplane.Icn rises with the increase of cementwater ratio(w/c)and the development of thecement hydration.The higher the w/c,themore the pores and micro-cracks in thezone,w/c bears an exact relation to the decreaseof bond strength.Icn is decreased when 5.0%silica fume is added to cement.Bond strengthsat early ages are reduced by adding 0.5% FDN(one kind of water-reducing admixture)due tothe retarded hydration of cement,but they willincrease continually and become greater than those of other samples after 28 days.     

碱-硅集料反应的模型研究Ⅱ.碱-二氧化硅溶液对氢氧化钙的侵蚀

为了深入研究氢氧化钙在碱-集料反应中的作用,专门研究了结晶氢氧化钙在各种SiO_2/Na_2O摩尔比及不同SiO_2浓度的碱-硅溶液中受侵蚀的情况。结果发现当溶液的R_(?s)=1.6—2时侵蚀最严重;在此范围以外的溶液造成的侵蚀轻微得多。表面反应生成的产物从沉淀到凝胶及C-S-H变化很大。溶液中的SiO_2浓度的增加也有利于对氢氧化钙侵蚀的增强。探讨了氢氧化钙在不同pH值下受侵蚀的机理。     

集料与硅酸盐水泥石界面区的若干研究

利用XRD层析法和SEM研究了集料与硅酸盐水泥石界面区的组成、CH晶体取向指数等。实验结果表明,界面区中水化产物主要是C-S-H凝胶、CH晶体、AFt、孔隙以及未水化的熟料矿物;界面区中CH晶体发育良好,取向作用较强,界面处它以(001)面平行于集料表面生长;水化龄期增长或水灰比提高时,CH晶体取向作用增强,而且水灰比高时,界面区中孔洞、裂纹增多,降低界面粘结强度;水泥中掺入5.0%wt硅灰后,CH晶体取向作用下降;水泥中掺入5.0wt%FDN减水剂,由于水泥早期水化程度低,对水泥石-集料早期粘结强度不利,但28d后其粘结强度就能赶上并超过其它试样。     

纳米SiO2与水泥硬化浆体中Ca(OH)2的反应

应用XRD相分析和强度试验对纳米SiO2与水泥硬化浆体中Ca(OH)2的反应进行了研究，从该体系中得到的纳米SiO2为无定形的物质。纳米SiO2与Ca(OH)2(晶体粉末)的反应至少始于1h龄期，而且在3d龄期内反应速度较快；在掺质量分数为3畅纳米SiO2的条件下，纳米SiO2能明显地降低在水泥硬化浆体和骨科界面中Ca(OH)2晶体的取向性，明显地减小界面Ca(OH)2晶粒的尺寸，并能减少界面Ca(OH)2晶体的含量，特别是在早期就能改善界面。纳米SiO2能明显地提高水泥硬化浆体的强度，特别是早期强度。     

Influence of Mineral Admixtures on Essential Properties of Ternary Cement Blends

Two CaCO3-based materials （limestone and clamshells） and steel slag were used as mineral admixtures in cement to produce ternary blends and their influences on hydration and portlandite formation were analyzed. Additionally, mechanical properties were determined. These properties were determined using X-ray diffraction and scanning electron microscopic/energy dispersive X-ray analytical techniques as well as applying methods specified by EN （European Standards） and ASTM （American Standards for Testing and Materials）. The portlandite （Ca（OH）E） content was considerably reduced from 36.9% of reference cement to between 13.79% and 15.5%. With the water demand and setting times of the cements containing up to 10%, admixtures did not change significantly. The mechanical tests results showed that ternary blends produced 2-day strengths higher than that specified by EN 197-1 and that blends containing up to 20% admixtures can be used to produce both Class 32.5N and 42.5N cements.     

THE STUDY OF MINERAL COMPONENTS IN KAOLINE FROM HUNAN JIEPAI

This paper finds out distributive pattern ofprincipal minerals in Jiepai kaoline and demonstrates theblack matter to be not onganic matter but fine scaly mica-hematite aggregate in black vein mud,the nanometer mineral-portlandite Ca(OH)_2 in this kaoline is discovered.