Formation of Microstructure in Low-clinker Binders Rich in Limestone

由于波特兰水泥熟料的生产是能源密集型的，开发低熟料结合剂对于未来环境友好型混凝土的生产具有高度的生态和经济意义。石灰石是一种合适的粘合剂组分，其已经在水泥生产商的采石场中，需要相对较少的能量用于研磨，并且几乎可以无限量地获得。然而，基于具有50%以上石灰石的熟料的合适粘合剂的生产超过了当前的技术极限，并且只能通过控制从密集堆积的新鲜糊状物形成固体结构的机理的知识来实现。这些包括颗粒表面和溶解的离子和高效减水剂分子之间的相互作用，它们决定了颗粒的分散、成核和生长过程，特别是石灰石颗粒之间的水化产物的空间分布，从而决定了相和强度的演变。在实验中，材料（CEM I，方解石粉）被分成一系列粒度级。然后将方解石级分以不同的比例合并以获得高度填充的颗粒混合物。接下来，将各种方解石尺寸级分体积地替换为等效的CEM I级分以产生不同的新鲜糊剂（CEM I粒度，w/c比）。在此，新鲜糊状物的水含量对应于填充颗粒之间的空间。除了研究新浆体的流动性（Minislump）外，还对早期水化过程进行了详细的研究：zeta电位（电声）、孔溶液的化学组成（包括TOC）（加压萃取）、水化热（热流量热法）和水化产物的形成（XRD分析）。测定了较长水化时间后的强度和孔径分布（MIP），并研究了其他矿物作为致密填充粘结剂体系中C-S-H成核的基质的影响。膏体是由与方解石类似的致密石英或文石制成的。预计微观结构的演变也取决于粘合剂中的铝含量，因为这会影响石灰石的化学反应性。为了研究这一点，将CEM I颗粒部分地用细粉煤灰代替。实验结果为粘结剂的粒度组成之间的关系提供了新的见解（组分、表面、粒度、填充、颗粒间分离）、孔溶液（离子、高效减水剂分子）、离子和高效减水剂分子的吸附及其对流动性和微观结构（相、孔隙率和强度）的影响。

Owing to the energy-intensive production of Portland cement clinker, the development of low-clinker binders is of high ecological and economic importance for the future production of environment friendly concrete. Limestone is a suitable binder component which is already in the quarries of the cement producers, requires relatively little energy for grinding and is available in virtually unlimited quantities. However, the production of suitable binders based on clinker with above 50% limestone exceeds the current limits of technology and can only be achieved through knowledge of the mechanisms that control the development of the solid structure from the densely packed fresh paste. These include the interactions between particle surfaces and dissolved ions and superplasticizer molecules which determine the dispersion of the particles, nucleation and growth processes and, in particular, the spatial distribution of hydration products between the limestone particles and thus the evolution of phases and strength.In the experiments, the materials (CEM I, calcite flour) are separated into a series of particle size fractions. The calcite fractions are then combined in different proportions to obtain a highly packed particle mixture. Next, various calcite size fractions are replaced volumetrically by equivalent CEM I fractions to produce different fresh pastes (CEM I particle sizes, w/c ratio). Here, the water content of the fresh pastes corresponds to the space between the packed particles. The type (nanostructure) and dosage of the superplasticizer are varied.Besides the flowability (Minislump) of the fresh pastes, the early hydration is studied in detail: zeta potential (electro-acoustic), chemical composition (including TOC) of the pore solution (extraction under pressure), heat of hydration (heat-flow calorimetry) and the formation of the hydration products (XRD analysis). Strengths and pore size distributions (MIP) are determined after longer hydration times.The effect of other minerals as a substrate for C-S-H nucleation in densely packed binder systems is also investigated. Pastes are produced with densely packed quartz or aragonite analogously to calcite.It is expected that the evolution of the microstructure also depends on the aluminium content of the binder because this affects the chemical reactivity of limestone. To investigate this, the CEM I particles are partly replaced by fine fly ash.The experimental results provide new insights into relationships between the granulometric composition of the binder (components, surfaces, particle sizes, packing, interparticle separation), the pore solution (ions, superplasticizer molecules), the adsorption of ions and superplasticizer molecules and their effect on flowability and microstructure (phases, porosity and strength).