Investigation on the replacement of lithium carbonate as accelerator for calcium aluminate cements and its underlying working mechanism

为了加速氧化铝粘结剂和三元粘结剂系统的发展，每年从供应链中提取1000多吨锂盐，并将其不可逆转地粘结到地板上。然而，电动汽车的生产大量需要这种宝贵的元素，导致建筑业的价格更高，供应不足。在此背景下，寻找锂在建筑应用中的替代品是非常可取的。令人惊讶的是，在建筑化学主席的早期研究中发现了这一点，该研究与微生物产生的和天然生物聚合物在藻酸盐加速氧化铝水泥水化的锌铝LDH中的插层行为有关。这是完全出乎意料的，因为众所周知，这种生物聚合物通常会延缓波特兰水泥的形成。这种加速效应也非常有趣，因为藻酸盐代表着一种可再生资源，几乎可以无限获得。此外，它们完全无毒，因为海藻酸盐通常用于食品工业(例如用作增稠剂)。对于藻类，可以区分不同的种类，如绿藻、红藻和褐藻，这些藻类又被划分为不同的亚纲和物种。商业海藻酸盐仅通过从褐藻(褐藻目)中提取获得。到目前为止进行的初步调查表明，在含有50m.%氧化铝的氧化铝水泥中加入0.1m%的随机选择的商品海藻酸钠，凝结时间减少了一半。因此，系统地检测选定的藻酸盐样品的加速效应、确定最佳的结构组成和研究这种效应的潜在机制似乎是很有希望的。为此，将通过各种方法来验证加速效果，包括热流量热法(检测水泥在水化过程中释放的热量)、超声波测量(根据声波通过介质的时间显示微结构的发展和致密化)和砂浆的机械强度测量。为了充分了解藻酸盐的作用机理，以及具体的藻酸盐衍生物，将研究藻酸盐对CA和C12A7等个别熟料相的影响，分析藻酸盐对水泥孔隙溶液中离子含量的影响以及C-A-H相的形核和结晶。最后，考察了海藻酸盐对OPC/CAC/AH三元粘结剂体系的影响。与纯铝酸盐水泥相比，该系统产生了根本不同的水合物。与纯CaC相比，没有形成C-A-H相，而主要水化产物为钙矾石。

For the acceleration of alumina cements and ternary binder systems, more than a thousand tons of lithium salts are taken each year from the supply chain and are irretrievably cemented e.g. into floorings. However, this precious element is needed in large quantities for the production of electric cars, resulting in a higher price and poor availability for the construction industry. Against this background, it is highly desirable to find an alternative for lithium in construction applications. Surprisingly, it was found in earlier research at the chair of construction chemistry relating to the intercalation behavior of microbially produced and natural biopolymers into Zn-Al-LDHs that alginates accelerate the hydration of alumina cement. This was completely unexpected as it is known that such biopolymers generally retard Portland cement. The accelerating effect is also very interesting because alginates represent a renewable resource with practically unlimited availability. Furthermore they are completely non-toxic as alginates are commonly used in the food industry (e.g. as thickener). As for the algae, different kinds can be differentiated, e.g. green algae, red algae and brown algae, which in turn are divided into different subclasses and species. Commercial alginates are obtained exclusively by extraction from brown algae (Phaeophyceae). Preliminary investigations conducted so far showed that addition of 0.1 M.% of a randomly selected commercial sodium alginate to an alumina cement containing 50 m.% Al2O3 reduced the setting time by half. It therefore seems to be promising to examine systematically selected alginate samples for their accelerating effect, to determine an optimal structural composition and to investigate the underlying mechanism of this effect. For this purpose, the accelerating effect is to be verified by various methods including heat flow calorimetry (detects the amount of heat released by the cement during hydration), ultrasound measurements (shows the development and densification of the microstructure based on the passage time of the sound wave through the medium) and mechanical strength measurements on mortars. To fully understand the mechanism of action of the alginate, also specific alginate derivations, an investigation of the effect on individual clinker phases such as CA and C12A7, an analysis of the influence of the alginate on the ion content in the cement pore solution and the nucleation and crystallization of the C-A-H phases will be studied. Finally, the effect of alginates on the ternary binder system OPC/CAC/AH will be investigated. This system produces fundamentally different hydrates compared to pure aluminate cement. In contrast to pure CAC, there no C-A-H phases are formed, but instead ettringite presents the main hydration product.