Cement additives for composite cements

The term “cement additives” is a general term for a group of products that are supplied to the cement industry with the goal of optimising the process and/or product performance.

The term “cement additives” is a general term for a group of products that are supplied to the cement industry with the goal of optimising the process and/or product performance. They can essentially be divided into three main groups; grinding aids, performance enhancers or quality improvers, and functional additives. The focus of this article will be on performance enhancers and in particular their application to composite cements. Composite cements Along with the ordinary Portland cement (OPC) clinker and a sulfate carrier, composite cements contain one or more additional materials such as limestone, granulated blast furnace slag (GBFS), pulverized fuel ash (PFA) or pozzolana. The actual material(s) used will depend on local availably, what is permitted in the respective cement and concrete standards as well as the desired performance and cost of the final product. In India, the two main type of composite cement are Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC) and these are the cements type that we will discuss in this article. Whereas ordinary Portland cement clinker is considered to be a hydraulic material i.e. it reacts on the addition of water, PFA (low calcium content) is a pozzolanic material and needs to be activated in order to react to a significant extent. GBFS sits between the two previously mentioned materials and has latent hydraulic properties in that it will react with water if it is given enough time, but the reaction proceeds much faster if it is activated in some way. The two main reactive minerals in OPC are Alite and Belite, which when combined make up about 70-80 per cent by mass of the clinker. Both Alite and Belite essentially follow the same reaction pathway with the main difference being in the amount of Portlandite they produce as shown in the below equations. Reaction of Alite (C3S) with water: 3CaO·SiO2 + H2O → 1.7CaO·SiO2·H2O + 1.3Ca(OH)2 Reaction of Belite (C2S) with water 2CaO·SiO2 + H2O → 1.7CaO·SiO2·H2O + 0.3Ca(OH)2 It is the production of this Portlandite that initiates the reaction with the PFA and accelerates the reaction with GBFS. Consequently, if we can somehow accelerate the rate at which the Portlandite is produced then we should also accelerate the hydration of the PFA and GBFS as well. In the next section of this article we will look at how performance enhancers can help achieve this aim. Performance Enhancers The are two ways in which a performance enhancer can accelerate the reaction of C3S/C2S, one is via physical means and the other is chemically. In most cases a customized performance enhancer will actually do both of these things but let us first of all focus on the physical effects. It is well known that one of the outcomes of using grinding aids is a reduction in the amount of coarse particles in the produced cement, but why is this important? Figure 1 helps to explain this. One 40µm particle occupies the same volume as eight 10µm particles, but eight 10µm particles have two times the surface area of the 40µm particle available for reaction. Assuming the depth of hydration of C3S is 3.5µm after three days, then only 39 per cent of the 40 µm particle will have reacted compared with 72.5 per cent of the 10µm particle. Consequently, it is fair to assume that the smaller particle, due to its higher reaction degree, will have produced a higher concentration of Portlandite and in turn this will have a greater accelerating effect on the reaction of the PFA and GBFS. This is obviously a very simplistic view and real life is much more complicated than this, but it illustrates the principle of physical enhancement well. If we now turn our attention to the mechanisms of chemical acceleration we need to first look at the components of a performance enhancer. Most commercial products are a customised blend of several different materials. These are typically alkanolamine and glycol based, but some products may also contain inorganic salts. When Alite and Belite react with water to form Calcium Silicate Hydrates (CSH), the CSH crystals start to form a barrier around the unreacted clinker particle. This ultimately slows down the reaction as it is more difficult for the water to penetrate to the unreacted clinker particle. Inorganic salts, such as Calcium Chloride have an ability to flocculate hydrophilic colloids, which results in the creation of a surface layer that is more permeable allowing the water to reach the unreacted clinker particle. This mechanism is only really viable during the early stages of hydration and hence the reason why chloride is well known as an early strength enhancer in cement and concrete. Some of the key alkanolamines used in performance enhancers are Triethanolamine (TEA), Diethanolisopropanolamine (DEIPA) and Triisopropanolamine (TIPA). Each of these materials interacts with the clinker hydration mechanism is a different way. For example, during the early stages of hydration DEIPA promotes the formation of ettringite, whereas at later stages it accelerates the reaction of the Alite and causes a reduction in both the pore size and porosity [2]. TIPA on the other hand predominantly interacts with one of the minor phases, Ferrite (C4AF), by complexing the iron at the surface and exposing more surface area resulting in enhanced hydration [3]. So far we have looked at the effects that performance enhancers have on Alite and Belite hydration and how that can help accelerate the hydration of GBFS and PFA by increasing the concentration of Portlandite in the pore solution, but was about the direct effects on the hydration of GBFS and PFA? When you compare the dissolution rate – the rate at which the solid elements are dissolved into solution – of the key ions, Calcium (Ca), Silicon (Si), Aluminium (Al) and Iron (Fe) from a PFA sample mixed in pure water to one containing a small amount of TEA (0.6g/L) research has shown that there is a significant effect the dissolution rate of calcium and iron during the initial few hours [4]. This would suggest that TEA does indeed have an effect of the hydration of PFA. A study by Chinese researchers [5] has shown that the use of TEA and TIPA with GBFS results in a higher reactivity and greater Portlandite generation than samples that do not use these alkanolamines. Furthermore, the hardened mixes with both TEA and TIPA show a denser microstructure than that those without. In the particular study presented in this research, TIPA increased the reactivity more than TEA.

Case Study In the previous section we have presented the key theoretical aspects of how performance enhancers can accelerate cement hydration and improve the performance of composite cements. However, real systems are much more complex that this and therefore the key question to be answered is how do these types of cement additives actually perform on a day to day basis? The following case study is an example taken from India of how we can use this information to help tailor our products to meet the customer’s needs. The customer was currently using PFA at a replacement level of 29 per cent and they wanted to increase this while maintaining the existing performance parameters such as setting time and compressive strength. Initial screening of the most suitable raw materials for the customized product was conducted using isothermal calorimetry to understand the effect on the hydration reaction and a sample of the results are shown in figure 2. This information was then used to develop a number of test formulations, the results of which are shown in Table 2. As can be seen from the results all three of the formulations improved the baseline strength performance. However, the optimum selection for this particular application was Formulation 3, which has consistent and significant improvements across the 1,3 and 7 day compressive strength measurements. It was this formulation that was then taken forward for successful industrial trials with the customer. Conclusions This article has presented a high-level overview of the hydration process of composite cements and how cement additives can be used to optimise the performance of them. It is clear that the hydration process and the strength development mechanisms are complex and in many cases specific to a particular combination of OPC clinker and PFA or GBFS. In order to obtain the optimum performance, it is essential to find a partner that understands the fundamental mechanism of cement hydration, the cement manufacturing process and how cement additives interact with both of these elements. References [1] Lea, F. M., The Chemistry of Cement and Concrete, London, Edward Arnold, (1970) and Mindess, S., Concrete materials, Journal of Materials Education, 4, (1983), 984-1046, in Sindhunata A conceptual model of geopolymerisation. PhD thesis, Department of Chemical and Biomolecular Engineering, The University of Melbourne (2006). [2] Suhua, M. et al., Study on the hydration and microstructure of Portland cement containing diethanol-isopropanolamine, Cement and Concrete Research, 67, (2015), 122-130 [3] Sandberg et al. On the mechanism of strength enhancement of cement paste and mortar with triisopropanolamine. Cement and Concrete Research 34 (2004) 973 – 976 [4] Heinz D., et al. Effect of TEA on fly ash solubility and early age strength of mortar. Cement and Concrete Research 40, (2010), 392–397 [5] Haoxin L., et al. Effect of different grinding aids on property of granulated blast furnace slag powder, Materials and Structures 48, (2015), 3885–3893 ABOUT THE AUTHOR: Martyn Whitehead of Fosroc International. P.O. Box 12276, City Tower 2, Sheikh Zayed Road, Dubai, UAE.