In a two-part series, Consultant and Advisor Shreesh A Khadilkar, discusses how advanced additive formulations allow for customised, high-performance and niche cements.
Cement additives are chemicals (inorganic and organic) added in small amounts (0.01 per cent to 0.2 per cent by weight) during cement grinding. Their main job? Reduce agglomeration, prevent pack-set, and keep the mill running smoother. Thus, these additions primarily improve, mill thru-puts, achieve lower clinker factor in blended cements PPC/PSC/PCC. Additionally, these additives improve concrete performance of cements or even for specific special premium cements with special USPs like lower setting times or for reduced water permeability in the resultant cement mortars and concrete (water repellent /permeation resistant cements), corrosion resistance etc
The Cement additives are materials which could be further differentiated as:
Grinding aids
- Bottlenecks in cement grinding capacity, such materials can enhance throughputs
- Low specific electrical energy consumption during cement grinding
- Reduce “Pack set” problem and improve powder flowability
Quality improvers
- Opportunity for further clinker factor reduction
- Solution for delayed cement setting or strength development issues at early or later ages.
- Others: Materials which are used for specific special cements with niche properties as discussed in the subsequent pages.
When cement additives are used as grinding aids or quality improvers, in general the additives reduce the inter-particle forces; reduce coating over grinding media and mill internals. Due to creation of like charges on cement particles, there is decreased agglomeration, much improved flowability, higher generation of fines better dispersion of particles in separator feed and reduction of mill filling level (decrease of residence time). However, in VRM grinding; actions need to be taken to have stable bed formation on the table.
It has been reported in literature and also substantiated by a number of detailed evaluations of different cement additive formulations in market, that the cement additive formulations are a combination of different chemical compounds, composed of:
1. Accelerator/s for the hydration reaction of cements which are dependent on the acceleration effect desired in mortar compressive strengths at early or later ages, the choice of the materials is also dependent on clinker quality and blending components (flyash / slag) or a mix of both.
2. Water reducer / workability / wet-ability enhancer, which would show impact on the resultant cement mortars and concrete. Some of the compounds (retarders) like polysaccharide derivatives, gluconates etc., show an initial retarding action towards hydration which result in reducing the water requirements for the cements thus act as water reducers, or it could be some appropriate polymeric molecules which show improved wet-ability and reduce water demand. These are selected based on the mineral component and type of Cements (PPC/PSC /PCC).
3. Grinding aids: Compounds that work as Grinding Aid i.e. which would enhance Mill thru-put on one hand as well as would increase the early strengths due to the higher fines generation/ or activation of cement components. These compounds could be like alkanol-amines such as TIPA, DEIPA, TEA etc. or could be compounds like glycols and other poly-ols, depending on whether it is OPC or PPC or PSC or PCC manufacture.
Mechanism of action
1. Reduce Agglomeration ; Cement particles get electrostatically charged during grinding; stick together ; form “flocs” ; block mill efficiency ; waste energy. Grinding aid molecules adsorb onto particle surfaces ; neutralise charge ; prevent re-agglomeration.
2. Improve Powder Flowability; Adsorbed molecules create a lubricating layer; particles slide past each other easier ; better mill throughput ; less “dead zone” buildup.
;Also reduces caking on mill liners, diaphragms, and separator screens ; less downtime for cleaning.
3. Enhance Grinding Efficiency (Finer Product Faster) ; By preventing agglomeration, particles stay dispersed ; more surface area exposed to grinding media ? finer grind achieved with same energy input ; Or: same fineness achieved with less energy ; huge savings.
Example:
- Without aid ? 3500 cm²/g Blaine needs 40 kWh/ton
- With use of optimum grinding aid ? same fineness at 32 kWh/ton ? 20 per cent energy savings
4. Reduce Pack Set and Silo Caking, Grinding aids (GA) inhibit hydration of free lime (CaO) during storage , prevents premature hardening or “pack set” in silos. , especially critical in humid climates or with high free lime clinker.
It may be stated here that overdosing of GA , can cause: – Foaming in mill (especially with glycols) ? reduces grinding efficiency, retardation of cement setting (especially with amines/acids), odor issues (in indoor mills) – Corrosion of mill components (if acidic aids used improperly)
The best practice to optimise use of GA is , Start with 0.02 per cent to 0.05 per cent dosage , test fineness, flow, and set time , adjust up/down. Due to static charge of particles, the sample may stick to the sides of sampler pipe and so sampling need to be properly done.
Depending on type of Cements i.e. OPC, PPC, PSC, PCC, the grinding aids combinations need to be optimised, a typical Poly carboxylate ether also could be a part of the combo grinding aids
Cement additives for niche properties of the Cement in Concrete.
The cement additives can also be tailor made to create specific niche properties in Cements, OPC, PPC, PSC and PCC to create premium or special brands. The special niche properties of the cement being its additional USP of such cement products, and are useful for customers to build a durable concrete structure with increased service life.
Such properties could be:
- Additives for improved Concrete performance of Cements, High early strength in PPC/PSC/PCC, much reduced water demand in cement, cements with improved slump retentivity in concrete, self-compacting, self levelling in concrete, cements with improved adhesion property of the cement mortar
- Water repellence / water proofing, permeability resistance in Mortars and Concrete.
- Biocidal cement
- Photo catalytic cements
- Cements with negligible ASR reactions etc.
Additives for cements for improved concrete performance
High early strengths: Use of Accelerators. These are chemical compounds which enhance the degree of hydration of cement. These can include setting or hardening accelerators depending on whether their action occurs in the plastic or hardened state respectively. Thus, the setting accelerators reduce the setting time, whereas the hardening accelerators increase the early age strengths. The setting accelerators act during the initial minutes of the cement hydration, whereas the hardening accelerators act mainly during the initial days of hydration.
Chloride salts are the best in class. However, use of chloride salts as hardening accelerators are strongly discouraged for their action in promoting the corrosion of rebar, thus, chloride-free accelerators are preferred. The hardening accelerators could be combinations of compounds like nitrate, nitrite and thiocyanate salts of alkali or alkaline earth metals or thiosulphate, formate, and alkanol amines depending on the cement types.
However, especially in blended Cements (PPC/PSC/PCC the increased early strengths invariably decrease the 28 Day Strengths. These aspects lead to creating combo additives along with organic polymers to achieve improved early strengths as well as either same or marginally improved 28 Days strengths with reduced clinker factor in the blended cement, special OPC with reduced admixture requirements. With use of appropriate combination of inorganic and organic additives we could create an OPC with substantially reduced water demand or improved slump retentivity. Use of such an OPC would show exceptional concrete performance in high grade concretes as it would exhibit lower admixture requirements in High Grade Concretes.
PPC with OPC like Properties: With the above concept we could have a PPC, having higher percentage flyash, with a combo cement additive which would have with concrete performance similar to OPC in say M40/M50 concrete. Such a PPC would produce a high-strength PPC concrete (= 60 MPa @ 28d) + Improved Workability, Durability and Sustainability.
Another interesting aspect could also be of using Ultrafine fine flyash /ultrafine slags as additions in OPC/PPC/PSC for achieving lower clinker factor as well as to achieve improved later age strengths with or without a combo cement additive.
The initial adhesion property at sites of especially PPC/PSC/PCC based mortars can be improved through use of appropriate organic polymers addition during the manufacture of these cements. Such Cements would have a better adhesion property for plastering/brick bonding etc., as it has much lower rebound loss of their Mortars in such applications.
It is needless to mention here that with use of additives, we could also have cement with viscosity modifying cement additives, for self-compaction and self-leveling concrete performance.
Use of Phosphogypsum retards the setting time of cements, we can use additive different additive combos to overcome retardation and improve the 1 day strengths of the cements and concretes.
The concluding part of this article will appear in the next issue of ICR.
About the author:
Shreesh Khadilkar, Consultant & Advisor, Former Director Quality & Product Development, ACC, a seasoned consultant and advisor, brings over 37 years of experience in cement manufacturing, having held leadership roles in R&D and product development at ACC Ltd. With deep expertise in innovative cement concepts, he is dedicated to sharing his knowledge and improving the performance of cement plants globally.