Frequently Asked Questions

Why is calcium chloride not permitted in reinforced concrete and how does Conplast PC achieve similar acceleration without the corrosion risk?

Calcium chloride has been used as a concrete set accelerator for over a century and is highly effective at accelerating cement hydration and early strength development — typically achieving the same or better early strength with calcium chloride that Conplast PC achieves, and at lower cost. The reason calcium chloride is absolutely prohibited in reinforced concrete (and in any concrete where corrosion of embedded steel is a concern) is that the chloride ion (CL minus) is the primary initiator of reinforcement corrosion in concrete. Concrete normally provides excellent corrosion protection to embedded steel reinforcement through two mechanisms: physical protection (the dense concrete matrix physically prevents oxygen and moisture from reaching the steel surface); and the alkaline pore solution (pH of approximately 12 to 13 in concrete creates a passive oxide film on the steel surface that prevents corrosion). The chloride ion is the specific chemical species that destroys this protective passive film — when the concentration of chloride at the steel surface exceeds a threshold value (the chloride threshold), the passive film is locally disrupted and active corrosion begins, eventually causing concrete spalling and structural deterioration. Adding calcium chloride to concrete introduces chloride ions directly and uniformly into the concrete at high concentration — well above the chloride threshold at the steel surface — effectively beginning the corrosion process immediately. In structures with calcium chloride-accelerated concrete, active reinforcement corrosion has been observed within 5 to 10 years of construction. IS 456:2000 and IS 1343 explicitly prohibit calcium chloride in reinforced and prestressed concrete. Conplast PC achieves set acceleration through a different chemistry — typically modified aluminate or sodium silicate compounds that accelerate the early dissolution and reaction of tricalcium aluminate and tricalcium silicate phases of cement without introducing any chloride species into the mix.

What is the maximum recommended dosage of Conplast PC and what are the risks of over-dosing the accelerator in concrete?

The maximum recommended dosage of Conplast PC is specified in the product TDS and typically falls in the range of 2.0 to 3.0 per cent by weight of cement for most applications — equivalent to approximately 700 to 1,050 grams per 100 kg cement at standard Indian concrete cement contents of 350 to 400 kg per cubic metre. This maximum is a firm limit: exceeding the maximum dosage of Conplast PC causes significant negative effects on both early concrete behaviour and final concrete quality. The most serious consequence of Conplast PC over-dosing is flash set or early stiffening — at very high dosages, the acceleration of aluminate hydration is so rapid that the concrete begins to stiffen immediately after mixing (within minutes), making it impossible to transport, pump, or place. This flash set produces concrete with a highly irregular, poorly consolidated internal structure and dramatically reduced strength. At moderately over-dosed levels (50 to 100 per cent above the maximum), the concrete remains workable for sufficient time to place but the excessive early hydration generates very high heat of hydration in the first few hours after casting — this temperature peak can cause delayed ettringite formation (DEF) in the interior of large mass concrete elements, which causes internal expansion and microcracking years after construction. Additionally, at high dosages, Conplast PC can reduce the 28-day compressive strength of concrete relative to the base mix without accelerator — the initial acceleration of hydration occurs at the expense of subsequent strength gain, producing concrete with good early strength but below-specification 28-day strength. All concrete mixes using Conplast PC should be trial-tested (with dosage confirmation at the intended level, measuring slump, initial and final set time, and compressive strength at 1, 3, 7, and 28 days) before use on structural concrete to verify both the early performance and the 28-day strength compliance.

Can Conplast PC and Conplast R (retarder) be used in the same concrete mix and is there a situation in India where both would be needed?

Conplast PC (accelerator) and Conplast R (retarder) have opposing effects on cement hydration — Conplast PC speeds up hydration and shortens set time, while Conplast R slows hydration and extends set time — and using them together in the same concrete mix would counteract their individual effects. In most situations, using both an accelerator and a retarder together does not make sense: if the concrete requires extended workability (which Conplast R provides), an accelerator counteracts this benefit; and if fast early strength is required (which Conplast PC provides), a retarder counteracts this. There is however one conceptually interesting scenario in Indian construction where both types of admixture have been used in the same concrete with different timing objectives, though this is an advanced technique and not a standard practice: in large mass concrete pours (mat foundations, large columns) in the Delhi NCR summer, the concrete simultaneously has a long-haul issue (transit from an RMC plant far from the site), requiring some retardation to maintain workability during transit, AND a heat management issue in the centre of the mass (the slow later hydration generates heat that accumulates in the core). However, adding both accelerator and retarder is not the standard engineering solution to this combined problem — instead, the standard approach is to use a retarder (Conplast R) with a supplementary cementitious material (GGBS or fly ash), which reduces total heat of hydration and provides the necessary retardation without the conflicting addition of an accelerator. If a contractor or site engineer encounters a situation where they believe both products are needed, this should be treated as a signal that the concrete mix design needs fundamental review rather than a justification for combining opposing admixtures.