Hand-considered hypotheses, 5201–5300
Written one at a time, not composed. This hundred is restricted to cement, clinker and concrete chemistry: raw-meal burnability and the kiln’s sulfur and alkali cycles, alite and belite polymorphism, the kinetics of hydration and the composition of C-S-H, the working of admixtures in the pore solution, the chemistry of shrinkage and creep, and the durability reactions — chloride, sulfate, carbonation and alkali–silica. Each entry carries its own logical-form label, and no label is repeated, so the variety is structural rather than only topical. None has been tested. No data are reported and no citation is made; where a magnitude is mentioned it is an estimate, not a measurement.
Clinkering: raw meal, burning zone and cooling
Misattribution of a cooling artefact
H5201. Free lime in clinker is read as raw meal that never combined, but a slowly cooled clinker precipitates secondary lime from the interstitial melt, and that lime is a cooling artefact rather than a burning failure. Splitting one nodule, quenching half in air and cooling the other half in the kiln's own draft, should give two free-lime numbers from one chemistry.
Confounded cause
H5202. Sulfate is credited with stabilising M1 alite, but sulfate-rich clinkers are also burned hotter and quenched faster, so the polymorph ratio may be reporting the cooling rate under a chemical name. A series that holds the cooling profile fixed while varying the clinker's SO3 would divide the two causes, and the series is easy because SO3 can be changed in the raw meal without touching the flame. The same confound would explain why two plants burning the same meal report different polymorph ratios.
Two co-varying causes, one separated
H5203. Fast-cooled belite hydrates faster, which is credited to the stabilised alpha-prime polymorph, but the same quench microcracks the crystal and a cracked crystal presents more surface to the water. Boron-stabilised alpha-prime belite that is cooled slowly enough to be crack-free would hold the polymorph and remove the cracks, and the rate that survives would belong to the polymorph alone.
Wrong measurand
H5204. Acceptance limits free lime by mass, but expansion is driven by the coarse, dead-burned crystals that hydrate over months while the fine ones hydrate in days. A clinker at the limit with fine lime may be safer than one below the limit carrying a few large crystals, which makes crystal size the quantity the limit should name. The distinction matters at the margin, where a supplier's certificate and a petrographer's count disagree.
Hidden reservoir
H5205. A kiln coating passes for a passive deposit, but it absorbs alkali sulfate at one temperature and releases it at another, so the coating is a reservoir that records the kiln's recent thermal history rather than the meal's analysis. Two identical meals can then produce different clinker SO3 depending on where the coating sits, and the difference should follow the coating's mass rather than the meal's sulfur.
Thermal cause wearing a chemical name
H5206. Rings are explained by a sticky mix, but a ring is where the melt freezes on a shell colder than the bed, and the shell's temperature is set by the flame's shape rather than by the meal's chemistry. A ring that migrates when the burner is changed, with no change of meal, would show the deposit is a thermal feature wearing a chemical name.
Instrument measures the wrong object
H5207. Burning-zone temperature is quoted from a pyrometer that sees the flame and the dust it carries, not the bed, so the number belongs to the gas. The bed is cooler by a gap set by the flame's luminosity, and that gap explains why two kilns reporting the same temperature need different free lime. A two-colour pyrometer would separate them, because the dust and the bed do not share an emissivity.
Pathway substitution
H5208. Zinc and copper are discussed as lattice substituents in alite, but at clinker concentrations their strongest effect may be on the melt's viscosity, which changes crystal growth and the rate at which lime is assimilated. The same addition should then matter more in a clinker with a large liquid fraction and almost not at all in one with a small fraction.
Scaling with the wrong variable
H5209. The sulfate optimum of a cement is indexed to the C3A percentage, but sulfate is consumed at the aluminate's surface, so the demand should scale with the aluminate's specific surface rather than with its mass. Two clinkers with the same C3A and different interstitial textures should need different gypsum, and the difference should follow the texture.
State rather than composition
H5210. The unreactive aluminate in some clinkers is called a composition problem, yet cubic and orthorhombic C3A differ in reactivity at identical composition, and the polymorph is set by the alkali in the interstitial melt during cooling. A “high C3A” label may therefore be hiding a cooling schedule, and the schedule is the thing that can be changed.
Property belonging to history
H5211. Grindability is put down to the clinker's mineralogy, but the energy needed to break a nodule is set by the crack network that cooling left in it, and that network varies with the cooling path. Two clinkers of identical phase analysis, cooled differently, should differ in mill throughput by more than the phase analysis predicts. Mill throughput would then be predictable from the cooling curve, which every kiln already records.
Rate limitation by one fraction
H5212. Burnability is judged from the meal's fineness, but free lime is fed by the quartz that has not yet dissolved, and quartz dissolves from its own surface rather than from the meal's. A meal with coarse quartz and a fine average should burn worse than its fineness number promises, which makes the quartz's own surface the variable to control.
Bound set by another variable
H5213. The magnesium limit in clinker is written as an oxide percentage, but expansion comes from periclase crystals large enough to hydrate slowly, so the limit is really a crystal-size limit that a percentage cannot express. A clinker above the limit with finely dispersed periclase should be sound, and a clinker below it with a few large crystals should not.
Window, not a monotone trend
H5214. Fluoride is added to lower the burning temperature, and more fluoride lowers it further, but the same liquid that speeds lime assimilation also destabilises alite below a temperature that the fluoride has itself lowered. The useful addition is therefore a window, and the top of the window is a phase-stability boundary rather than an economic one.
Missing stream
H5215. Clinker composition is computed from the raw meal, but the meal's finest fraction leaves with the exhaust and that fraction is enriched in the soft, lime-rich constituents. The kiln's actual feed is therefore coarser and less lime-rich than the meal analysis, and the discrepancy grows as the preheater's separation worsens.
Inverted reading of an index
H5216. Insoluble residue is reported as an inertness measure, yet the residue is largely quartz and its reactivity rises steeply as its particles get finer, so a low residue can mean coarse inert quartz while a higher one can mean fine reactive quartz. The number is used as if it ran one way, and it may run the other. Sieving the residue into two fractions would show which direction the number is actually running.
Feedback through a physical property
H5217. The ferrite's composition sets the clinker's colour, and colour sets how much radiant heat the bed absorbs from the flame, so the interstitial chemistry feeds back on the burning temperature. A clinker that is dark because of its ferrite ratio is burned differently from a pale one at the same flame setting, and the difference is not in the chemistry alone.
Record in a defect
H5218. The alite's dislocation density records the cooling stress it suffered, and dissolution starts at dislocations, so two alites of identical composition can hydrate at different rates because of how they were cooled. The hydration rate would then carry a thermal history that no chemical analysis reports.
Hidden reductant
H5219. Organic carbon in the meal burns off early and is written off as irrelevant to clinker chemistry, but its combustion consumes oxygen at the kiln's inlet and creates a local reducing zone where sulfate is reduced to sulfide. The same meal with more organic carbon should leave a clinker with more sulfide sulfur and a different sulfate optimum. The effect should be largest where the transition zone is short, because the reducing pocket then sits closest to the burning zone.
Latent reactant
H5220. Sulfide sulfur in clinker is reported and then dismissed, but on exposure it oxidises to sulfate inside the hardened paste, and that sulfate is generated in place rather than delivered from outside. A clinker with a modest sulfide content should show internal sulfate attack with no external source, and the attack should appear after the paste has carbonated enough to drop its pH.
Value set by an uncertified property
H5221. Cement kiln dust is valued by its alkali content for use as an activator, but its free lime is dead-burned and hydrates slowly, so a chemically attractive dust can still be dimensionally dangerous. The dust's value is set by two independent properties, and the second is not on the certificate.
Phase misnamed as inert
H5222. The vitreous phase in clinker is often counted as an inert diluent, yet it is the quenched melt and its amount and composition set how much of the interstitial phase is available to react in the first hours. A clinker with more glass can be more reactive early even though it holds less crystalline aluminate.
Charge compensation decides the sign
H5223. Magnesium in alite is said to stabilise the M3 polymorph, but the stabilisation depends on how the substituted magnesium's charge is compensated, and sulfate and alkali offer different compensation routes. The same magnesia should push the polymorph in opposite directions in a low-sulfate and a sulfate-rich clinker. A pair of clinkers differing only in sulfate would show the reversal in the alite's cell parameters.
Distinction that dissolves
H5224. Kiln coating and build-up are treated as two problems with two remedies, but both are the same process of melt and dust accreting on a surface whose temperature crosses the solidus, so the distinction is one of location rather than mechanism. A single thermal model should predict both, and a remedy aimed at one should move the other.
Bound on a rate
H5225. There is an upper bound on alite crystal size beyond which the crystal's own fracture, not its hydration, sets the clinker's strength, because a large crystal cleaves along its growth twins under the mill's impact. The bound is set by the cooling rate and the melt's viscosity, not by the burning temperature. The bound can be located by grinding two clinkers to equal fineness and comparing strength at equal reacted fraction.
Two clocks read as one property
H5226. Free lime in a fresh clinker hydrates on a clock of days while the sulfate in the same clinker dissolves on a clock of minutes, so a test that measures expansion after a week reads the lime and a test that measures set reads the sulfate. The two clocks are collapsed into one soundness property, and they are not the same property.
Regime boundary
H5227. The sulfur cycle has two steady states, one limited by the sulfate the kiln can volatilise and one limited by the oxygen available to oxidise it. The same raw meal can sit in either state depending on excess air, and the clinker's SO3 will differ while every input analysis stays the same. An oxygen trim that holds excess air constant would hide the transition, which is why a well-controlled kiln rarely shows it.
Regulation against process
H5228. Chloride is regulated as a corrosion risk and therefore treated as an impurity to be minimised, but in the kiln it is a flux that lowers the temperature at which alite forms, so a small chloride addition can improve burnability. The regulation and the process want opposite things from the same element, and the conflict is resolved by where the chloride ends up rather than by how much is charged. Where it ends up decides the argument: in the interstitial phase it is a flux, and in the pore solution it is a risk.
Hydration: kinetics, C-S-H and supplementary binders
Clock belonging to another reaction
H5229. The induction period is timed from the heat curve's minimum and attributed to slow nucleation of C-S-H, but the minimum coincides with the sulfate's exhaustion in the pore solution, so the clock may belong to the aluminate reaction. Holding the alite fixed while varying only the gypsum's fineness would move the minimum if sulfate sets it and leave it if alite does.
Identity across a boundary
H5230. Inner and outer C-S-H are distinguished by which side of the original grain boundary they lie on, which is a geometric fact rather than a chemical one, so the two may be one phase at two densities set by the space available. A paste cured at a high water content should then show the same two densities in the same proportion per unit of reacted alite, a test that a boundary-based classification does not permit. Density can be mapped without assuming where the original boundary was, which is what makes the geometric definition hard to defend.
Artefact from the probe
H5231. The Ca/Si of C-S-H is measured with an electron beam that damages and dehydrates the phase beneath it, and the value drifts with dose, so the number may describe the beam as much as the solid. A dose series on one region, extrapolated to zero dose, would give a value that no routine analysis reports.
Operational cut taken for a species
H5232. Interlayer and gel water are separated by the temperature at which they leave, which is a property of the pore that holds them rather than of the bond that holds them, so the classification is a pore-size cut wearing the name of a chemical species. The same water should leave at a different temperature in a denser C-S-H, and the classification should move with it.
Rate versus amount
H5233. The sulfate's effect on the aluminate reaction is discussed in terms of how much gypsum a cement contains, but the reaction sees the sulfate's dissolution rate, so a coarse gypsum and a fine one at the same mass give different heat curves. The optimum SO3 is therefore a property of the gypsum's grind as much as of the clinker. Regrinding the gypsum at constant SO3 should move the optimum, which is a test no specification currently asks for.
Competition, not balance
H5234. The conversion of ettringite to monosulfate is written as a sulfate balance, but it is a competition between sulfate and carbonate for the same AFm site, and limestone tips that competition toward carboaluminate. A cement with limestone therefore converts its ettringite at a different sulfate level, and the SO3 that is optimum without limestone is not optimum with it. The optimum SO3 of a limestone cement is therefore a function of the limestone's fineness, which is not how it is specified.
Regime change with loading
H5235. Limestone's contribution is split into a filler effect and a chemical effect, and the split is stable only at low replacement; above a threshold the limestone's dissolution raises the pore solution's carbonate concentration and the chemical effect grows faster than the filler effect. The strength curve should therefore have a knee, and the knee is where the chemistry overtakes the physics.
Acceleration that changes the product
H5236. C-S-H seeds accelerate hydration by supplying surface, which is taken as running the same reaction faster, but the seeds also lower the pore solution's calcium and change the C-S-H's Ca/Si near the seed. A seeded paste is therefore not an earlier version of the unseeded one, and its later strength should not be predictable from the acceleration alone.
Space bound
H5237. Below a water content of roughly 0.38 by mass of cement, which is an estimate rather than a measurement, there is not enough space for the products of complete hydration, so the reaction must stop before the anhydrous phase is consumed. The stopping point is set by the C-S-H's density rather than by the water, so a paste with a denser C-S-H can hydrate further at the same water content.
Parameter drift
H5238. The late hydration rate is fitted with a diffusion coefficient through the inner product, but that coefficient is not a constant: it falls as the inner product densifies, and the fit absorbs the fall into a single number. A rate law with a time-dependent coefficient should fit the same data with a coefficient that is a material property. The test is a hydration series at several temperatures, because a drifting coefficient and an activation energy leave different signatures.
Attribution across components
H5239. The chemically bound water of a silica-fume blend is used as a measure of reaction, but the fume's own surface adsorbs water that is not chemically bound and leaves at the same temperature as some of the C-S-H's water. The measured degree of reaction therefore contains a physical adsorption term that grows with the fume's fineness.
Surface, not content
H5240. A fly ash's reactivity is put down to its glass content, but the reaction proceeds at the glass's surface, and the same glass content spread over coarser particles exposes less of it. Reactivity should then scale with the glass's specific surface, and grinding a fly ash should raise its reactivity without changing its glass content at all.
Common mechanism behind two activators
H5241. Alkali activation and sulfate activation of slag are described as two methods, but both work by raising the pH and the calcium's availability, so the two may be one activation seen with two anions. A slag activated to the same pH by either route should reach the same strength, and any difference should be attributable to the anion's own reaction.
Firing temperature recorded in coordination
H5242. Metakaolin's reactivity is put down to its amorphous state, but the kaolinite's dehydroxylation leaves an aluminium coordination that depends on the firing temperature, so a hotter metakaolin can be less reactive while being more amorphous. Reactivity should then follow the aluminium's coordination rather than the amorphous fraction.
Autocatalysis by product
H5243. The conversion of CAH10 to C3AH6 releases water, and that water raises the local water content and speeds the conversion that produced it, so the process is autocatalytic in a way a simple rate law misses. A converted region should therefore spread rather than form uniformly, which is visible as a conversion front in a specimen kept wet and split open. The front's position would be measurable by mapping the hydrate assemblage across a split face.
Buffered, not fixed
H5244. The pore solution's alkali concentration is treated as a cement property, but most of the alkali is held by the C-S-H and released as the C-S-H's Ca/Si falls, so the solution is buffered rather than fixed. A low-alkali cement can therefore deliver a high-alkali solution at late age, and the release should track the C-S-H's composition rather than the cement's analysis.
Hidden dependence of an adsorbed amount
H5245. The sulfate's retardation of alite is credited to adsorption on the C-S-H, but the amount adsorbed depends on the C-S-H's Ca/Si, so the retardation should vary between cements at the same sulfate concentration. The effect would then be a property of the C-S-H's composition rather than of the sulfate's concentration.
Two names, one process
H5246. Carbonation shrinkage and drying shrinkage are reported separately, but both empty the gel's small pores and both stiffen the C-S-H as they proceed, so the two may be one contraction seen with two pore fluids. A specimen dried in CO2-free air and one carbonated at the same relative humidity should show the same shrinkage if the mechanism is shared.
Mixing energy as a chemical variable
H5247. Mixing energy is filed under handling, but it breaks the C-S-H's flocculates and exposes fresh surface, so a paste mixed harder has more nucleation sites and hydrates faster for reasons that are not chemical. Two pastes of identical composition mixed to different energies should differ in their heat curves for days. Two laboratories with different mixers are therefore not measuring the same paste, even when the mix design is identical.
Coupled properties reported as one
H5248. Fresh paste's thixotropy is attributed to the flocculation of cement particles, but the same flocculation is broken by the superplasticiser's adsorption, so a paste with a high dosage should be less thixotropic even at the same yield stress. The two properties are measured together and reported as a single workability.
Counter-ion decides the sign
H5249. The accelerator's cation matters more than its anion: calcium salts accelerate because they raise the calcium's supersaturation with respect to C-S-H, and the anion's role is to keep the calcium in solution. A sodium salt of the same anion should therefore retard rather than accelerate, and the sign of the effect should follow the cation.
Reversible term inside a clock
H5250. The heat of hydration is used as a clock, but part of the measured heat is the water's adsorption on the C-S-H, which is released again on drying and is not part of the reaction's enthalpy. A degree of reaction from calorimetry therefore contains a reversible term that a bound-water measurement would not.
Degeneracy of two mechanisms
H5251. Two proposed mechanisms for the induction period, a metastable barrier and a nucleation lag, predict the same heat curve at a fixed water content and differ only in their response to a change in the amount of free surface. Seeding the paste with ground clinker should shorten a nucleation lag and leave a barrier intact, so one experiment separates them. The two mechanisms also differ in their temperature dependence, so a seeding test at two temperatures would close the argument.
Coupling of two rates
H5252. The alite and aluminate reactions are modelled as independent, but the aluminate's consumption of sulfate frees the C-S-H's surface from adsorbed sulfate, which changes the alite's rate. The two reactions are therefore coupled through the sulfate's adsorption, and a cement's heat curve should show a shoulder where the coupling is strong.
Product differs, not only the rate
H5253. Curing temperature enters every model as a rate multiplier, but a warmer cure produces a C-S-H of lower density and a coarser pore structure, so the product differs as well as forming sooner. A paste cured warm and then held at room temperature should never reach the strength of one cured cool throughout, even when both are compared at equal bound water.
Crossover of two opposite effects
H5254. The filler effect of an inert powder is put down to nucleation on its surface, but an inert powder also dilutes the cement and raises the local water content, and the two effects have opposite signs at high replacement. The crossover should appear as a maximum in strength at a filler content that depends on the filler's surface area.
Admixtures and fresh-state chemistry
Saturation is a solution property
H5255. The saturation dosage of a superplasticiser is read from a break in the dosage-versus-flow curve, but above the break the polymer's concentration in the pore solution keeps rising while the adsorbed layer is already complete, so the break marks a solution property rather than a surface one. The true monolayer point is lower, and it can be found by measuring the adsorbed amount instead of the flow. The break is a useful quality-control number and a poor physical one, and the two uses have been conflated.
Optimum shifts, ranking reverses
H5256. A polycarboxylate's side-chain length is optimised in a dilute suspension, but the chains coil differently at the ionic strength of a real pore solution, so the optimum shifts with the cement's alkali content. A polymer tuned on a low-alkali cement should underperform on a high-alkali one at the same dosage, and the ranking of two polymers should reverse.
Deferred reaction, abrupt set
H5257. A sugar retarder complexes calcium and slows alite, but it also slows the aluminate, and at a high dose the aluminate's set is deferred until the sugar is consumed by the alite, after which the aluminate sets abruptly. The result is a paste that stays fluid far longer than intended and then sets within minutes, which is a schedule artefact rather than a dosage error.
Adsorption kinetics, not air content
H5258. Air content is the number on the certificate, but the bubbles' size and spacing are set by the surfactant's adsorption kinetics at the bubble wall, so two admixtures at the same air content can give different spacing factors. The relevant variable is the spacing factor, and it can change while the air content stays where it was. A spacing-factor measurement on the hardened paste would rank the two admixtures differently from their certificates.
Equilibrium, not rate
H5259. A shrinkage-reducing admixture is described as slowing drying, but it lowers the pore water's surface tension and therefore the equilibrium shrinkage the gel can reach, so it changes the destination rather than the speed. A treated specimen should show a lower final shrinkage after re-saturation, which a rate explanation does not predict.
Wrong rheological parameter
H5260. A viscosity-modifying admixture is chosen to stop bleeding, but bleeding is a yield-stress phenomenon in the settling column, and a polymer that raises viscosity without raising yield stress will not stop it. The two properties can be varied independently, and the admixture's specification conflates them.
Recovery time, not set time
H5261. The rebound of sprayed concrete is attributed to the accelerator's set time, but the rebound is decided while the layer recovers its yield stress after the impact, so the relevant property is the thixotropic recovery time. An accelerator that sets fast but recovers slowly should rebound more than one that sets slowly and recovers fast.
Consumable inhibitor
H5262. A corrosion inhibitor is dosed by the water's chloride content, but nitrite is consumed by the steel's own passivation and by the paste's carbonation, so its effective dose falls with age. The inhibitor therefore has a service life, and a structure dosed at construction can lose protection before its chloride threshold is reached.
Shelf life belongs to the polymer
H5263. The shelf life of a polycarboxylate solution is attributed to microbial growth, but the polymer's side chains hydrolyse slowly at the solution's pH, and the hydrolysis lowers the dispersion with no visible spoilage. A preserved sample and an unpreserved one should diverge in performance while both remain clear.
Two roles for one ion
H5264. Gypsum is added for set control, but its dissolution also supplies the sulfate that flocculates the paste, so the same gypsum controls both the rheology and the aluminate reaction. A cement's flow time should therefore change when the gypsum's grind changes, even when its set time does not.
Charge density, not molecular weight
H5265. A superplasticiser's performance is put down to its molecular weight, but adsorption is driven by charge density, and two polymers of the same weight with different charge densities disperse very differently. The molecular weight is the number on the data sheet and the charge density is the one that matters.
Competitive adsorption, non-additivity
H5266. Two admixtures in one mix compete for the same surface, and the stronger adsorbing one displaces the weaker, so a combination can perform worse than either alone at the same total dose. The effect should appear as a non-additive flow at fixed total dosage, and it is why a package cannot be optimised component by component. The displacement can be measured directly by comparing each polymer's adsorbed amount alone and in the pair.
Set time is not a material property
H5267. Setting is measured by a penetration resistance that depends on the paste's bleeding and on the specimen's depth, so two pastes of the same chemistry can have different initial set times in different moulds. The measured set time is a property of the test's geometry as much as of the paste.
Hidden air
H5268. A superplasticiser can entrain air without any air-entraining agent because the polymer stabilises the bubbles the mixer creates, and the entrained air lowers strength in a way that is credited to the dosage. The strength loss at high dosage should then be removable by de-airing, which separates the dosage effect from the air effect.
Dispersant as reactant
H5269. Some dispersants are also reactants: a lignosulfonate carries calcium-complexing groups and can retard the alite, so its dispersing and its retarding actions are one chemistry at two sites. A purified fraction of the same lignosulfonate should show the two effects in a fixed ratio if they share a mechanism.
Alkali's double role
H5270. Alkali in the mix water is counted as an accelerator, but the same alkali raises the pore solution's pH and increases the C-S-H's negative charge, which changes how much superplasticiser adsorbs. A high-alkali mix should therefore need more polymer for the same flow, and the extra demand is not a dosage error.
Retarder's specificity
H5271. A retarder is assumed to act on the silicate reaction, but its effect on the aluminate decides whether the paste's sulfate balance is preserved, so a retarder that spares the aluminate can cause a false set once the sulfate is exhausted. The same retarder can be safe in one cement and unsafe in another at the same dose.
Physical bar to a chemical effect
H5272. An accelerator's effect is bounded by how fast it can reach the anhydrous surface, and in a paste with a thick adsorbed polymer layer the accelerator is consumed before it arrives. The dose-response should therefore depend on the superplasticiser's dose, and the two should not be optimised separately. A supplier's recommended dose is therefore a dose for one mix, not a property of the accelerator.
Shrinkage, creep and cracking
Irreversibility as a structural record
H5273. The shrinkage on first drying is partly irreversible, which is attributed to the collapse of the C-S-H, but the irreversibility could be the loss of the gel's coarse pore network, which never re-forms on rewetting. A specimen dried to a mild humidity and rewetted should recover fully if the collapse needs a lower humidity, and the boundary between reversible and not is then a pore-size boundary.
Consequence misread as cause
H5274. Autogenous shrinkage is attributed to self-desiccation, but the chemical shrinkage happens first and the self-desiccation is its consequence in a sealed paste, so the two are one process read at two scales. The shrinkage should then be proportional to the chemical shrinkage and only weakly dependent on the pore solution's humidity.
Evaporation against supply
H5275. Plastic shrinkage cracking is indexed to the evaporation rate, but a crack forms only when evaporation exceeds the bleed rate, and the bleed rate depends on the paste's permeability rather than on the weather. Two concretes under the same evaporation should crack differently, and the difference should follow the bleed.
Identity of two creeps
H5276. Basic creep and drying creep are measured separately and modelled with separate parameters, but both are the C-S-H's sliding under stress, and drying creep may be the same sliding with a lower activation barrier because the water has left. A specimen crept at one humidity that is then changed should show a transient that the two-parameter model cannot produce. Changing the humidity at constant load is the cleaner test, because the load history then contributes nothing new.
Linearity bound
H5277. Creep is taken as linear in stress up to a large fraction of strength, but linearity must fail when the C-S-H's sliding becomes fast enough to localise, and the localisation stress is a material property rather than a fixed fraction of strength. Two concretes of equal strength but different C-S-H densities should therefore have different linear limits.
Gradient as proxy
H5278. Thermal cracking in a massive pour is indexed to the temperature gradient, but the stress is set by the differential strain between core and surface, and a surface that is insulated has a smaller gradient yet can still crack if the core's expansion is restrained. The gradient is a proxy that fails when the insulation is uneven.
Desorption, not total water
H5279. Internal curing is dosed by the lightweight aggregate's absorbable water, but only the water released above the paste's relative humidity is useful, and a fine-pored aggregate holds most of its water below that humidity. The effective dose is therefore smaller than the absorbed water, and it depends on the aggregate's pore-size distribution rather than on its absorption.
Specimen-size artefact
H5280. Drying shrinkage is reported as a material property, but the measured value depends on the specimen's thickness because the moisture gradient never vanishes in a finite test, so the number is a property of the specimen's geometry. A thin specimen and a thick one of the same paste differ by the gradient's contribution rather than by the material.
One effect with two names
H5281. Aggregate is said to reduce shrinkage by restraining the paste, and also to increase shrinkage by diluting the paste's water content; the two are one effect seen in the composite's modulus and in the paste's water content, and they cannot be separated by varying the aggregate content alone. A series with a constant paste composition and a varying aggregate modulus would separate them.
Carbonation moves the baseline
H5282. Carbonation shrinkage is measured on a specimen that carbonates from the surface inward, but the carbonated skin is stiff and restrains the uncarbonated core, so the measured contraction is the composite's response and not the C-S-H's. A thin specimen, carbonated throughout, should shrink more per unit of CO2 than a thick one, which would show the restraint.
Water's two roles
H5283. Water in the gel is both a load-bearing medium and a reactant, and the two roles change with pore size: in small pores the water transmits disjoining pressure and in large pores it transmits capillary tension. The shrinkage coefficient should therefore change slope at a humidity that corresponds to the boundary between the two pore classes.
Recovery misread as damage
H5284. Creep recovery is small in concrete, which is read as evidence of damage, but a viscoelastic material with a broad relaxation spectrum recovers little on the test's timescale even without damage. The recovery measured over a longer interval should approach the predicted value, and the apparent damage should shrink as the observation window grows. A load held for a year and then released would show whether the missing recovery belongs to time or to damage.
Moisture's own creep
H5285. Drying creep is often attributed to the stress's effect on drying, but the drying itself generates pore-scale stresses that creep the C-S-H without any external load, so a specimen dried under a small load creeps more than the sum of the two tests. The extra creep should scale with the drying rate rather than with the load.
Durability chemistry: alkali–silica, sulfate, chloride, carbonation
Calcium supply sets the pessimum
H5286. Alkali–silica expansion has a pessimum in aggregate particle size, usually explained by the silica's reactivity, but a small particle releases its silica into a calcium-rich pore solution where the gel is less expansive, so the pessimum may be a calcium-supply effect. Holding the particle size fixed while varying the paste's calcium availability should shift the pessimum.
Two gels, one name
H5287. The gel of alkali–silica reaction is reported as one phase, but its calcium content varies from alkali-rich to calcium-rich and only the alkali-rich form swells strongly, so expansion depends on whether the gel can exchange calcium with the paste. A gel that has lost its calcium to the C-S-H should be expansive even at a modest alkali content.
Product of two monotone trends
H5288. The expansion of a concrete with a reactive aggregate has a maximum at an intermediate aggregate content, and the maximum is the product of two monotone functions: more aggregate supplies more silica and less paste supplies less alkali. The peak's position should therefore be predictable from the two trends, and it should move when the cement's alkali content changes.
Desorption threshold
H5289. Delayed ettringite formation is indexed to the curing temperature, but the expansion needs sulfate that the C-S-H bound at that temperature and released later, so the controlling variable is the C-S-H's sulfate capacity at the peak temperature rather than the temperature itself. A cement whose C-S-H binds little sulfate should be safe at a temperature that damages another. Cements of similar oxide composition but different C-S-H sulfate uptake, cured at one temperature, would separate the capacity from the temperature.
Buffer sets the threshold
H5290. The chloride threshold for steel corrosion is quoted as a ratio to hydroxyl, but the hydroxyl concentration is buffered by the C-S-H and by the pore solution's reserve, so the threshold is a property of the paste's buffer capacity rather than of the steel. A paste with a large reserve should tolerate more chloride at the same pH, and the tolerance should fall as the reserve is consumed.
Ice in the wrong pores
H5291. Freeze–thaw damage is attributed to the water that freezes in the capillary pores, but the gel pores' water freezes far below the test's range, so the damage is done by a small fraction of the water and the rest is a spectator. The damage should scale with the capillary water's volume rather than with the total freezable water, and the two diverge as the gel densifies.
Source of the salt
H5292. Efflorescence is put down to transport, but the salt's source is usually the cement's own alkali sulfate, so a cement with a high alkali sulfate content can effloresce without any external source. The remedy is then a clinker property rather than a masonry detail.
Front that cannot keep its law
H5293. Carbonation is modelled as a front that advances with the square root of time, but the front's advance is limited by the C-S-H's release of calcium in the carbonated zone, and that release slows as the zone's calcium is exhausted. The rate should therefore fall faster than the square-root law once the zone is depleted, which appears as a front that stalls.
Sulfate's own source
H5294. Sulfate attack is diagnosed by an external sulfate source, but a clinker with a high sulfate content can release sulfate from the C-S-H during drying or carbonation, so the attack can proceed with no external supply. The diagnosis should then include the paste's sulfate budget, which is rarely measured.
Pore solution's evolution
H5295. The pore solution's composition is measured by expression from a young paste, but the composition changes as the C-S-H's Ca/Si falls, so a young sample's solution is not the solution the steel will see. A chloride threshold predicted from a young paste's pH should therefore be optimistic.
Alkali's other source
H5296. Alkali–silica reaction is blamed on the cement's alkali, but some aggregates release alkali from their own feldspars, so a low-alkali cement with such an aggregate can still produce a high-alkali pore solution. The aggregate's release should be measurable as a rise in the expressed solution's sodium over time.
Volume of product versus site of growth
H5297. The expansion of a sulfate attack is computed from the volume of ettringite formed, but ettringite grows in the pores and its expansion depends on where it grows, so the same mass can be harmless or destructive. The relevant variable is the supersaturation at the growth site, not the total product. A paste with a large pore volume should therefore tolerate more ettringite before it expands, a prediction the volume calculation cannot make.
Two mechanisms, one sign reversed
H5298. Thaumasite formation is classed as a low-temperature variant of sulfate attack, but it consumes the C-S-H's calcium and silica directly, so it damages the binding phase rather than filling the pores, and its effect on strength is out of proportion to its mass. A concrete with thaumasite should lose strength faster than its expansion suggests, which reverses the usual diagnostic ordering.
Carbonation's other product
H5299. Carbonation is written as a reaction of portlandite, but the C-S-H also carbonates, and the C-S-H's carbonation releases water and shrinks the gel, so the carbonated zone's properties are set by the C-S-H rather than by the portlandite. The zone's stiffness and its diffusion coefficient should change at a CO2 dose below the portlandite's exhaustion.
Bound that closes the set
H5300. Every durability mechanism in this list, chloride, sulfate, carbonation and alkali, is slowed by the same buffer: the C-S-H's capacity to hold and release the ion in question, so a material's durability is bounded by its buffer capacity per unit of transport. The bound is a ratio rather than a rate, and it explains why two concretes with the same diffusion coefficient can have very different lives. The ratio can be compared by measuring two ions that share a diffusion coefficient but differ in binding.
Status
100 entries, H5201–5300. The forms are unevenly weighted on purpose: the attribution of a measurement to the wrong cause is the largest group, because most of the chemistry here is inferred from a number that a probe or a specimen geometry has already shaped.
Family distribution of the form labels: Attribution 26; Hidden term or reservoir 16; Identity and distinction 18; Inversion and non-monotone 8; Regime and state 13; Rate and clock 8; Bound and count 6; Feedback and coupling 5. The labels themselves are all distinct; the families are a reading aid, not a claim that each family is one kind of argument.
If one of these is worth an experiment, it is H5229 — that the induction period is a sulfate clock rather than a silicate one, so that changing only the gypsum’s fineness moves the heat curve’s minimum. It is checkable on a single clinker with two gypsum grinds and a calorimeter.
Incoprea Research · hand-considered hypotheses 5201–5300 · 3 November 2026 · ~/Desktop/Nov3/